Keywords: consciousness, attention, vision, audition, EEG

What mechanisms lay the ground for conscious experience? Different accounts aim at resolving the issue. Some theories of conscious access posit that conscious experience is a graded phenomenon, reliant on activity within the sensory cortices. Other theories suggest that conscious access depends on the involvement of a supra-modal network which broadcasts information in an all-or-none fashion. The present thesis aims at better determining the mechanisms underlying conscious access by probing experimentally these different theoretical options. The work presented here is split according to two axes: the first is behavioral and focuses on how contextual information can modulate the timing and content of consciousness, the second relies on neuroimaging to better understand the neural dynamics of access, and help provide means to better care for non-communicative patients.

Behavioral work:

Recent studies conducted in the lab have shown that cueing spatial attention using a Posner-like paradigm, but after the offset of a target stimulus still facilitated conscious perception of the target. This effect was called "retroperception", and highlighted the existence of temporal flexibility in conscious access. Here, we aimed at further understanding the properties of retro-perception in a cross-modal speeded response task. We show that retroperception can occur using cross-modal stimuli (effect of a spatialized sound on visual perception), and model response time distributions in a drift-diffusion analysis. This analysis reveals that cue congruence has an effect both in pre-and post-cueing, and affects perceptual processing rather that the response decision process.

Previous studies show that the perception of a stimulus may concern only a fraction of its features: participants can perceive low-level information about a degraded stimulus, but not its higher-level features which are then inferred, a phenomenon called partial awareness. We explore the claim that the reverse is also possible: if consciousness relies on amodal processes, participants may have semantic experience of a stimulus without its sensory features. We show in retro-cueing that masked words, when primed using a congruent semantic cue, are seen and identified more often than with incongruent cues. This contrasted with an absence of effect of congruence on visual feature perception, together with evidence that word-identification was less dependent on visual feature recognition in the congruent condition. We interpret this as evidence that conscious access can act on amodal information even when its sensory counterpart is degraded. This validates a strong prediction of models postulating the existence of general amodal mechanisms for conscious awareness.

Neuroimaging work:

The neural dynamics of conscious access at the perceptual threshold have been studied in several EEG studies, but these dynamics remain to be explored at a finer scale. We thus extended these studies to intracranial-EEG patients. Participants were presented with auditory vowels in ambient noise, at different signal-to-noise ratios ranging from inaudible to clearly identifiable. Our hypothesis was that at the perceptual threshold, if conscious access relies on bifurcation or "all or none" dynamics, neural activity and behavioral audibility ratings in different trials would be split bi-modally according to whether the stimulus was consciously perceived or not. We explore this possibility using evoked potentials, decoding, and data visualization. Future analyses will focus on modeling the data according to our hypotheses.

Titre : Variabilité et prise de conscience : une exploration dans le temps, contenu et les signatures cérébrales Résumé : Quels mécanismes permettent la prise de conscience ? Différentes théories visent à résoudre la question. Certaines supposent que l'expérience consciente est un phénomène graduel, qui dépend de l'activité dans les cortex sensoriels. D'autres suggèrent que l'accès conscient dépend de l'implication d'un réseau supra-modal qui permet l'échange d'information de manière tout-ou-rien. Cette thèse cherche à mieux déterminer les mécanismes sous-jacents à la prise de conscience en testant expérimentalement ces deux options. Le travail présenté ici est divisé selon deux axes: le premier est comportemental et s'intéresse à l'influence du contexte sur le moment et le contenu de conscience, tandis que le second emploie une méthode de neuroimagerie afin de mieux comprendre les dynamiques neurales de l'accès conscient, et aider à mieux prendre en charge les patients non-communicants.

Axe comportemental : Des études récentes menées au laboratoire ont montré qu'orienter l'attention spatiale dans le contexte d'un paradigme de Posner, mais après la disparition d'un stimulus facilitait malgré tout la prise de conscience de ce stimulus. Cet effet, appelé "rétroperception" met en évidence une haute flexibilité temporelle dans la prise de conscience. Ici, nous avons cherché à mieux comprendre les propriétés de la rétro-perception dans une tâche usant deux modalités perceptives (effet d'un son spatialisé sur la perception visuelle). Nous montrons que la rétro-perception se produit dans ce cas, et modélisons les temps de réaction dans le cadre d'une analyse de "drift-diffusion". Cette analyse montre que l'indiçage sonore spatial affecte la perception visuelle plutôt que la prise de décision, et ce quand l'indice est présenté avant, ou après la cible. D'autres études montrent que la perception d'un stimulus peut ne concerner qu'une fraction de ses attributs: les participants perçoivent de l'information de bas niveau au sujet d'un stimulus dégradé, mais pas ses attributs de haut niveau qui sont alors inférées, dans une situation de "conscience partielle". Nous explorons la possibilité que l'inverse puisse aussi se produire : si la conscience repose sur des mécanismes amodaux, les participants pourraient avoir l'expérience d'attributs sémantiques d'un stimulus sans ses propriétés sensorielles. Nous montrons en rétro-indiçage que des mots masqués, s'ils sont indicés avec un stimulus congruent sémantiquement, sont vus et identifiés plus souvent que si l'indice est incongruent. Cela n'était cependant pas vrai pour la perception des attributs de bas niveau. Ces résultats sont en faveur du fait que l'accès conscient peut agir sur de l'information amodale, même quand les propriétés sensorielles associées sont dégradées par un masque. Cela valide une prédiction forte concernant les mécanismes de la prise de conscience.

Axe d'imagerie cérébrale :

La dynamique de l'accès conscient au seuil de perception a beaucoup été étudiée en EEG, mais mérite d'être étudiée à une échelle plus fine. Nous avons donc mené un protocole en EEG intracrânien. Les participants ont écouté des voyelles dans un bruit continu, à différents niveaux de signal-sur-bruit allant de parfaitement inaudible à tout à fait identifiable. Notre hypothèse était qu'au seuil de perception, si la conscience repose sur des mécanismes tout-ou-rien, l'activité cérébrale ainsi que les rapports subjectifs d'audibilité devraient être divisés de façon bimodale selon si le stimulus a été entendu consciemment ou non. Nous explorons cette possibilité à l'aide de potentiels évoqués, de la classification par apprentissage machine, et modélisation. Des analyses futures s'intéresseront à la modélisation de ces données en accord avec nos hypothèses.

Mots-clés : conscience, attention, vision, audition, EEG

Acknowledgements

First and foremost, I would like to thank my supervisor Claire Sergent, who hired me when I had (almost!) given up on doing a PhD early in the course of my MD-PhD cursus, for lack of grant money. Your trust, positive attitude and guidance were invaluable over the last four years, without which I would not have made it through. I learned so much, and plan to make good use of that knowledge in the future. I am very happy to have been there for your HDR defense and become your first administratively official PhD student. I'm still sorry I managed to fall down 4 meters the next day. I promise, I didn't do it on purpose. Now, this journey is coming to an end and I would like to thank you from the bottom of my heart.

I extend my gratitude to my collaborators during the course of my PhD, Matteo Lisi Viola Störmer, and Jérôme Sackur, as well as the people who provided me with punctual but much needed help, Charlotte Delporte for her help running experiments, Benoît Laslier and his expertise in probabilities, Jean-Christophe Thalabard and his guidance with statistical analyses, Louis Thibault who helped me perform online experiments, Axel Garnier-Allain whose strong background in psychology and critical thinking have proven invaluable, Théophile Rimsky-Robert who not only got me interested in cognitive science to begin with, but also shared a fraction of his vast knowledge in philosophy with me, and Antoine Dieulesaint, without whom I couldn't have hoped to make my online data visualization dream a reality. I would also like to thank all INCC Vision team members, and particularly the various people who have shared an office with me. This includes those who were almost done when I arrived and taught me skills I would not have succeeded without: Sergiu Popescu, Marianne Duyck, Solène Le Bars, Cécile Issard, Louis Thibault, Nika Adamian, Romain Vincent and Arielle Veenemans. This also comprises those who arrived a couple of years or more after me, and provided me with support in the face of upcoming deadlines, or a welcome ear when I was struggling: Axel Garnier-Allain, Cédric Foucault, Camille Fakche and Garance Merholtz. Some of us will get to remember how Notre-Dame smells when it is rapidly burning down. Thank I thank my friends outside the lab, whether I met them before, or after starting my PhD.

I would also like to thank Antoine again, for cooking me delicious meals and his unwavering support.

Lydie Uro holds a special place in my heart, with whom we decided we would get our PhDs before finishing medical school -although we now can't exactly remember why. I hope we will keep supporting each other through hardship, administrative pains, professional and personal setbacks.

My parents, of course, deserve my profound and unending gratitude for helping me navigate the intricacies of doing a PhD. I am so lucky to have you in my life.

Table of contents

Chapter I.

General Introduction

I.1 Is an experimental science of consciousness possible?

I.1.1 Who cares about consciousness?

Popular culture in the 21 st century has incorporated several themes centered on the question of consciousness. One of the most dominant of them is the mindless zombie represented in all kinds of media: from video-games (Resident Evil, 1996), comic books [START_REF] Kirkman | The Walking Dead (Image Comics)[END_REF], pop songs (The Cranberries, 1994), to the more traditional medium that is cinema, which in itself encompasses a variety of different types of zombies. Variations of this popular representation are present across the globe, and have been for a very long time: among the most well-known are the Jiangshi in China, or the Haitian Zombi, which is at the origin of the more familiar North-American zombie. While this representation has been used as a metaphor for various plagues, such as consumerism (Dawn of the Dead [START_REF] Snyder | Reappraising the relationship between working memory and conscious awareness[END_REF])), social unrest (Night of the Living Dead [START_REF] Romero | A general framework for parallel distributed processing[END_REF])), disease (Outbreak [START_REF] Petersen | Bottom-up and top-down attention are independent[END_REF]) or war (e.g. World War Z (M. [START_REF] Forster | World War Z. Skydance Productions[END_REF]), zombies first invoke a universal question, often discussed in philosophy of mind: could real zombies exist [START_REF] Chalmers | The conscious mind : In search of a fundamental theory[END_REF]?

The philosophical zombie is only defined by its unconsciousness: it isn't ugly, rotten, aggressive, or dumb. It does everything we do, it just doesn't feel. It isn't aware, it does not know what pain is, it isn't moved when looking at the sun setting over the hills. It can act appropriately in each case, but does not have any experience of these things. The philosophical zombie is a very high-functioning robot. Something about the possibility of their existence is intuitively frightening: what if there existed someone just like me, who wasn't conscious? What exactly could distinguish it from myself, since it functions exactly the same?

Recently, history, psychology, neuroscience and biology have acquired the means to start questioning consciousness with new tools, and ask different questions: does consciousness have any biological grounds? If so, can it be impaired? Are there clinical zombies? Can a biological process account for the richness of experience, and explain what makes me be myself?

Many would argue it cannot. Some researchers however, readily oppose this view, and in fact, the question is not definitely resolved. A second question is whether consciousness is accessible at all by scientific means: is it a biological phenomenon? This would entail that it is bounded by natural laws and that it can be studied like any other. Before we can review the experimental work presented in the current dissertation, we may want to discuss why and how we can consider a scientific study of consciousness is possible.

We will try and provide the reader with some key concepts to understand the reasons which allow some to claim that we can, in fact, study everything there is to know about conscious experience. This first section will be followed by a more detailed description of cognitive processing, focused on sensory processing and consciousness.

I.1.2 A definition of terms

There is now a growing body of work in the fields of psychology, neuroscience and biology in general which also attempt at resolving the question of what consciousness is, what purpose it serves if there is one, and what biological process is the cause of subjective awareness. Before diving into whether and how one can experimentally study conscious experience, it is important to define what exactly is being studied. The word consciousness has the somewhat interesting property of having an intuitive meaning, yet being particularly hard to define. Although this doesn't seem to be an issue for debates after family dinners, the lack of appropriate vocabulary to describe the various phenomena occurring under its scope leads to comically obvious problems when one chooses to try and understand what is conscious.

A first step, for example, may be to try and look in the dictionary, the entire purpose of which is to define the meaning of words. The Collins, first contender on Google, provides us with the following:

Your consciousness is your mind and your thoughts. (« Consciousness definition and meaning | Collins English Dictionary », s. d.)

The second step, then, may be to try and look up the definition of mind, which, unsurprisingly, leads to some more confusion:

You refer to someone's mind when talking about their thoughts. For example, […] if you say that something is at the back of your mind, you mean that you are aware of it […].

(« Mind definition and meaning | Collins English Dictionary », s. d.)

The third step would naturally be to look up the definition of aware: Knowing or realizing; conscious; informed. (« Aware definition and meaning | Collins English Dictionary », s. d.)

The less courageous researcher may give up on the obvious circularity, or may make one last effort to read the synonym note listed on the web page, which attempts at determining what exactly separates aware and conscious, when both words are defined by one another.

While the Collins Dictionary -among others -has an unfortunately difficult time with phenomenology, a thorough read of several sources from current research on the topic indicates that, besides the lack of clarity, most experts converge towards a threefold definition [START_REF] Dehaene | What is consciousness, and could machines have it?[END_REF]:

1) Being awake as opposed to asleep -with the special case of dreams and postcomatose state [START_REF] Dehaene | Conscious, preconscious, and subliminal processing : A testable taxonomy[END_REF]Laureys & Boly, 2007).

2) Experiencing an external or internal event, such as a smell or a thought (Dehaene et al., 2006;[START_REF] Lamme | Separate neural definitions of visual consciousness and visual attention; a case for phenomenal awareness[END_REF].

3) Having free will as in making conscious decisions [START_REF] Libet | Time of conscious intention to act in relation to onset of cerebral activity (readiness-potential)[END_REF][START_REF] Schurger | Neural Antecedents of Spontaneous Voluntary Movement : A New Perspective[END_REF].

All three parts of the phenomenon are studied under the scope of psychology and neuroscience, however, they are most often examined separately, and later related to the others, as studying each one is already a large enough trial at present time. The current dissertation focuses mainly on the second aspect, experiencing.

If consciousness is herein the experiencing of things, it first must be noted to be a transitive phenomenon. There is no experiencing without a thing, within the terms of our definition. This may require some nuance, however: meditative practices, for example, are often thought to dramatically reduce the dichotomy between a subject and their experiences [START_REF] Josipovic | Neural correlates of nondual awareness in meditation[END_REF]. Whether meditation allows for the production of version 1 of consciousness without version 2 remains an open question. For this reason and several others that are discussed in section I.3, the experiencing of a thing will rather be called conscious access than consciousness, in order to make explicit the transitivity of the process we are studying. The thing one can be conscious of can be any event occurring within the self or outside, so that any sensation can be consciously accessed. The experiencing itself refers at first glance to the hardto-define part of consciousness, the subjective feeling of the thing one is conscious of.

I.1.3 All elusive consciousness

Most of our modern take on what this type of consciousness is starts with René Descartes' philosophical dwellings in the 17 th century (although the school of thought can easily be traced to Saint Augustine (Taylor, 1989 p. 111)), in which he describes in some detail that our reflexive behaviors are very much like the movements of complex automata, which react to the external environment thanks to mechanistic processes [START_REF] Descartes | Les passions de l'âme[END_REF]. The nerves were then supposed to be hollow tubes filled with pressurized air [START_REF] Adam | Oeuvres de Descartes[END_REF] connected to the brain, which acted as a hub for emitting or receiving airflow from the sensory organs, or to the motor system.

One issue with this model is that it is too deterministic: an automaton always reacts to an external force in the same way with little noise, but this obviously isn't true for all animal behavior, and most importantly, this type of system seems wholly inappropriate for consciousness. Nothing should feel like anything if we are a series of pipes and valves. In brief, this consideration led Descartes to the conclusion that the mind could not be contained within the body, because the biological machinery he found there was unsuitable. He formulated that what constitutes one's mental life, the res cogitans or "thinking thing", should thus be located outside the physical world, but able to interact with the body in order to act on it. He suggested that this was done though a specific sub-part of the brain, and designated the pineal gland as the place where the Cartesian soul and the body interacted, because of its central location.

Cartesian dualism, which places the mind beyond the physical realm, places consciousness beyond biology. If we choose to believe this to be true, then the most we can do is find out where our extra-corporeal soul interacts with our body, and how the latter responds to the former. This would not explain consciousness, but simply posit its existence and study its influence on our zombie brain if it has one. Anything more is beyond our scope: we can leave the rest to philosophy or theology, and start working on something else.

Cartesian dualism is currently not considered a valid theory by many [START_REF] Dennett | Consciousness explained[END_REF][START_REF] Kim | Philosophy of mind[END_REF][START_REF] Parfit | Reasons and persons[END_REF][START_REF] Searle | Dualism revisited[END_REF]. In History, it coexisted with other movements, particularly in the 19 th and 20 th centuries, which tried bridging mind and body by appealing to current knowledge in biology, such as vitalism [START_REF] Bechtel | Vitalism. Routledge encyclopedia of philosophy[END_REF]. These were also dualist, but appealed to natural laws in their attempts at describing the mind. These forms of Cartesian materialism [START_REF] Dennett | Consciousness explained[END_REF] hold that the mind is a separate entity from the rest of our biological function, but that it must be produced somewhere in the physical body, likely within the brain1 . In this case, there still is a dichotomy between mind and body, and conscious experience is not exactly of the same cloth as the rest of biological processes, because other biological processes do not feel like anything. However, biology could explain, or find, what makes us conscious of things, because all of these are inside the brain. This view of consciousness is intuitive: we feel we are someone inside our body, and we place ourselves, for the most part, as living somewhere behind our two eyes, like a hermit crab, but with a much nicer shell. When working under the assumptions of Cartesian materialism, we can find out where the mind is in the body, and seemingly describe, in part, how it works: there is a place where experiences are experienced. This seems to make consciousness accessible to biology, but at second glance, how or why experiences in the brain feel like anything at all is incredibly complicated to probe.

Understanding the mechanisms which allow the brain to access sensory information does not explain how it experiences. This distinction between the accessing to information and the experiencing of what has been accessed is non-trivial, and was most recently formalized by philosopher David Chalmers [START_REF] Chalmers | The conscious mind : In search of a fundamental theory[END_REF]. Finding a place in the brain where things are felt explains the location of experiences, not experience itself. This question allows us to introduce an important property of our definition of consciousness, "Experiencing an external or internal event, such as a smell or a thought" [START_REF] Dehaene | Conscious, preconscious, and subliminal processing : A testable taxonomy[END_REF][START_REF] Lamme | Separate neural definitions of visual consciousness and visual attention; a case for phenomenal awareness[END_REF]: it is actually twofold. There are two problems we are trying to resolve at once according to Chalmers: the Easy Problem, what the brain does that makes it able to access external information, process it, and make conscious decisions about it, and the Hard Problem, what the brain does that makes it have phenomenal experience, and why it is so [START_REF] Chalmers | Facing up to the problem of consciousness[END_REF]. All conscious decisions could be imagined to have been made by a zombie designed to act like a person, so what is it that makes the difference? Many brain functions do not elicit experience, and even if we found some brain area, with a specific activity associated with a specific experience, why would this activity help us be more than a sophisticated robot? Finding where and what are not enough, just like explaining how we access information does not explain why we experience.

It seems, now, that our working definition of conscious access may not study consciousness at all, depending on what we are focusing. Indeed, if conscious access is split in two phenomena, the first being some kind of information processing up to some thing that makes this information conscious (conscious access), and the second being the subjective experience of this information (conscious experience), then studying access is not studying consciousness. We can imagine biology being able to explain the Easy Problem: what cerebral processes lead to conscious experience of sensory information. But could we imagine it is able to explain the Hard Problem, how and why these processes lead to subjective experience? What is it that makes me feel heat any differently than, say, a toaster, or my own heat-sensitive neurons?

Chalmers, among others [START_REF] Levine | Materialism and qualia : The explanatory gap[END_REF][START_REF] Nagel | What is it like to be a bat? The philosophical review[END_REF], claims that biology cannot answer these questions. It can only examine the processes allowing for experience to arise, but no brain mechanism will explain why they lead to qualitatively different states of mind: biology falls short, and explains next-to-nothing.

I.1.4 Obstacles to a scientific study of consciousness

An important sidetrack on the journey to experimentally study consciousness is to ponder whether and how we can resolve this issue. It appears that studying the processes correlating with conscious access does not explain consciousness at all, because it does not explain the irreducible properties of subjective experience, the "what it is like to feel something". These properties have been called qualia [START_REF] Lewis | Mind and the world-order : Outline of a theory of knowledge[END_REF], and correspond to experiences which cannot be described except by appealing to themselves. Some examples of them are the color red, the smell of a rose, pain, or what it would be like to use echolocation instead of light vision, like bats do [START_REF] Jackson | Epiphenomenal qualia[END_REF][START_REF] Nagel | What is it like to be a bat? The philosophical review[END_REF]. According to contemporary dualists such as David Chalmers or Franck Jackson, the major issue when studying consciousness is that explaining how we become conscious is not sufficient, because it is not the root problem of consciousness, we must also explain why this particular physiological phenomenon leads to conscious experience [START_REF] Chalmers | Facing up to the problem of consciousness[END_REF].

Considered in a dualist manner, the problem of consciousness seems intractable by means of biological study. For hundreds of years, dualism, Cartesian or otherwise, has been the bedrock of how we view consciousness, but a biological account of consciousness cannot operate within this framework. Some recent philosophers argue that separating mind from body, or thought from cerebral processing, is misguided. We will focus on the arguments put forth by Daniel Dennett and Derek Parfit in the next paragraphs to understand what they propose instead.

To start untangling how we can circumvent this issue, we have to dissociate the feeling of Self, and the rest of the qualia: the Self experiences qualia, it is that which accesses and feels, but it is also a quale in itself. We will distinguish the two from this point on.

I.1.5 Qualia experienced by the Self

The term qualia in its modern sense was first formally introduced by Clarence Irving Lewis in Mind and the World Order [START_REF] Lewis | Mind and the world-order : Outline of a theory of knowledge[END_REF], and refers to the atomic (as in "smallest unit of") properties of subjective experience, which no amount of purely physical information includes [START_REF] Jackson | Epiphenomenal qualia[END_REF]. One common example would be the color red, which cannot be described to a person who has never seen such a color, as it would be the case for a congenitally blind person. Qualia hold four properties [START_REF] Dennett | Quining qualia[END_REF]: they are ineffable and can only be apprehended by experience, intrinsic as experiencing them leads to the same sensation in all cases, private as one cannot compare their qualia to those of another, and directly apprehensible in consciousness, so that experiencing a quale leads to knowledge of everything concerning said quale. Qualia are, as stated earlier, one of the main argument against a physicalist description of consciousness, one that would only rely on properties of the physical world. I will here cite one thought experiment in favor of qualia formulated by Jackson.

Mary the color scientist knows all the physical facts about color, including every physical fact about the experience of color in other people, from the behavior a particular color is likely to elicit to the specific sequence of neurological firings that register that a color has been seen. However, she has been confined from birth to a room that is black and white, and is only allowed to observe the outside world through a black and white monitor. When she is allowed to leave the room, it must be admitted that she learns something about the color red the first time she sees it -specifically, she learns what it is like to see that color. (p. 130) This thought experiment first exemplifies what a quale is, here the feeling of seeing red, and exposes the "knowledge argument", which is that Mary does learn something when she sees the color for the first time, indicating that there is an intrinsic property of red that she didn't know about before experiencing it: its quale. This is what is argued to be inaccessible by biology. There are many other examples of thought experiments in favor of the existence of qualia, but for our purpose of exposing arguments, only this one is required. It was conveniently chosen because philosopher Daniel Dennett proposes a concise counter-argument to it: the story simply doesn't have to go this way [START_REF] Dennett | Consciousness explained[END_REF].

And so, one day, Mary's captors decided it was time for her to see colors. As a trick, they prepared a bright blue banana to present as her first color experience ever.

Mary took one look at it and said 'Hey! You tried to trick me! Bananas are yellow, but this one is blue!" Her captors were dumfounded. How did she do it? "Simple," she replied. "You have to remember that I know everything-absolutely everythingthat could ever be known about the physical causes and effects of color vision. So of course before you brought the banana in, I had already written down, in exquisite detail, exactly what physical impression a yellow object or a blue object (or a green object, etc.) would make on my nervous system. So I already knew exactly what thoughts I would have (because, after all, the 'mere disposition' to think about this or that is not one of your famous qualia, is it?). I was not in the slightest surprised by my experience of blue (what surprised me was that you would try such a secondrate trick on me). I realize it is hard for you to imagine that I could know so much about my reactive dispositions that the way blue affected me came as no surprise.

Of course it's hard for you to imagine. It's hard for anyone to imagine the consequences of someone knowing absolutely everything physical about anything! Brown and Co Edition) In this case, there is no particular reason, beyond intuition, why Mary couldn't know what red feels like. The whole idea behind qualia would be misguided: when a person feels pain, or sees the color red, the description of what occurs in the brain suffices to encompass everything there is to know about seeing red [START_REF] Minsky | The emotion machine : Commonsense thinking, artificial intelligence, and the future of the human mind[END_REF]. The feeling of red is not something that emerges out of brain activity or that differs from it in any conceivable way, it is the ensemble of changes in brain activity itself.

Many will object that it is just a trick: this explanation negates experience entirely. There are no more qualia, and as such these are no longer problematic, but there also isn't anyone anymore. Where am I in there? Dennett, among other physicalists, argue that this postulate, according to which there is an I who feels, and that this I is also something beyond what can be described by biology, is false [START_REF] Dennett | Consciousness explained[END_REF][START_REF] Graziano | Consciousness and the social brain[END_REF][START_REF] Minsky | The emotion machine : Commonsense thinking, artificial intelligence, and the future of the human mind[END_REF][START_REF] Parfit | Reasons and persons[END_REF].

I.1.6 The Self experiencing qualia

I.1.6.a

The Cartesian Theater To tackle to question of who is I who feels, we need to retrace our steps back to Descartes, or at least to modern dualism. We saw earlier the distinction between Cartesian dualism, which required the existence of a soul in a different plane, and forms of nonreductionist materialism such as vitalism, which assumes the conscious mind to be located inside the brain, and bound by the physical laws of the world. Dennett calls these forms of nonreductionist materialism "Cartesian materialism" because they still imply a strong distinction between mind and body. This view remains a logical endpoint to most of our intuitive reasoning about the brain. Let's indulge in a simple description of Cartesian materialism. What it means is that somewhere inside the brain, there is a center for experiencing, a true self of some sort.

What composes this true self is indeterminate, apart from its necessary physicality, and its location. It must be somewhere. This little Me in a place in the brain must then receive the images and sounds of the external world as they are conveyed and rebuilt by other places in the brain. In a way, it watches them, and it is this watching that experiences. But soon, one might wonder what allows this little Me, in a place, in the brain, to watch what other things in the brain have produced. Is there a smaller Me, in that little Me, somewhere, that makes it able to see, watch and make sense of things?

The fallacy soon becomes apparent: these little Me, or homunculi, are inside one another, in an infinite regress situation [START_REF] Gregory | The Oxford companion to the mind[END_REF]. This doesn't explain anything. More unsettling but less apparent, they are watching something, but we know there is no television in there. And, as Dennett indicates, a brain that processes information for interpretation (access) and then processes it for displaying it all together in another brain region (experience) does the job twice: once for information processing, and a second time for broadcasting to the audience, but what would be the point of this second step, if all the information was already processed and understood in the first step? [START_REF] Dennett | Time and the observer : The where and when of consciousness in the brain[END_REF]. It is not a parsimonious account, regardless of whether it is intuitive.

Having a homunculus does not make much computational sense, and we have no particular evidence there exists a brain region associated with the Self. We will thus reject this possibility. We will reject the notion that there could be someone inside of me because it simply delocalizes the problem of consciousness into a smaller version of myself within me without explaining anything, and we will reject the television the homunculus watches -the Cartesian Theater [START_REF] Dennett | Consciousness explained[END_REF], or the idea that there is a place in the brain where images, sounds and other feelings are shown, heard and felt, because if there is, there also must be an audience to watch, hear and feel, and we rejected that just a sentence ago.

What then, should we substitute to this true self? If we're naively interested in consciousness, then we may want to look into what we know about brain function, maybe there will be some hints in there. We haven't mentioned the brain beyond what we learned with Descartes, it may be time to catch up to 21st century knowledge. From Figure I-1, we can discriminate 4 colored cerebral lobes and one cerebellum, with different subparts and each of these seems to have a specific function.

We can identify areas, which seemingly deal with single functions, and we can imagine these areas communicate. It feels reasonable to assume that, if we study the brain in enough detail, we will find a region, or a series of regions, whose activity may correlate with participants being aware a given stimulus was present in their environment, as was shown for several areas at different timings in the brain [START_REF] Koch | Neural correlates of consciousness : Progress and problems[END_REF].

Potentially, some of them will be identified as indispensable for consciousness. Many would argue nothing could: the Hard Problem is simply unattainable, even with a full description of the underlying processes [START_REF] Chalmers | Facing up to the problem of consciousness[END_REF]. We know one possible conclusion: that consciousness is not something we can explain with strict reductionism and physicalism. Among those who accepted this conclusion, some have argued that consciousness just is: we do not ask why atoms exist, we should not ask why consciousness exists, it is a fundamental part of the physical world [START_REF] Nagel | What is it like to be a bat? The philosophical review[END_REF], some kind of elementary particle. Others have, in the same vein, argued that consciousness is the result of quantum mechanics [START_REF] Hameroff | Orchestrated reduction of quantum coherence in brain microtubules : A model for consciousness[END_REF][START_REF] Tegmark | Importance of quantum decoherence in brain processes[END_REF]but see Baars & Edelman, 2012;[START_REF] Koch | The relation between quantum mechanics and higher brain functions : Lessons from quantum computation and neurobiology[END_REF], especially because quantum indetermination -i.e. a certain kind of randomness, may allow for free-will in a world driven by causality. Others appeal to divine intervention (like Descartes), or loosen the definition of consciousness enough to advocate for panpsychism, where consciousness is an intrinsic property of everything (see Lamme, 2018 for a discussion).

None of these paths leads us to a biological study of consciousness, but some philosophers have argued there is another: we can't find what makes cerebral function different from other biological processes, because it isn't. The mind is not an entity beyond what can be observed, and the full description of the cerebral mechanisms underlying conscious experience would indeed describe all there is to describe about biological consciousness [START_REF] Dennett | Consciousness explained[END_REF][START_REF] Minsky | The emotion machine : Commonsense thinking, artificial intelligence, and the future of the human mind[END_REF][START_REF] Parfit | Reasons and persons[END_REF]. This was already proposed at the end of section I.1.5, but not explained in detail:

We feel we are one Self, experiencing qualia, a united person implying a necessary dichotomy between mind and body, which must not be of the same cloth. We keep going back to the Cartesian theater because any explanation we find with the help of biology doesn't comply with our dualist intuitions about consciousness. There must be more, because I feel more, presupposes that the I who feels is something beyond regular qualia. Itself, it is a higherorder type of essential experience of the world, it is the experience that experiences. But beliefs are not necessarily true. What if there was no I? Or at least, not in the way we have imagined until now, which makes it impossible to study consciousness, because we just can't find or explain it without explicitly denying its ineffable nature, deep within our otherwise zombie brain.

A good philosophical conversation on what could be the premise for a biological science of consciousness may want to include a discussion on self-identity, because it is a major stopping point: the consciousness we seem to explain in cognitive science does not explain experiences, because all description of the cerebral processes accompanying them forgets to explain, or account for, the person experiencing. To introduce a brief overview on the question of self-identity and consciousness, we can review a famous aphorism written in the 3 rd century B.C. [START_REF] Watson | The complete works of Chuang Tzu[END_REF]):

Once, Zhuang Zhou dreamed he was a butterfly, a butterfly flitting and fluttering about, happy with himself and doing as he pleased. He didn't know that he was Zhuang Zhou. Suddenly he woke up and there he was, solid and unmistakable Zhuang Zhou. But he didn't know if he was Zhuang Zhou who had dreamt he was a butterfly, or a butterfly dreaming that he was Zhuang Zhou. Between Zhuang Zhou and the butterfly there must be some distinction! - Zhuangzi, chapter 2 (Watson translation, In this aphorism, The Butterfly Dream, a man dreams he is a butterfly, and that he experiences all the things a butterfly experiences. When he wakes, the poem states that for a short period, the man no longer knows whether he is himself who has dreamt he was a butterfly, or a butterfly dreaming that it is a man. This aphorism is often used to discuss the Dream argument: when can a person know that they are dreaming or experiencing reality? But for the current issue, it is not the point. When Zhuang Zhou wakes, during a certain period of time, he can be either the butterfly or Zhuang Zhou, dreaming of the other. His self-identity is indeterminate [START_REF] Parfit | Reasons and persons[END_REF]. We could discuss this question and find a suitable argument for either response to "Who is Zhuang Zhou during that time?", but there is no true answer, it all depends on what we consider important for being a continued Self. Yet, once the moment of indetermination has passed, we will all agree that Zhuang Zhou is Zhuang Zhou, and no longer a butterfly, if he ever was one. What helps determine, after the haze, that Zhuang Zhou is Zhuang Zhou, and not the butterfly, and why is this important to us?

I.1.6.b
Away with the Cartesian Soul Faced with the repeated argument that the processes correlating with experience are not the experience itself, and that studying the brain does not provide us with a suitable explanation of how and why we feel, we have to wonder, then, what would. If we accept Cartesian dualism, then the answer is, quite simply, nothing that we can find in biology. As this would bring the current dissertation to an end, we will reject this argument with little more explanation than what was provided until now, but several philosophers have made detailed arguments against Cartesian dualism [START_REF] Dennett | Consciousness explained[END_REF][START_REF] Lichtenberg | Schriften und Briefe[END_REF]: see in Parfit, 1984, p. 224).

The key determinant between those who believe there is a Hard problem of consciousness and those who do not lies in what matters according to them [START_REF] Parfit | Reasons and persons[END_REF]. The first claim that what matters is personal identity, the I we have talked about before, and that as long as we haven't explained this, we haven't explained consciousness. Parfit claims that what matters is not personal identity because this notion makes little sense, what matters is physical and psychological continuity. We will thus go on to explain this last sentence, and expose why this could be what matters, and what it means for a biological study of consciousness. Consider Hume's analogy [START_REF] Hume | A treatise of human nature Oxford[END_REF]: a nation's existence consists in the existence of its citizens on its territory. Is a nation the exact result of the sum of its parts (citizens + territory), or does the combination of these parts lead to nations being something more than them, a further fact beyond citizens on a territory? This can be applied easily to persons: a person's existence involves the existence of a brain and body, and a series of interrelated physical and mental events. Is there something more to it? Those who believe there is a Hard Problem would readily argue that it is so, there is something more, and that is personal identity, the Self we have mentioned earlier. Parfit argues, to the contrary, that there is nothing more to a person than what was described in the sentence before. This does not mean that people don't exist and cannot be referred to, but that a complete description of the brain, the body, and the set of interrelated physical and mental events is a complete description of what a person is. This declaration may resonate with what Dennett said about qualia in section 1.1.5, with Mary the color scientist. They argue that people exist, but not in any way beyond their biological function: there is nothing more to account for when describing them (pp. 110-112 and appendix D).

Parfit uses a set of thought experiments to develop his argument that there is no further fact to existence than psychological and physical continuity in time. We will now review one of them, the case of Teleportation (pp. 265-275): let's assume that humanity has designed a teleporter than can take us to Mars. This teleporter works by making a full-body scan of the person, after which it will simultaneously destroy the being on Earth, as well as re-creating it on Mars. In this case, there are never two of the same people existing at once on Earth and Mars. The person created on Mars will have all the psychological and physical states of the Earthling, carry its train of thought from the point where it ended on Earth, and keep on going thinking that is it, indeed, the person that was on Earth, but is this really true? Proponents that self-identity is what matters, such as Thomas Nagel, would argue that a situation like this one is just like ordinary death, that the two people are not the same, and the belief that the Marsling holds about his identity is wrong [START_REF] Nagel | The absurd, Mortal Questions[END_REF] examines a similar thought experiment). Parfit argues however that whether the two people are the same or not is indeterminate, like Zhuang Zhou's identity was when he woke up from his dream of the butterfly. There is no answer to this question. In cases like this one, self-identity is not clearly continuous in time. Unless we embrace the notion that Cartesian Souls exist, this does not matter because there is no individual soul to be attributed to anyone. Parfit argues that whether the Earthling and Marsling are the same person makes no difference to who they are, as such the important point is not personal identity, it is the fact that the Marsling picked up all mental states of the Earthling. This is one example where self-identity can matter less than psychological and/or physical continuity for determining who a person is.

That there is a further fact beyond our physical and mental states is wrong, he argues, because more complicated cases start making no sense at all. He proposes an extension on William's example [START_REF] Williams | The self and the future[END_REF]): let's imagine that we are tortured by a very knowledgeable, but sadistic neurosurgeon, who likes to replace the cells in our body and tamper with our brain in order to disrupt our physical and psychological states respectively. If the neurosurgeon chose to replace nothing of us, then we would remain one and the same person.

If the neurosurgeon chose to replace every cell in our body by destroying us, and recreating a copy an hour later, with all of our psychological states intact, we'd be in the teleporter case and we could consider it to be death or not death depending on what matters. If the neurosurgeon chose to leave our body intact but to alter all of our psychological states, so that we had the beliefs, memories and character of Napoleon, we'd most often consider ourselves to have died in the process. What is unclear is when the neurosurgeon chooses to alter parts of each. If he replaced 99% of our cells, and changed 99% of our character, then we'd be someone else. What if this number was 80%, or 70%, or 50% of both, or any random number of each? When would we stop being ourselves, and start being someone else? Parfit argues again that there is no answer to this question, and this is where self-identity stops being evident. In these cases, who is who is not determinate, and does it matter? What is tangible is whether there is continuity between our physical and psychological states in time. We can quantify how many of these have changed or not changed after the neurosurgeon's experiment, but we couldn't possibly determine whether the person is enough of the same to still be itself. A complete description, in this case, of psychological and physical states in time of the person is all that can be said with certainty about this person. There is no further fact about ourselves that we haven't described if we have described these states. The question of self-identity here makes no sense. Parfit argues this is not only true in cases where self-identity is indeterminate, it is true in all cases: there is no further fact about ourselves we must explain beyond psychological and physical states. There is no I as in a Cartesian Soul, as such, there is no Hard problem. There is nothing more than coherent physical and psychological continuity.

Thomas Nagel, who defended that taking the teleporter would be as good as ordinary death, admits that the view stated here may be true. It is, according to him, just not possible to believe (Parfit, 1984, p. 274). Taking the teleporter nevertheless feels like he is about to die. To this, and the current dissertation will not go into much more detail, we could reply that taking the teleporter is just as good as ordinary survival, because the latter involves the gradual replacement of all physical and psychological states. All there is, is continuity between them.

The teleporter experiment only makes this fact more apparent. If we were to die and be replaced by a replica, the replica would have a very difficult time believing that he or she is not I.

Whether this is true, remains indeterminate as well as unimportant because it says nothing about the nature of the person. Of course, it is a Pascalian wager: anyone who chooses to believe what Parfit believes stands to lose much more than they could gain by taking the Teleporter. Derek Parfit is not the only contemporary philosopher to endorse a variation of this view: Daniel Dennett also states that what we believe about consciousness is an illusion, and that refusing this view is currently a great obstacle to the development of a science of consciousness. He compares it to studying magic: imagine a researcher who studies magic, and thus travels around the world to talk about and examine magic tricks and shows, and that he or she aims at explaining them. An interlocutor who believes magic is real would immediately object: if you are studying what is behind magic tricks, then you're not explaining real magic.

The interlocutor here rejects the explanation the researcher has to provide, because he or she did not account for all the real magic, the one that cannot be explained. Dennett argues that people who believe current attempts at studying and explaining consciousness fail to account for qualia and self-identity suffer from the same fallacy: there is nothing besides what is currently being studied and described, there is no real consciousness beyond what can be observed [START_REF] Dennett | Sweet dreams : Philosophical obstacles to a science of consciousness[END_REF].

I.1.7 Summary

If consciousness is not the place in the brain where it all happens, if it isn't the existence of a little me, inside of me, looking at what I am doing from behind my own eyes, if we postulate that conscious experience is a biological phenomenon, neither existing in a different plane than the rest of the world, nor the result of a further fact to ourselves other than the continued existence of our physical and psychological states then there must be ways to study it, like any other biological phenomenon. This section did not mean to demonstrate that the philosophers cited are right, and does not provide the reader with enough explanations to fully understand the question of qualia, the Self, and consciousness from a philosophical standpoint. The objective was however to have a brief overview of what conceptual issues readily arise when developing a scientific study of consciousness, as well as offering some ideas in favor of the view that what the neuroscience of consciousness studies is, in fact, real consciousness, and not some watered-down version of the phenomenon. The next section will undertake a short description of sensory processing, regardless of this information became conscious or not, while the last section will focus on what we know about higher-order cerebral function, and its relation to consciousness.

I.2 Sensory perception: the brain working on the world

I.2.1 Blindsight, and the discovery of unconscious processing

The brain was thought to be the center for experience and intelligence as early as Hippocrates, who reported alterations in behavior after his patients suffered brain injury. The manifestations of brain lesion are varied, and one of them in particular piqued the interest of the scientific community. Patients suffering from large lesions in one or both lobes of the occipital cortex, which is located at the back of the brain, reported a partial or total loss of vision. As we will see later when we describe the visual system in more detail, the occipital cortex receives most of its input from the retina, our visual sensory organ. In 1973, a seminal experiment for consciousness research took place with the help of now famous patient GY who presented with what seemed to be total loss of conscious vision in one half of his visual field. The experimental protocol consisted of asking this partially blind patient to try and move his eye in the direction of a luminous dot, placed in the half of the world he couldn't see. The instructions were, of course, quite baffling for the patient, who couldn't see the point in trying to look where he was blind. Execution was however near-perfect [START_REF] Pöppel | Residual visual function after brain wounds involving the central visual pathways in man[END_REF]: the patient perfectly moved his eyes so that they would land on a dot in space which he swore he couldn't see initially. This phenomenon, repeated a number of times with various well-executed motor tasks in the blind field of patients [START_REF] Sahraie | Increased sensitivity after repeated stimulation of residual spatial channels in blindsight[END_REF] was termed blindsight [START_REF] Sahraie | Spatial channels of visual processing in cortical blindness[END_REF][START_REF] Weiskrantz | Parameters affecting conscious versus unconscious visual discrimination with damage to the visual cortex (V1)[END_REF], and brings into real life the type of incomprehensible situations we've mentioned before: who is seeing this dot, if not the patient? Imagining someone else inside of him who was able to see the dot, but with whom he couldn't communicate seems ridiculous, so we must admit that for some reason, part of his brain was working without fault outside the scope of conscious experience. The visual stimulus must have been processed unconsciously. But what does that mean?

The history of the discovery of unconscious processing is twofold. The first was already evoked by Descartes' descriptions of automatic behavior, and indeed, unconscious processing has historically been considered a dry set of reflexes until the 20 th century. Until then, unconscious processing was concerned with stereotypical reactions to familiar tasks, and was described in detail by Hughlings Jackson during the 19 th century. Beyond Descartes' simple description of the brain as a hub from communicating pressure changes in response to external stimuli, Jackson formulated that the brain was composed of several structures, which were organized along a strong hierarchy. At the top, the most recently evolved and outer cerebral cortex, which functions as an inhibitory organ over its subalterns, and deals with conscious thought and decision. Below the cortex were all sub-cortical structures, such as the basal ganglia, thalamus, hippocampus, brainstem etc. which dealt only in reflexive behavior, some of which are implicated in blindsight2 : shifting one's eyes towards a location, reacting to painful stimuli, and various motor behaviors not concerned with intelligent thought. For Jackson, mental life took its roots within the cortex, but was governed by the Self, so that no complex behavior could arise outside of consciousness [START_REF] York | Hughlings Jackson's neurological ideas[END_REF].

At the end of the 20 th century, the scientific community gained access to new technical and conceptual means for exploring cognitive functions. As computers became more widespread and complex, the scientific community gained new means of investigation, from precise measuring tools for reaction times to brain imaging techniques such as electroencephalography (EEG) or functional magnetic resonance imaging (fMRI). In order to fully understand what Dennett and others mean when they propose we are highly functional robots, and what other physically grounded theories of consciousness have come to life at the end of the 20 th century, we will take a short tour of human perceptual processing, and highlight important notions for the coming chapters.

I.2.2 A few tools for studying brain function

First off, we will shortly review the two techniques used in the present manuscript, their advantages and shortcomings, in order to better assess what has been done.

I.2.2.a

Electroencephalography Electroencephalography (EEG) takes advantage of the fact that when neurons send information, they do it via electrical impulses. While these can be recorded in vivo by inserting electrodes with a sub-millimetric tip in live brain tissue since the 1940s, this method did not allow us to study human cognition for obvious ethical constraints. However, informational exchange between neurons at the level of the synapse produces local-field potentials (LFP), which are variations in the local electric field due to cerebral activity. These can be recorded by using metallic electrodes placed on a person's scalp: electrical impulses are, schematically speaking, produced thanks to exchanges of charged (+ or -) atoms, or ions, in and out of the neuron. Initially, a neuron and its extra-cellular the medium do not have the same amount of ions of each polarity. The neuron electrical potential is negative: it holds more negative charges on the inside. In appropriate situations (see 1.2.4.a and 1.2.5.a for examples), the neuron allows for negative and positive charges to reverse between its inside and outside near its core. Inside the neuron, this causes a chain of events displacing this reversal along its long tail, or axon, until it reaches the tip. The impulse allows for the release of chemicals, caught by adjacent neurons. These chemicals are the information transmitted by the neuron, and cause large electric potential changes by means of ion exchange in the vicinity of the receiving neuron (at the dendrites). On the outside, this causes a disturbance in the originally well-organized positive and negative ions in the extra-cellular medium, which behave like magnets and repel those of similar polarity. Ions have to re-organize to avoid being too close to their kin and this leads to widespread but transient changes in polarity across brain tissue (on the outside of other neurons). If this disturbance in the electrical field is strong enough, when thousands of neurons send signal together because they respond to the same input, it can be recorded through the skull [START_REF] Buzsáki | The origin of extracellular fields and currents -EEG, ECoG, LFP and spikes[END_REF]. This technique is very precise in time, but because the electrical field is conducted throughout different types of tissue (grey and white matter, the meninges and the skull), it can be very difficult to determine its location of origin. EEG allows for dynamic monitoring of brain function in response to stimuli. In the present manuscript, we will use intracranial EEG (iEEG) as investigative means. This technique involves the surgical placement of electrodes inside the skull of patients suffering from drug-resistant epilepsy. These electrodes are inserted for clinical purpose, in order to determine the location of epileptogenic tissues that will be resected. This technique is as precise in time as scalp EEG, and because we know where the electrodes are implanted, we can locate where the information is coming from.

However, because the regions of implantation are dissimilar across patients and determined for clinical purposes, patients are not strictly comparable, and not all areas of the brain are explored at once. Last, these patients have a disorder that affects cerebral functioning, which can impair the quality of recorded data and its interpretation.

I.2.2.b

Behavioral measures EEG, and other imaging techniques, are mostly used in combination with behavioral tasks involving sensory stimulation in controlled settings and reports about what was perceived in the form of motor expressions, such as language or button presses. In the relevant type of experimental setting for this dissertation, responses recorded can be of three kinds: 1) Objective measures, in which participants are forced to choose between alternatives, among which not all are correct. An example would be a discrimination task, where a participant should determine whether a shortly presented square was green or red.

2) Subjective measures, in which participants are encouraged, with certain rules, to report what cannot be measured without introspection. In the discrimination task, we might ask the participant to report whether they saw the square or not. These are of particularly high importance for the study of conscious awareness because they reflect experience (although arguably not always, see Section I.3).

3) Reaction times, which are the amount of time a participant took before they provided an expected response during a task. In our example, participants may have been prompted to respond as fast as possible to the objective task. The time it took them to do so can be measured, and can indirectly reflect properties of cerebral processing.

Behavioral measures are the basis for a neuroscientific study of consciousness. There is no better judge of whether an event was consciously perceived than the subject of the experience. Neuroimaging helps inform the putative mechanisms behind these behavioral phenomena.

I.2.2.c

The contrastive method applied to consciousness The most widespread methodology in cognitive science is to contrast two experimental situations with only one variable change. In studying consciousness, instead of contrasting brain activity for different stimulations, we can contrast activity for identical stimulation according to the participant's subjective report: did the participant report having had a sensory experience, and what difference did this make in the underlying cerebral activity? [START_REF] Baars | Global workspace theory of consciousness : Toward a cognitive neuroscience of human experience[END_REF]

I.2.3 The brain as an informational relay

Before trying to assert what in the brain resonates with subjective perceptual experience, it is important to have a basic understanding of how the brain handles information. Perception usually refers to the conscious experiencing of an external or internal event, and can be studied in itself. In most cases, consciousness is a given in all experimental situations, and differences in how events are experienced is studied. For the sake of simplicity, we will only consider the perception of externally provoked events, and focus on two sensory organs: vision, and audition, for the pragmatic reason that these are the two sensory modalities used in the experimental work reported in the current dissertation.

Generally speaking, the brain can be viewed as an informational relay, in the sense that it receives input from places, and emits outputs to the same or other places in response. The human body is filled with sensory neurons, which are outside the skull, in almost all organs.

Different types of sensory neuron monitors different types of sensation. The most obvious types give rise to well identified sensations: some neurons monitor light for vision, other sense changes in air pressure for audition, chemicals for smell and taste, or soft tissue pressure for touch. Many sensory neurons however lead to more ambiguous subjective states, usually monitoring internal function: some keep track of oxygen levels in the bloodstream or blood pressure in various places, for example. Most sensory neurons in the human body connect with the spinal cord, which in turn transmits the information to the brain. Neurons transmitting information to the brain are called afferent neurons. Most input reaching the brain transits in a structure called the thalamus before going to the cortex. It is there decoded, interpreted alone or together with other types of input by many types of neurons in various steps, and the resulting information is sent back from the brain, via efferent neurons, from the cortex to the thalamus again, and down the spinal cord. It should be noted, because we will mostly be concerned with these in the dissertation that specific types of neurons communicate directly with the brain, without help of the spinal cord. Vision, audition, olfaction, taste, and several other functions are regulated by nerves that make no or little use of the spinal cord (at least for afferent neurons).

The brain can be seen as a relay, where information is transformed into appropriate outformation. Our current primary means of understanding what consciousness is in the brain is to study what happens between the time when input is received and output is provided, within the central nervous system. To this aim, we usually make use of stimuli leading to nonambiguous sensations, such as vision or audition, and compare situations where an input leads to experience, and others where it doesn't.

I.2.4 Visual perception

I.2.4.a

Anatomy and physiology Vision originates in the retina, which is composed of several layers of cells, covering the inner part of the back of the eye. Among these several types of cells, some are sensory neurons, in that they respond to physical stimuli of a certain kind, and send information in the form of electric impulses to the brain. These neurons respond specifically to light, and code its intensity by sending more or less electrical impulses (or action potentials). Sensory neurons possess proteins on their surface which have a low-level physical reaction to what they are supposed to detect: when a photon of the appropriate wavelength (i.e. color) reaches the exposed part of the sensory neuron, it hits a colored pigment, which absorbs it like filters on 3-D glasses. This pigment is attached to a derivative of vitamin-A, which changes shape when it interacts with light. This change of shape results in a series of events affecting ion channels and leads to the reversal of electrical charges in and out of the neuron as described earlier. This allows the cell to send electric signal to another layer of cells (ganglion cells) in the retina. These, in turn, communicate this signal to the brain. Ganglion cells do not only relay what was perceived by sensory neurons to the brain, but tune each other with horizontal connections among themselves and other types of cells present in the retina, so that what reaches the brain is not a mere translation of how many photons reached each cell at one point in time: the signal is processed within the eye. These phenomena occurring before the brain can do anything about them lead to a variety of well-known optical illusions [START_REF] Eagleman | Visual illusions and neurobiology[END_REF].

All axons of ganglion cells meet in a small spot near the center of the retina in order to leave the eye and travel to the brain. There, axons form what is called the optic nerve. The optic nerve travels towards the back of brain from both eyes, and nerves from each eye meet soon at the optic chiasma. There, they split in two: the half that received information from the left side of the world from either eye will travel towards the right hemisphere, and vice-versa, so that each newly formed tract contains information from one visual hemi-field. This process is called decussation, and occurs once again later for the upper and lower halves of the visual field. In short, nerves travel from the eye to the occipital lobe at the very back of the brain, and the upper left quadrant of the visual scene is processed by the lower right primary visual cortex (V1, or striate cortex), etc. [START_REF] Widmaier | Vander's Human physiology : The mechanisms of body function[END_REF]. Part of the optic nerve however does not go to the optic chiasma, but leaves for several other places, including the brainstem.

In the case of blindsight patients, whose primary visual cortex is impaired, this is what allows them to have basic reactions to external stimuli [START_REF] Bridge | Changes in connectivity after visual cortical brain damage underlie altered visual function[END_REF]: a part of the nerve going to the superior colliculus in the brainstem allows for some reaction to external events, but without the subjective experience we get when information relays through V1. This curious fact has not eluded neuroscientists, and the role of V1 in visual awareness is the cornerstone of some major research avenues and debates in consciousness research [START_REF] Lamme | The role of primary visual cortex (V1) in visual awareness[END_REF][START_REF] Silvanto | Why is "blindsight" blind? A new perspective on primary visual cortex, recurrent activity and visual awareness[END_REF][START_REF] Silvanto | Striate cortex (V1) activity gates awareness of motion[END_REF]Silvanto, Lavie, & Walsh, 2005).

I.2.4.b

Cortical processing The major part of cortical visual processing starts in V1, at the very end of the occipital lobe, and is then organized around several other regions which are functionally specialized: V1 codes for the orientation (and spatial frequency) of bars in the visual scene [START_REF] Hubel | Receptive fields of single neurones in the cat's striate cortex[END_REF][START_REF] Watson | The complete works of Chuang Tzu[END_REF], V2 the detection of contours [START_REF] Hegdé | Selectivity for complex shapes in primate visual area V2[END_REF], as V4 which is also assumed play a role in color perception [START_REF] Heywood | Cortical area V4 and its role in the perception of color[END_REF][START_REF] Pasupathy | Responses to contour features in macaque area V4[END_REF], and V5 (or MT) is specialized in motion perception [START_REF] Newsome | A selective impairment of motion perception following lesions of the middle temporal visual area (MT)[END_REF].

There are plenty of cerebral regions thought to be implicated in visual processing, spanning the occipital, temporal, and parietal lobes [START_REF] Brodmann | Vergleichende Lokalisationslehre der Grosshirnrinde in ihren Prinzipien dargestellt auf Grund des Zellenbaues[END_REF][START_REF] Felleman | Distributed hierarchical processing in the primate cerebral cortex[END_REF][START_REF] Maunsell | Visual processing in monkey extrastriate cortex[END_REF]

(Figure I-2).
Visual perception is hierarchically organized: processing starts in V1, and moves on to different regions in order. This was evidenced through specific, reversible deactivations of subparts of V1 or other regions of the visual cortex. When V1 is deactivated by invasive cooling, regions thought to be higher in the hierarchy do not display much activity in response to a visual stimulus [START_REF] Girard | Response selectivity of neurons in area MT of the macaque monkey during reversible inactivation of area V1[END_REF]. This led the scientific community to think the visual processing is, in part, serial: whether activity is elicited in high-level areas depends in part on activity in low-level areas.

From V1, information is transmitted through two major streams of processing [START_REF] Goodale | Separate visual pathways for perception and action[END_REF][START_REF] Milner | Two visual systems re-viewed[END_REF][START_REF] Mishkin | Object vision and spatial vision : Two cortical pathways[END_REF]: the ventral and the dorsal streams. The ventral stream is, simplistically, thought to be specialized in the identification of objects and their different features ("what" pathway), such as color, texture etc. It spans from V1 to the inferior part of the temporal cortex (IT) through V2, V4, and many other functionally specific subdivisions of the cortex, such as the visual word form area (VWFA) which detects written words (Laurent [START_REF] Cohen | Specialization within the ventral stream : The case for the visual word form area[END_REF][START_REF] Cohen | Language-specific tuning of visual cortex? Functional properties of the Visual Word Form Area[END_REF], or the fusiform face area (FFA), which specializes in face detection (N. Kanwisher, Figure I-2: Functional anatomy of the visual system. Cortical visual processing begins in the occipital lobe in V1, and is hierarchically organized. It is classically thought to be divided in two streams. The ventral stream goes towards the infero-temporal cortex and deals with object identification (in red), whereas the dorsal stream is in the parietal cortex and processes object location (in blue). Both streams interact with one another, and with more frontal areas dedicated to semantics and decision-making.

(From http://fourier.eng.hmc.edu/e180/lectures/visualcortex/node7.html).

The dorsal stream of visual processing, in contrast, is thought to mainly deal with spatial information relating to the objects identified in the ventral stream ("where" pathway), so that it helps determine the location of stimuli in space, notably for preparation of actions relating to these stimuli. It spans from V1 to the parietal lobe through V2 and V5, as well as a variety of frontal areas [START_REF] Mishkin | Object vision and spatial vision : Two cortical pathways[END_REF]. Of course, those two streams of processing are permeable and exchange information at various levels [START_REF] Cloutman | Interaction between dorsal and ventral processing streams : Where, when and how?[END_REF][START_REF] Creem | Grasping objects by their handles : A necessary interaction between cognition and action[END_REF].

When visual information reaches the cortex in V1, it is thus serially processed through the ventral and dorsal streams for identification, location and response. Neurons in V1 are tuned to respond to oriented bars [START_REF] Hubel | Receptive fields of single neurones in the cat's striate cortex[END_REF][START_REF] Watson | The complete works of Chuang Tzu[END_REF], and will specifically respond to their preferred orientation: a neuron whose preferred orientation is 90°, if presented with a high contrast 90° bar in the portion of space it receives input from, will have a very high response frequency, and send many electrical impulses per second. The same neuron will respond to an 80° oriented bar, but with fewer action potentials per second, etc. Besides their preferred orientation (orientation receptive field), neurons have a spatial receptive field: they respond to subparts of the visual scene. V1 is retinotopically organized: adjacent neurons code for adjacent locations in the visual field [START_REF] Engel | Retinotopic organization in human visual cortex and the spatial precision of functional MRI[END_REF][START_REF] Wandell | Visual field maps in human cortex[END_REF].

Through the ventral stream, specialized networks higher in the hierarchy gradually increase their receptive fields (for an example in V4, see Gattass, Sousa, & Gross, 1988), meaning that neurons will respond to stimuli in larger portions of the visual field. The spatial resolution of what these neurons are coding for will decrease, but the complexity of their preferred stimulus will increase in return [START_REF] Booth | View-invariant representations of familiar objects by neurons in the inferior temporal visual cortex[END_REF][START_REF] Riesenhuber | Hierarchical models of object recognition in cortex[END_REF][START_REF] Rolls | Functions of the primate temporal lobe cortical visual areas in invariant visual object and face recognition[END_REF][START_REF] Serre | A feedforward architecture accounts for rapid categorization[END_REF], which is how areas such as the VWFA and FFA respond specifically to written words and faces: these are complex stimuli, usually spanning large portions of the visual field. This process going up the hierarchy has been called the feedforward sweep (Lamme & Roelfsema, 2000), and results in already deep processing of the visual input (i.e. from oriented bars in space to face detection), but in itself doesn't compose the whole processing chain, nor does it particularly correlate with subjective experience as will be detailed in section I.3. The feedforward sweep is followed by recurrent processing, or feedback loops, where higher levels in the hierarchy send back information to the lower levels, and information is exchanged between them. Feedback connections are numerous [START_REF] Felleman | Distributed hierarchical processing in the primate cerebral cortex[END_REF], and also interact with lateral (same-level) connections [START_REF] Liang | Interactions between feedback and lateral connections in the primary visual cortex[END_REF]. Recurrent processing is thought to help regulate neuronal activity and integrate information [START_REF] Gilbert | Brain states : Top-down influences in sensory processing[END_REF].

Recurrent processes have been shown to play a pivotal role in figure-ground segregation [START_REF] Hupé | Cortical feedback improves discrimination between figure and background by V1, V2 and V3 neurons[END_REF] and perceptual processing [START_REF] Lamme | Feedforward, horizontal, and feedback processing in the visual cortex[END_REF][START_REF] Supèr | Two distinct modes of sensory processing observed in monkey primary visual cortex (V1)[END_REF]. It has been recently proposed that in the context of visual object recognition, the feedforward sweep leads to the identification, in high levels modules, of statistically probable templates for the current object in the visual scene, and that communication with lower levels in the hierarchy serves as means for verification and matching of this template to the actual data in the system (Di Lollo, 2018), because lower levels have better spatial resolution to perform this task. In other words, high-level areas make probabilistic estimations of what is being seen because they have low spatial resolution but very specific tuning, and send their guess downwards to regions holding more detailed but less global information for matching. In the context of visual awareness, this recurrent processing is thought to be highly related to the emergence of subjective experience (Lamme & Roelfsema, 2000).

I.2.4.c

An example of complex visual stimulus processing: written words In order to review the vast amount of information we have just overviewed about visual processing, we will walk through an example of visual stimulus processing that is, conveniently, very important for one of the following chapters (Chapter 3). What happens when we read (Figure I-3)? In the primary visual cortex (V1), oriented bars are detected, and neurons have very small receptive fields, meaning that they integrate information about a very small region of retinal space, of typically 0.5 to 2 degrees of visual angle near the fovea [START_REF] Spillmann | A comparison of perceptive and receptive fields in man and monkey[END_REF]. Information then moves on to V2, where contours are detected, so that letter fragments can be identified. Higher in the visual hierarchy, in V4, it is hypothesized that there are neurons that respond to letters in an abstracted way, meaning that they are insensitive to case. Of course, to be able to recognize entire letters, these neurons must have larger receptive fields and cover bigger portions of space. The feedforward sweep of processing then goes on in the infero-temporal cortex (IT), in the ventral pathway, and more specifically, in the Visual Word Form Area (VWFA), where combinations of letters, or small words and frequent substrings are recognized. The information will then move on from IT towards two different pathways in the temporal and frontal lobes: one for semantics, the other for phonological processing of the word (Laurent [START_REF] Cohen | Specialization within the ventral stream : The case for the visual word form area[END_REF][START_REF] Cohen | The visual word form area : Spatial and temporal characterization of an initial stage of reading in normal subjects and posterior split-brain patients[END_REF][START_REF] Dehaene | The neural code for written words : A proposal[END_REF][START_REF] Vinckier | Hierarchical Coding of Letter Strings in the Ventral Stream : Dissecting the Inner Organization of the Visual Word-Form System[END_REF]. Of course, information also travels through the dorsal pathway, towards the parietal and frontal lobes, for localization. At this point, if processing was successful, a visual word is recognized, localized and interpreted, but it is not necessarily conscious (Dehaene, Naccache, Cohen, Bihan, et al., 2001). Recurrent processes as well as conscious awareness of the stimulus may or may not take place depending on multiple factors addressed in more detail in section I.3.

I.2.5 Auditory perception

I.2.5.a

Anatomy and physiology Audition originates in the inner ear, near the temporal lobe of the brain. Sensory neurons there are receptive to changes in air pressure due to sound waves traveling in space. Sounds reaching the ear are transduced by the eardrum and amplified by a series of bones before reaching the cochlea. Schematically, the cochlea contains a basilar membrane, which is more or less rigid depending on the distance from the outside. It is surrounded by a liquid medium. The occurrence of a sound will lead the membrane to vibrate in different places depending on the sound's pitch, thanks to its variability in rigidness. On the membrane's surface are auditory sensory neurons, which display cilia, little tubes laying in liquid medium. These cilia at the top of each cell are connected to each other: when the membrane vibrates, they will start oscillating together. Movement in one direction will lead the molecular links between one cilium and the next to pull and release the opening of ion channels on the adjacent cilium. This phenomenon allows for the cell to send signal in the form of an electric impulse to the brain. The auditory nerve is the collection of all axons from these neurons, and leaves the inner ear for the brainstem in the cochlear nucleus and superior olivary complex, where it move on to the midbrain tectum (lateral lemniscus and inferior colliculus), and the thalamus (medial geniculate body), eventually reaching the temporal lobe on the same or the opposite side of the brain [START_REF] Biacabe | Functional anatomy of auditory brainstem nuclei : Application to the anatomical basis of brainstem auditory evoked potentials[END_REF]. The auditory nerve relays in several places in the brainstem, and the different functions of these pathways in auditory perception are still not well known [START_REF] Hackett | Chapter 2âAnatomic organization of the auditory cortex[END_REF][START_REF] Hall | Relationships between Human Auditory Cortical Structure and Function[END_REF][START_REF] Kaas | Subdivisions of auditory cortex and processing streams in primates[END_REF].

I.2.5.b

Cortical processing Contrary to visual perception for which animal studies have been very informative to describe the visual cortex, similarities across species for auditory perception are harder to define beyond the general organization of the auditory cortex [START_REF] Hackett | Information flow in the auditory cortical network[END_REF], which occupies the upper two-thirds of the temporal lobe, as described in early anatomical maps of the brain [START_REF] Brodmann | Vergleichende Lokalisationslehre der Grosshirnrinde in ihren Prinzipien dargestellt auf Grund des Zellenbaues[END_REF], although important information can still be drawn from animal electrophysiology [START_REF] Rauschecker | Auditory and visual cortex of primates : A comparison of two sensory systems[END_REF]. Auditory processing appears to be more parallel than in the visual cortex from the start, with different auditory regions processing different aspects of the auditory stimulus in parallel [START_REF] Kaas | Subdivisions of auditory cortex and processing streams in primates[END_REF][START_REF] Rauschecker | Serial and parallel processing in rhesus monkey auditory cortex[END_REF], which makes it more difficult to determine which places do what. Lesion studies have been less informative, possibly for this reason [START_REF] Griffith | Disorders of human complex sound processing[END_REF].

Neurons in the primary auditory cortex (A1) are tonotopically organized, so that regions of the basilar membrane (coding pitch) are mapped onto specific regions of A1 [START_REF] Widmaier | Vander's Human physiology : The mechanisms of body function[END_REF]. There are other types of maps for different dimensions of auditory stimuli such as timbre (the spectral properties of complex sounds) [START_REF] Bizley | Interdependent encoding of pitch, timbre, and spatial location in auditory cortex[END_REF][START_REF] Menon | Neural correlates of timbre change in harmonic sounds[END_REF][START_REF] Warren | Analysis of the spectral envelope of sounds by the human brain[END_REF]. Akin to visual neurons, auditory neurons have receptive fields, which can relate to pitch, timbre, etc. [START_REF] Bizley | The what, where and how of auditory-object perception[END_REF]. In the macaque monkey for example, we know the core of the auditory cortex maps sounds according to frequency, with varying bandwidth depending on location [START_REF] Rauschecker | Cortical processing of complex sounds[END_REF][START_REF] Rauschecker | Processing of complex sounds in the macaque nonprimary auditory cortex[END_REF]. This can be expressed as frequency selectivity, with varying band receptive field (bandwidth), like visual neurons can have orientation selectivity and spatial receptive fields [START_REF] Rauschecker | Auditory and visual cortex of primates : A comparison of two sensory systems[END_REF].

Auditory processing has been described as taking place in two parallel pathways which process identity and location in space, akin to those present in the visual cortex [START_REF] Kaas | Subdivisions of auditory cortex and processing streams in primates[END_REF][START_REF] Rauschecker | Auditory and visual cortex of primates : A comparison of two sensory systems[END_REF][START_REF] Rauschecker | Mechanisms and streams for processing of "what" and "where" in auditory cortex[END_REF][START_REF] Rauschecker | Serial and parallel processing in rhesus monkey auditory cortex[END_REF][START_REF] Romanski | Dual streams of auditory afferents target multiple domains in the primate prefrontal cortex[END_REF]. The ventral pathway starts in A1 in the temporal cortex (Heschl's gyrus), which is the core of auditory processing (Figure I-4), and extends to the anterior belt of the auditory cortex, which is more or less circularly organized around its core, before reaching the frontal cortex. Neuroimaging studies suggest some subdivision of labor like in the visual cortex, but less so. Timbre [START_REF] Bizley | Interdependent encoding of pitch, timbre, and spatial location in auditory cortex[END_REF] and pitch [START_REF] Griffiths | Direct recordings of pitch responses from human auditory cortex[END_REF][START_REF] Patterson | The processing of temporal pitch and melody information in auditory cortex[END_REF][START_REF] Staeren | Sound categories are represented as distributed patterns in the human auditory cortex[END_REF] seem to be coded by several regions with varying specificities (i.e. K. M. M. [START_REF] Walker | Multiplexed and Robust Representations of Sound Features in Auditory Cortex[END_REF], from A1 to the posterior superior temporal gyrus which handles phonetic parsing with different populations of neurons selectively responding to different phonemes (language units such as /ba/ or /ga/) [START_REF] Chang | Categorical speech representation in human superior temporal gyrus[END_REF], before extending to the inferior frontal cortex and Broca's area [START_REF] Hickok | Dorsal and ventral streams : A framework for understanding aspects of the functional anatomy of language[END_REF][START_REF] Rauschecker | Where, When, and How : Are they all sensorimotor? Towards a unified view of the dorsal pathway in vision and audition[END_REF].

The auditory ventral pathway is supposed to be hierarchically organized like its visual counterpart, with less obvious modularity, at least for pitch processing. This evidence comes from experiments in neuroimaging showing that while activity in A1 is modulated by pitch [START_REF] Garcia | The effect of stimulus context on pitch representations in the human auditory cortex[END_REF], activity in the planum temporale and other extra-core regions could be pitch independent [START_REF] Hackett | Information flow in the auditory cortical network[END_REF], suggesting the same kind of tuning to more complex stimuli seen along the visual ventral pathway (from oriented bars to faces).

The dorsal pathway is thought to originate in A1 as well, but extends through the posterior part of the belt region before reaching the parietal cortex and the premotor cortex, before extending to Broca's area as well [START_REF] Rauschecker | Where, When, and How : Are they all sensorimotor? Towards a unified view of the dorsal pathway in vision and audition[END_REF]. The first studies suggesting a dissociation between these pathways showed that while the ventral stream was more responsive to the type of vocalization produced, the dorsal one was selective for their spatial locations [START_REF] Tian | Functional specialization in rhesus monkey auditory cortex[END_REF]. Again, and maybe more so than in the visual pathway, many interconnections have been demonstrated to exist between dorsal and ventral streams [START_REF] Bizley | Interdependent encoding of pitch, timbre, and spatial location in auditory cortex[END_REF].

Figure I-4:

Functional anatomy of the auditory system. Auditory processing starts in Brodmann area 41, called Heschl's gyrus. The cortical auditory system is not well-known, but is thought to be organized around a core region (Heschl's gyrus) and its belt (area 42), in a less hierarchical manner than the visual system. The auditory cortex is located in the superior part of the temporal lobe, and divided in a ventral stream (green) and a dorsal stream (red). The first is thought to process pitch, timbre, and lead to auditory object identification, whereas the dorsal stream is thought to handle sound location (red). Auditory processing includes areas involved in understanding and producing language, such as Broca's area (areas 44 and 45), and the premotor cortex (PMC) as well as areas dealing with phonological processing. It can involve semantic areas depending on the stimulus. (From Rauschecker 2018)

I.2.6 Multisensory interactions

I.2.6.a

Audio-visual interactions: multisensory processing. The interactions that exist between auditory and visual input are crucially relevant for the experiments presented in the following chapters. The combination of the two are of great help when it comes to making sense of the world around us: combining different sources of information with different properties is an optimal strategy in many instances. There are two kinds of audio-visual interactions to account for here: direct interactions, and interactions mediated by a potentially amodal (as in non-sensory) higher-order support system. The first are called multisensory integration, while the second rely on cross-modal attention, which will be described in section I.3.

Historically, cortical function was mostly investigated unimodally, because its organization was thought to have unimodal sensory integration occur first, followed by multisensory processing. The prevailing notion at the time was that different features of stimuli were bound together at a late stage in processing [START_REF] Treisman | A feature-integration theory of attention[END_REF] in "polysensory" areas [START_REF] Felleman | Distributed hierarchical processing in the primate cerebral cortex[END_REF][START_REF] Penfield | The cerebral cortex of man; a clinical study of localization of function[END_REF]. What this type of view would entail is that primary sensory cortices receive input from outside, convert this information, and send it to the associative areas, which only have cortico-cortical connections, and these take care of the rest. More recent research has shown that things are actually much more complicated: V1 responds to auditory stimulation [START_REF] Wang | Visuo-auditory interactions in the primary visual cortex of the behaving monkey : Electrophysiological evidence[END_REF], or to Braille in blind people [START_REF] Burton | Adaptive changes in early and late blind : A fMRI study of Braille reading[END_REF][START_REF] Sadato | Activation of the primary visual cortex by Braille reading in blind subjects[END_REF]. Just as well, A1 responds to somatosensory inputs (tactile stimuli) [START_REF] Foxe | Auditory-somatosensory multisensory processing in auditory association cortex : An fMRI study[END_REF][START_REF] Kayser | Integration of touch and sound in auditory cortex[END_REF]. Audio-visual interactions are actually mapped from early visual and auditory cortices, which allows for very fast multisensory integration [START_REF] Rockland | Multisensory convergence in calcarine visual areas in macaque monkey[END_REF].

Multisensory integration even occurs in the superior colliculus in very early, pre-cortical stages, to deal mainly with stimulus location [START_REF] Wallace | Converging influences from visual, auditory, and somatosensory cortices onto output neurons of the superior colliculus[END_REF].

Areas subtending multisensory integration are mainly in the parietal cortex, and some overlap, of course, with streams of processing we have identified earlier: in the posterior parietal cortex, the dorsal stream of visual processing (the one supposed to take care of visual object location and goal directed behavior towards the object) actually includes partially dynamic [START_REF] Schlack | Multisensory space representations in the macaque ventral intraparietal area[END_REF] audio-visual and visuosomatosensory maps (Y. E. [START_REF] Cohen | A common reference frame for movement plans in the posterior parietal cortex[END_REF]. The superior temporal cortex (auditory processing) also includes such multisensory crossroads [START_REF] Hickok | Area Spt in the human planum temporale supports sensory-motor integration for speech processing[END_REF]). An interesting feature of cross-modal interactions is that they also exist between primary cortices, particularly for audio-visual interactions [START_REF] Cappe | Heteromodal connections supporting multisensory integration at low levels of cortical processing in the monkey[END_REF][START_REF] Falchier | Anatomical evidence of multimodal integration in primate striate cortex[END_REF][START_REF] Rockland | Multisensory convergence in calcarine visual areas in macaque monkey[END_REF]. Lip-reading activates the auditory cortex in the absence of sound [START_REF] Calvert | Activation of auditory cortex during silent lipreading[END_REF], and indeed, there is evidence that low-level interactions between these two senses occur, which is what is hypothesized to produce the ventriloquist effect, in which reasonable spatiotemporal congruence between a visual speech source (such as a well-manipulated puppet) and an auditory speech source (such as the ventriloquist) make us feel like the puppet is talking [START_REF] Howard | Human spatial orientation[END_REF]. Of course, when this congruence is reduced temporally or spatially, the illusion partially dissipates [START_REF] Alais | The ventriloquist effect results from near-optimal bimodal integration[END_REF].

Similarly, temporal synchrony produces remarkable effects on behavioral performance: a sound with no spatial localization can help find the location of a visual target appearing at the same time in a crowded display (Van der Burg, Olivers, Bronkhorst, & Theeuwes, 2008).

Most cross-modal interactions described here do not require conscious evaluation, but allow for better reaction to events on the outside, and fast processing. Multisensory integration occurs within short windows of time [START_REF] Wallace | Converging influences from visual, auditory, and somatosensory cortices onto output neurons of the superior colliculus[END_REF], and is gradually less effective beyond 100ms of asynchrony between two stimuli. Multisensory integration can be insensitive to location, so that the temporal coherence between two stimuli of interest in different modalities is enough for multisensory integration to take place.

I.2.6.b

An example of tripartite sensory interactions: phonological processing in the (left) planum temporale Cross-modal interactions can involve more than two senses at once. It is the case for phonological processing, for example, which involves the visual, auditory and motor systems.

Phonological processing deals with the information carried by the sounds produced in language.

As such, they are involved in both language comprehension and language production. The motor system will not be described in detail in this document, because it is hardly involved in the experiments described in the following chapters. However, language is a major component of the present work and particularly phonological processing in Chapter 4.

We long surmised that the phonological pathways involved in language comprehension actually make use of motor components also involved in speech production [START_REF] Hickok | Auditory-motor interaction revealed by fMRI : Speech, music, and working memory in area Spt[END_REF]. The planum temporale, mentioned earlier as being strongly involved in auditory sensory processing, was also evidenced to be active when speech is produced internally and no actual sounds are spoken [START_REF] Buchsbaum | Reading, hearing, and the planum temporale[END_REF][START_REF] Hickok | Auditory-motor interaction revealed by fMRI : Speech, music, and working memory in area Spt[END_REF], or when the participant is humming [START_REF] Pa | A parietal-temporal sensory-motor integration area for the human vocal tract : Evidence from an fMRI study of skilled musicians[END_REF]. A broader region including the planum temporale was shown to be functionally connected to motor speech areas in the frontal cortex [START_REF] Hickok | Auditory-motor interaction revealed by fMRI : Speech, music, and working memory in area Spt[END_REF], and it also responds to visual stimuli when these are related to speech. These include lip-reading (Okada & Hickok, 2009), with the surprising illusion induced by the McGurk effect: when a video of lips showing the visual component of a sound (such as /ba/) is paired with the auditory component of another sound (such as /ga/), participants report hearing a third, different phoneme (in this case, /da/) [START_REF] Mcgurk | Hearing lips and seeing voices[END_REF].

Strikingly, the planum temporale responds when processing more distantly speech-related sounds: written words [START_REF] Jobard | Evaluation of the dual route theory of reading : A metanalysis of 35 neuroimaging studies[END_REF].

This short description of planum temporale's function is one example that heterogeneous inputs can be processed at one point or another by the same module, and goes to show the potential complexity cognitive neuroscientists have to explain when dealing with sensory processing.

I.2.7 Making sense of the input: semantic processing

Once an event has been uni-or multimodally processed on the basis of its physical properties, the brain is supposed to make sense of it. Most elements of the world are meaningful: a cubic wooden object with little dots on it is a die, an animal with four legs, fur, and who barks is a dog. Dice have equal side-lengths, and same-surface sides. They are great tools to study the basics of probability, and are mostly used to play games with a random component. Dogs are man's best friend, mammals, can be male or female, and social animals, although one should be careful not to scare them because they can be dangerous.

Objects in the external world have meaning, because they are associated with abstract concepts that have more features than what is immediately accessible by means of our sensory organs. So far, we have only described processes which seem to represent meaningless features such as bars or pitch, but the brain must make sense of them, to produce appropriate responses in thought, action, or emotion.

How the brain makes sense of things has been tested in neuroimaging studies contrasting meaningful objects with others, which strongly resemble them but have no meaning. For example, when looking for a region in the brain that selectively makes sense of words, we are looking for a place selectively responding to "arousal" versus "aosular". Variations of this manipulation have been found for many different meaningful and complex objects, and used in neuroimaging studies such as fMRI, looking for specific regions coding for meaning. There turns out to be many different places involved: the left fusiform gyrus responds to words versus non-words [START_REF] Dehaene | The unique role of the visual word form area in reading[END_REF], buildings, scenes or large objects are coded in the parahippocampal place area [START_REF] Epstein | The parahippocampal place area : Recognition, navigation, or encoding?[END_REF][START_REF] Epstein | A cortical representation of the local visual environment[END_REF], faces are found in the fusiform and occipital face areas [START_REF] Gauthier | Expertise for cars and birds recruits brain areas involved in face recognition[END_REF][START_REF] Kanwisher | The fusiform face area : A module in human extrastriate cortex specialized for face perception[END_REF]. Many different places have been associated with semantic processing across the brain, mostly in temporal [START_REF] Fairhall | Person-and place-selective neural substrates for entity-specific semantic access[END_REF][START_REF] Price | Converging evidence for the neuroanatomic basis of combinatorial semantics in the angular gyrus[END_REF][START_REF] Visser | Semantic processing in the anterior temporal lobes : A meta-analysis of the functional neuroimaging literature[END_REF] and frontal [START_REF] Devlin | Semantic processing in the left inferior prefrontal cortex : A combined functional magnetic resonance imaging and transcranial magnetic stimulation study[END_REF] areas.

On the theoretical level, how semantics are processed is quite puzzling: there cannot be one module or neuron coding for each possible object, because these are virtually infinite. One large theory of semantic processing was proposed in the 1970s: for the brain to make sense of the word "dog", it needs to bring all elements relevant to it in play, so that "has four legs", "is a mammal", "is friendly", "can be dangerous", "is man's best friend" are immediately associated to it. This way, any specific dog will be coded with shared properties belonging to all dogs plus what makes them special ("I love" or "bit me"), without the need for a network dedicated to each dog.

The spreading semantic association (SSA) theory of semantic processing was first put forward by Quillian [START_REF] Quillian | A revised design for an understanding machine[END_REF][START_REF] Quillian | Word concepts : A theory and simulation of some basic semantic capabilities[END_REF], and made popular in a revised version by Collins & Loftus [START_REF] Collins | A spreading-activation theory of semantic processing[END_REF]. It is one of several models describing parallel distributed processing (PDP) [START_REF] Romero | A general framework for parallel distributed processing[END_REF]. In this framework, a concept such as "dog" can be represented as a node in a framework, with bidirectional links to other nodes representing properties of the concept ("has four legs"). These links are weighted, so that "has four legs" is more related to "dog" than "is brown" would be, because it is more essential to dogness. When a dog is seen, visual processing takes place, and somehow activates the "dog" semantic node. Activation of this node leads to the activation of all first-order related nodes, then from these to the second-order related nodes etc. The theory presupposes that information can be traced back to its starting point, and that there can only be one starting point at a time. This last assumption is based on the apparent seriality of conscious processing: the stream of consciousness, or the central processor only handles one thing at a time. From the starting point, the spreading activation decreases in strength outwards, depending on the number of nodes it has traveled through, link weights (the weaker the link, the more the activity decreases when it travels through it), time, and depending on whether new meaningful content was introduced to the semantic network.

I.2.8 Summary

Blindsight is a condition in which patients have some representations of objects but do not experience them. This find was baffling because until the 20 th century, non-conscious processes were thought to be very limited. Some elements of knowledge of how the brain works might provide us with constraints and tools to start wondering what makes a stimulus conscious or not. As such, we reviewed elements of sensory processing. The brain can be viewed as an information processing hub, with inputs and outputs. In visual perception, information is sent by the retina to the brain, after some local processing. In the visual cortex, we have seen there exists a hierarchy, in the sense that an input is serially processed in subsequent modules with different functions. Of course, there are several streams of processing, so that one input is serially processed in several chains of modular processing. This step is called the feedforward sweep, and is not thought to correlate with experience. After the feedforward sweep, there are recurrent loops of processing taking place, in which information is exchanged from the topdown, and back and forth, as well as there are horizontal (same level in the hierarchy) connections for communication. This step is thought to be crucial for experience [START_REF] Di Lollo | Attention is a sterile concept; iterative reentry is a fertile substitute[END_REF]Lamme & Roelfsema, 2000). Auditory processing shares some organization principles with visual processing in the sense that we have evidenced cortical maps coding for different features of stimuli (pitch in the auditory, and space in the visual system), and some hierarchical organization, although it is less clear. There are many interconnections between sensory cortices, and some of these interactions occur before any voluntary control can be exerted, but cross-modal voluntary phenomena also exist [START_REF] Driver | Cross-modal links in spatial attention[END_REF]. At the top of the hierarchy, semantic processing takes place, under the assumption that there are interconnected featural nodes representing concepts in some way within the brain.

I.3 Cortico-cortical phenomena: the brain working on itself

I.3.1 Unconscious versus conscious processing

So far, we have seen a brief overview of how information is processed in the brain. We have given some bases for how sensory processing is completed, from early sensory areas, to multimodal integration, followed by semantic interpretation of a stimulus. This process ends when the brain provides the appropriate output via, for example, motor production (in speech or action). But this is foregoing two important properties of how we interact with our proximal environment: the multiplicity of stimuli arriving at our senses at all times, and the fact that not all stimuli are consciously perceived. Until now, we have reviewed what we know about perception, and described the steps of sensory processing up to its most complex and abstract levels, semantics. Not all processed stimuli become conscious, but unconscious processing is actually quite complex on its own: it isn't restricted, as Jackson thought in the 19 th century, to reflexive behavior.

To study unconscious perception, there are several tricks we can use in a laboratory setting. Stimuli can be presented below the threshold for conscious experience and report: in this case they are subliminal. To achieve this, stimuli can be masked by the concurrent or subsequent presentation of one or several stimuli that shares some of its physical features. One example is visual pattern masking, where a visual stimulus is closely followed by an overlapping, similar stimulus (for example the word "VIDEO" followed by "OEIVRG" in rapid succession). In other cases, a stimulus can be presented at the perceptual threshold: its intensity over the background noise is very low, so that it is very difficult to perceive. In such cases, a stimulus can be consciously perceived or not, depending on the internal state of the brain.

Studies using subliminal paradigms have shown that unconscious stimuli can be processed at the sensory level: using neuroimaging techniques such as fMRI, activations in the relevant regions of the cortex have been found for the visual (Dehaene, Naccache, Cohen, Bihan, et al., 2001;[START_REF] Fang | Cortical responses to invisible objects in the human dorsal and ventral pathways[END_REF][START_REF] Jiang | Processing of invisible stimuli : Advantage of upright faces and recognizable words in overcoming interocular suppression[END_REF], auditory [START_REF] Diekhof | Top-down and bottomup modulation of brain structures involved in auditory discrimination[END_REF][START_REF] Kouider | Cerebral bases of subliminal speech priming[END_REF], and tactile [START_REF] Taskin | Inhibitory impact of subliminal electrical finger stimulation on SI representation and perceptual sensitivity of an adjacent finger[END_REF] modalities.

Studies highlighting the depth of unconscious processing can use imaging techniques to

see what brain regions are involved when presented with an unconscious stimulus, but behavioral methods are also essential to probe the depth of unconscious processing. In experimental paradigms using priming, the influence of a first stimulus on the behavioral or cerebral response to a second one have shown this can occur unconsciously: participants respond faster (Naccache & Dehaene 2001), or more accurately [START_REF] Bar | Subliminal visual priming[END_REF] when presented with unconscious primes during a behavioral task.

It has been shown with and without imaging techniques that high-level perceptual processing without awareness ranges from motor planning [START_REF] Dehaene | Imaging unconscious semantic priming[END_REF][START_REF] Eimer | Effects of masked stimuli on motor activation : Behavioral and electrophysiological evidence[END_REF]Naccache & Dehaene, 2001;[START_REF] Vorberg | Different time courses for visual perception and action priming[END_REF], repetition priming of words (Dehaene, Naccache, Cohen, Bihan, et al., 2001;[START_REF] Forster | Repetition priming and frequency attenuation in lexical access[END_REF][START_REF] Humphreys | Event perception and the word repetition effect[END_REF][START_REF] Humphreys | Automatic phonological priming in visual word recognition[END_REF][START_REF] Kanwisher | Repetition blindness : Type recognition without token individuation[END_REF], semantic priming of words [START_REF] Abrams | Unconscious semantic priming in the absence of partial awareness[END_REF][START_REF] Fowler | Lexical access with and without awareness[END_REF][START_REF] Koriat | Semantic facilitation in lexical decision as a function of prime-target association[END_REF][START_REF] Marcel | Conscious and unconscious perception : Experiments on visual masking and word recognition[END_REF][START_REF] Perea | Associative and semantic priming effects occur at very short stimulus-onset asynchronies in lexical decision and naming[END_REF], to emotional processing of masked words or faces [START_REF] Dimberg | Unconscious Facial Reactions to Emotional Facial Expressions[END_REF][START_REF] Gaillard | Nonconscious semantic processing of emotional words modulates conscious access[END_REF][START_REF] Morris | Conscious and unconscious emotional learning in the human amygdala[END_REF][START_REF] Naccache | A direct intracranial record of emotions evoked by subliminal words[END_REF]. See [START_REF] Kouider | Levels of processing during non-conscious perception : A critical review of visual masking[END_REF] for a comprehensive review. Beyond sensory processing in its entirety, studies evidence unconscious effects on cognitive control [START_REF] Lau | Unconscious activation of the cognitive control system in the human prefrontal cortex[END_REF]

I.3.2 Controlling and experiencing cerebral representations

We have considered the brain as some kind of supercomputer, without exploring experience, or voluntary control. In this section, we will review them both, as they are interconnected, under the broad concepts of conscious access and attentional control. We have tried defining consciousness in section I, but we haven't talked about attention at all until now, voluntarily.

If we imagine being at a cocktail party, with several dozens of people, all engaged in conversations, then the auditory stimuli reaching our ears do not make sense all together: there is a large chance that we are, in fact, hearing more than one coherent stream of speech, because our auditory cortex integrates both the conversation we are engaged in, but also others around.

In the case of the cocktail party, we can choose to pay attention to only one conversation at a time, and actually not have any clear perception of what is being said behind us. All that is perceived is noise in the background, or the experience of noisiness, with the vague knowledge that there exists another stream of speech in another location. We can choose to attend to the one we were attending before, or shift our attention to another. Either task requires voluntary inhibition of irrelevant stimuli [START_REF] Cherry | Some Experiments on the Recognition of Speech, with One and with Two Ears[END_REF].

If we imagine being at the wheel inside a car, the multiplicity of visual inputs to the retina is huge: we have to discriminate fast changing objects in a moving environment, some of them having motion of their own. If a deer suddenly crosses the road, our attention will be caught by it and we may even respond before we have really understood what has happened. A very different scenario can occur if, distracted by a conversation on our mobile phone -even when using a hands-free kit, we unfortunately fail to notice a no-entry sign and find ourselves facing another car in what was, in fact, a one-way street in the opposite direction. We just can't believe it: we just haven't seen the sign.

With these examples, we raised intuitive questions on attentional control, conscious experience, and their interrelation. The deer crossing the road pops-out, it is a case of exogenous attention: driven by an external event, and involuntary. The inhibitory control we exert on noise to keep it from misunderstanding other inputs we are attending to is endogenous attention: it is goal-driven, and fully voluntary. Last, making the wrong turn in a one-way street when the sign was within our sight is a sign that when we don't pay attention, sometimes we have no conscious experience of things that are however processed by our sensory system. The following subsections will detail some of the features of these phenomena, in the context of visual stimuli.

I.3.3 Differentiating attention and consciousness

I.3.3.a

Exogenous and endogenous visual spatial attention Attention is classically split between two related phenomena: one that is reflexive automatic and stimulus driven, whereas the other is voluntary and goal-oriented [START_REF] James | The principles of psychology[END_REF]. Respectively, these are called exogenous attention, and endogenous attention.

Exogenous attention is the term used when a stimulus attracts resources to its location in space without any voluntary input [START_REF] Carrasco | Covert attention effects on spatial resolution[END_REF][START_REF] Yantis | Abrupt visual onsets and selective attention : Voluntary versus automatic allocation[END_REF]. It is when some element "pops-out" and the brain automatically reacts to it. This type of stimulus thus reaches conscious awareness immediately. It is thought to emerge in a bottom-up fashion, so that events occurring low in the hierarchy drive the phenomenon, however it does not always lead to a conscious experience (C. [START_REF] Feng | Invisible images can influence saccadic eye movements[END_REF][START_REF] Jiang | A gender-and sexual orientation-dependent spatial attentional effect of invisible images[END_REF]. It is short-lived, with a larger effect around 100-120ms after stimulus onset, whereas endogenous attention takes about 300ms [START_REF] Cheal | Does attention have different effects on line orientation and line arrangement discrimination[END_REF][START_REF] Hein | Visual attention and temporal discrimination : Differential effects of automatic and voluntary cueing[END_REF][START_REF] Ling | Sustained and transient covert attention enhance the signal via different contrast response functions[END_REF][START_REF] Posner | Attention and the detection of signals[END_REF][START_REF] Yantis | Abrupt visual onsets and selective attention : Voluntary versus automatic allocation[END_REF].

On the other hand, endogenous attention is voluntary and goal-oriented: it is deployed when one is trying to perform a specific task, and is thought to depend on top-down mechanisms regulating expectations or knowledge for example (de Fockert, [START_REF] De Fockert | The role of working memory in visual selective attention[END_REF][START_REF] Egeth | Visual attention : Control, representation, and time course[END_REF][START_REF] Posner | Attention and the detection of signals[END_REF][START_REF] Yantis | Abrupt visual onsets and selective attention : Voluntary versus automatic allocation[END_REF]. The putative neural mechanisms underlying attention in general are partially shared between endogenous and exogenous attention [START_REF] Corbetta | Control of goal-directed and stimulus-driven attention in the brain[END_REF][START_REF] Wright | Orienting of attention[END_REF]. This short section on spatial attention allows us to make an important distinction in the field of cognitive science: bottom-up, and top-down mechanisms. They rely on the notion that the brain is organized along a hierarchy, with low-levels taking care of elementary functions relating to sensory processing, and higher-levels dealing with more complex or abstract functions, such as memory, attentional control, or awareness. Bottom-up and top-down attention deeply affect other levels of processing: pop-out distractor stimuli impair performance in behavioral tasks because they affect how attentional resources are used [START_REF] Lavie | Attention, distraction, and cognitive control under load[END_REF][START_REF] Lavie | Perceptual load as a major determinant of the locus of selection in visual attention[END_REF], and deploying visual spatial attention in a location in space will affect processing of stimuli down to V1: neurons will respond more specifically to the attended stimulus, rather than the ones surrounding it [START_REF] Desimone | Neural mechanisms of selective visual attention[END_REF][START_REF] Luck | Neural mechanisms of spatial selective attention in areas V1, V2, and V4 of macaque visual cortex[END_REF][START_REF] Moran | Selective attention gates visual processing in the extrastriate cortex[END_REF].

I.3.3.b

Cross-modal exogenous attention Attention can be deployed unimodally, so that a visual event happening in one location in space will attract spatial attention and facilitate processing of a subsequent visual event occurring there [START_REF] Posner | Attention and the detection of signals[END_REF]. We have seen that two relatively co-occurring stimuli in space and time in two different modalities can be processed in light of one another, through what is called multisensory integration, but attention can also be deployed multimodally: it is generally thought to occur for longer latencies than multisensory integration (50 -300ms as opposed to within 100ms) [START_REF] Bolognini | Acoustical vision" of below threshold stimuli : Interaction among spatially converging audiovisual inputs[END_REF][START_REF] Müller | Reflexive and voluntary orienting of visual attention : Time course of activation and resistance to interruption[END_REF], including an overlap with multisensory integration, so that both effects can be additive.

Exogenous cross-modal orienting of spatial attention has been shown to reduce reaction time to target stimuli and increase d' detection sensitivity [START_REF] Mcdonald | Involuntary orienting to sound improves visual perception[END_REF], which is an objective measurement of detection performance [START_REF] Green | Signal detection theory and psychophysics[END_REF], and affect judgement of time order between stimuli [START_REF] Spence | Capturing spatial attention with multisensory cues : A review[END_REF].

It should be noted however, that exogenous attention, under some specific cases where attention is endogenously and wholly deployed elsewhere, seems to be less effective, or not effective at all [START_REF] Lavie | Perceptual load as a necessary condition for selective attention[END_REF][START_REF] Santangelo | The suppression of reflexive visual and auditory orienting when attention is otherwise engaged[END_REF], which moderates our earlier claim that it is fully automatic. Attentional needs in one modality can also negatively affect sensitivity to stimuli in others (Boot and Simons 2005, Cosman 2009, Alais and Burr 2006[START_REF] De Fockert | The role of working memory in visual selective attention[END_REF]. These accounts support the hypothesis according to which the attentional network is amodal and globally shared among sensory modalities. Importantly, cueing in a different modality from the target stimulus' is often less effective than cueing attention within the same modality [START_REF] Koelewijn | Auditory and visual capture during focused visual attention[END_REF][START_REF] Spence | A new approach to the design of multimodal warning signals[END_REF], although this depends on the task [START_REF] Spence | Capturing spatial attention with multisensory cues : A review[END_REF].

I.3.3.c

Attention and consciousness We have seen with the one-way street example one instance where a person is not consciously aware of an event even though it is accessible through their senses. In this case, there is a body of evidence pointing at the notion that sensory processing did occur, but that nevertheless the person wasn't conscious of its result as seen in section I.3.1. This can occur when stimuli are very difficult to perceive, but also when they are very accessible, like the oneway sign, but are not the object of attention (O'Regan, [START_REF] O'regan | Change-blindness as a result of 'mudsplashes[END_REF][START_REF] Simons | Gorillas in our midst : Sustained inattentional blindness for dynamic events[END_REF]. This type of reflection of the relationship between conscious access and attentional control was the subject of many experiments in attentional cueing, where true or false information about a target stimulus is provided before its appearance, such as a possible location on-screen [START_REF] Posner | Components of visual orienting[END_REF][START_REF] Posner | Attention and the detection of signals[END_REF], or, simply, experiments testing for conscious access of a stimulus when all attentional resources are directed elsewhere [START_REF] Bahrami | Unconscious orientation processing depends on perceptual load[END_REF][START_REF] Bahrami | Attentional load modulates responses of human primary visual cortex to invisible stimuli[END_REF][START_REF] Lavie | Perceptual load as a major determinant of the locus of selection in visual attention[END_REF]. Because attention and consciousness co-occur so often, it was sometimes argued that they were one and the same phenomenon [START_REF] James | The principles of psychology[END_REF][START_REF] Merikle | Parallels between perception without attention and perception without awareness[END_REF][START_REF] O'regan | A sensorimotor account of vision and visual consciousness[END_REF][START_REF] Posner | Attention : The mechanisms of consciousness[END_REF], but others insisted that their separation was not just a matter of semantics [START_REF] Baars | Global workspace theory of consciousness : Toward a cognitive neuroscience of human experience[END_REF][START_REF] Block | Two neural correlates of consciousness[END_REF][START_REF] Dehaene | Conscious, preconscious, and subliminal processing : A testable taxonomy[END_REF][START_REF] Iwasaki | Spatial attention and two modes of visual consciousness[END_REF][START_REF] Kentridge | Spatial attention speeds discrimination without awareness in blindsight[END_REF][START_REF] Koch | Attention and consciousness : Two distinct brain processes[END_REF][START_REF] Naccache | Unconscious masked priming depends on temporal attention[END_REF][START_REF] Woodman | Serial deployment of attention during visual search[END_REF]: they were truly different, and as such, dissociable.

Many studies point to the importance of attentional input for conscious experience: patients affected with lesions of the fronto-parietal cortex can display attentional impairments in some regions of space. Most of the time, these impairments concern one hemi-field of the visual space. These patients, although they might have a working visual system and be able to experience visual stimuli in one half on the world, do not experience stimuli in the affected region in space. They are thought to have a specific attentional deficit, which leads them to neglect perceptual events originating from regions of space contralateral to their lesion: this phenomenon is called neglect or hemi-neglect [START_REF] Kooistra | Hemispatial visual inattention masquerading as hemianopia[END_REF][START_REF] Walker | Disentangling neglect and hemianopia[END_REF][START_REF] Ward | Grouping reduces visual extinction : Neuropsychological evidence for weight-linkage in visual selection[END_REF]. In experimental settings, we have reproduced phenomena that resemble a form of neglect in healthy volunteers, as in the case of inattentional blindness [START_REF] Mack | Inattentional blindness[END_REF][START_REF] Most | What's "inattentional" about inattentional blindness?[END_REF][START_REF] Simons | Gorillas in our midst : Sustained inattentional blindness for dynamic events[END_REF]: having participants focus all of their attentional resources on one task led to a complete inability to report even the most unusual events when they were unrelated (such as a gorilla walking by). Some experiments have evidenced that daydreaming leads to unconsciousness of external events, in report and cerebral activity (Bekinschtein et al., 2009a;[START_REF] Marti | How does the extraction of local and global auditory regularities vary with context?[END_REF], others have shown that participants can become blind to visual changes if these occur very slowly [START_REF] Simons | Change blindness in the absence of a visual disruption[END_REF], or if these visual changes are introduced at the same time as a much more compelling stimulus, which wholly attracts attention [START_REF] O'regan | Change-blindness as a result of 'mudsplashes[END_REF][START_REF] Rensink | Change detection[END_REF][START_REF] Simons | Gorillas in our midst : Sustained inattentional blindness for dynamic events[END_REF]. These experiments highlight the importance of attentional involvement for consciousness. It seems, from there, that they are either one and the same, or that attention might be necessary for consciousness.

However, further investigation revealed this was not as simple: when presenting participants with short-lived visual stimuli, such as a word, which was followed by a mask (e.g. the word "VIDEO" followed by "#####"), and asking them to pay full attention to the word, they were unable to report what it was [START_REF] Naccache | Unconscious masked priming depends on temporal attention[END_REF]. Conversely, studies using salient subliminal (unconsciously perceived) stimuli, such as images of nude people, show that these attract attention in the absence of consciousness (C. [START_REF] Feng | Invisible images can influence saccadic eye movements[END_REF][START_REF] Jiang | A gender-and sexual orientation-dependent spatial attentional effect of invisible images[END_REF]. These studies revealed that depending on the stimuli presented, attention was not always sufficient to elicit conscious awareness, but is it necessary? Some of the studies reviewed until now show that in the absence of attention, there is no conscious access [START_REF] Mack | Inattentional blindness[END_REF][START_REF] Rensink | To see or not to see : The need for attention to perceive changes in scenes[END_REF], but others argue that consciousness can indeed take place in the absence of attention. Arguments in favor of this claim rely on experimental tasks during which participants cannot deploy endogenous attention towards the stimulus-of-interest. In inattentional blindness experiments, although participants cannot report single changes in the scene, they have experience of its gist, or general properties [START_REF] Mack | Inattentional blindness[END_REF]. In a different paradigm, participants were strongly engaged in a demanding task and presented at the same time with irrelevant peripheral stimuli. They were supposed to deploy almost all their attention to the primary task, and still they were able to report some of the irrelevant stimuli [START_REF] Reddy | Face identification in the near-absence of focal attention[END_REF]. Finally, when participants were presented with complex images of natural scenes for 53ms followed by a blank, a delay too short for the involvement of endogenous attention, they remained able to report the gist of the images presented, suggesting some experience of the stimulus [START_REF] Li | Rapid natural scene categorization in the near absence of attention[END_REF]. These results were thus reinterpreted as evidence for phenomenal awareness in the absence of attention [START_REF] Koch | Attention and consciousness : Two distinct brain processes[END_REF]: participants have experience of the visual stimulus, but cannot remember or report everything there was to say about it. It was argued there is a double-dissociation between attention and awareness: they are fully independent processes, so that any combination of presence and absence of each is possible. Other experiments however provide evidence to the contrary, and the issue is not definitely settled (M. A. [START_REF] Cohen | The attentional requirements of consciousness[END_REF].

I.3.4 Where and when to look for consciousness

I.3.4.a

Attention as a gating mechanism: early or late occurring consciousness? Regardless of whether attention is sufficient, the question at hand is whether it is necessary, for consciousness to emerge. There are two distinct possibilities here:

1) Attention acts are a gatekeeper for consciousness.

2) Attention does not gate experience, but the reportability of experience.

The first case goes as follows: if attention is not deployed, there cannot be experience of a stimulus [START_REF] Dehaene | Conscious, preconscious, and subliminal processing : A testable taxonomy[END_REF]. When attention is deployed, other factors come into play to determine whether the percept will in fact become conscious (such as stimulus strength) and be reported (Figure I-5a). In the second case, attention is thought to gate the mechanisms that allow for report, but not experience. What this means is that a person can experience something but be wholly unable to report it. In this second account, things schematically go as follows:

experience occurs according to its own terms, and attention allows for report. Experience The two accounts lead to two very different putative mechanisms in terms of cerebral processing: the first account requires the input of attention, thus the occurrence of fairly lateonset phenomena [START_REF] Baars | Global workspace theory of consciousness : Toward a cognitive neuroscience of human experience[END_REF][START_REF] Dehaene | Conscious, preconscious, and subliminal processing : A testable taxonomy[END_REF]Dehaene & Naccache, 2001), whereas in the second, consciousness is dependent of processes occurring pre-attentively, approximately within the first 150ms after the onset of the stimulus [START_REF] Lamme | Why visual attention and awareness are different[END_REF][START_REF] Lamme | Separate neural definitions of visual consciousness and visual attention; a case for phenomenal awareness[END_REF]Lamme & Roelfsema, 2000).

A second important distinction is that, if attention gates experience, there are no nonreportable experiences. In the case of the second account, however, phenomenal consciousness overflows access: there are more experiences than can be reported thanks to attentional orienting [START_REF] Block | Consciousness, accessibility, and the mesh between psychology and neuroscience[END_REF][START_REF] Koch | Attention and consciousness : Two distinct brain processes[END_REF]. The predictions made by these different models will be tested within the experimental chapters of this manuscript. As such, we will review them in some detail. 2001). In either case, it is the sustained and strong exchange of information between coalitions of neurons that supports conscious access, the difference is which, and where.

The full extent of biological theories of consciousness will not be covered here, we will rather provide a brief overview of what has been proposed, and follow-up with a more in-depth description of two competing models, which help inform the work described in the following chapters (see A. Seth, 2007 for a review). Although we will focus on theories that consider consciousness to be supported by a distributed process (how much is however debated), there are instances of models that consider that consciousness arises from localized processes. One of them considers, for example, that each cortical module is, in itself, micro-conscious [START_REF] Zeki | The disunity of consciousness[END_REF], so that the activity of any given module can be construed as consciousness of whatever it codes for.

Cerebral activity has been shown to display rhythmic activity in the form of oscillations, which have been related to a variety of cognitive functions [START_REF] Buzsáki | Neuronal oscillations in cortical networks[END_REF]). These range from very slow (<1Hz) to quite fast (>100 Hz), and are thought to reflect respectively more global or local trends in processing [START_REF] Bauer | Attentional modulation of alpha/beta and gamma oscillations reflect functionally distinct processes[END_REF][START_REF] Buzsáki | The origin of extracellular fields and currents -EEG, ECoG, LFP and spikes[END_REF][START_REF] Varela | The brainweb : Phase synchronization and large-scale integration[END_REF]) that are interdependent. Empirical findings as well as computational models have hypothesized that neurons will tend to integrate information more efficiently when they fire action potentials in a synchronized manner. For this reason, mid-range to high oscillations have been thought to reflect locally optimal integration of information. For example, gamma oscillations (30-80 Hz) have been hypothesized to reflect local and coherent exchanges of information in the cortex, thus efficient processing [START_REF] Fries | Modulation of oscillatory neuronal synchronization by selective visual attention[END_REF][START_REF] Singer | Visual feature integration and the temporal correlation hypothesis[END_REF]. More recently, these were thought to be a neural correlate of conscious awareness (F. C. [START_REF] Crick | What are the neuronal correlates of consciousness[END_REF]. This view was met with some amount of criticism [START_REF] Shadlen | Synchrony unbound : A critical evaluation of the temporal binding hypothesis[END_REF], it is now more commonly assumed that oscillatory synchronization is a marker of efficient processing, rather than a causal mechanism in itself. Slower oscillations have been hypothesized to reflect global trends across larger regions of the cerebral cortex, and related to top-down processes in general, such as attentional control, although it is also known to play a role in perception [START_REF] Dugué | The phase of ongoing oscillations mediates the causal relation between brain excitation and visual perception[END_REF][START_REF] Jensen | Shaping functional architecture by oscillatory alpha activity : Gating by inhibition[END_REF][START_REF] Sadaghiani | Alpha-band phase synchrony is related to activity in the frontoparietal adaptive control network[END_REF].

Distributed theories relying on recurrent processes involve efficient communication between different areas or modules as well, but not necessarily in the form of oscillations. There are currently three models of this kind. Generally speaking, these suppose that the interaction between feedforward and feedback exchanges of information sustained in time allows for the maintenance of information in the brain, as well as in-depth processing. The first of these theories, Information Integration Theory (IIT) considers that this sustained and recurrent information processing across one or several regions in the cortex forms a dynamic core. The amount of information exchanged and maintained in time in this core can be correlated with levels of consciousness (A. K. [START_REF] Seth | Theories and measures of consciousness : An extended framework[END_REF][START_REF] Tononi | A measure for brain complexity : Relating functional segregation and integration in the nervous system[END_REF]. A mathematical formula was proposed to quantify the amount of information integrated in time, in the hope to provide an objective numerical measure of consciousness [START_REF] Tononi | An information integration theory of consciousness[END_REF]. The other two models, because they are highly relevant to the experimental work presented here, will benefit from individual sections.

I.3.5 Recurrent processes

In section I.2, we saw that recurrent processes arise when higher levels in the cortical hierarchy start sending back information downwards [START_REF] Felleman | Distributed hierarchical processing in the primate cerebral cortex[END_REF]. Some have hypothesized in general, that because receptive fields in the visual system become larger as we go up in the hierarchy, higher levels need to feed information backwards to match what they have likely recognized with the more detailed information held in lower levels, especially in terms of location [START_REF] Di Lollo | Attention is a sterile concept; iterative reentry is a fertile substitute[END_REF]. For awareness, we now know they are thought to be a crucial element to the emergence of conscious experience in the two models we are most interested in [START_REF] Baars | Global workspace theory of consciousness : Toward a cognitive neuroscience of human experience[END_REF]Dehaene & Naccache, 2001;Lamme & Roelfsema, 2000). Recurrent processes were evidenced thanks to series of experiments using electrophysiology, which showed that neurons low in the hierarchy displayed strong modulations in their preferred stimulus and receptive fields some amount of time after they first respond to a stimulus, i.e. after 100ms poststimulus, which is commonly thought to be the window for the feedforward sweep [START_REF] Nowak | The timing of information transfer in the visual system[END_REF][START_REF] Rodman | Afferent basis of visual response properties in area MT of the macaque. II. Effects of superior colliculus removal[END_REF][START_REF] Schmolesky | Signal timing across the macaque visual system[END_REF]. These phenomena were then related to conscious experience when further evidence showed that low-energy stimuli (e.g. short-lived, or of weak intensity) did not elicit recurrent activity and were also unconscious, and that a disruption in the establishment of recurrent loops was potentially the mechanism behind visual masking: when two similar stimuli are presented in rapid succession, the first is often not reported [START_REF] Luck | Word meanings can be accessed but not reported during the attentional blink[END_REF][START_REF] Sergent | Timing of the brain events underlying access to consciousness during the attentional blink[END_REF], potentially because when high levels in the hierarchy send information back about stimulus one, the information held in the lower levels concerns stimulus two, and the mismatch prevents recurrent processes from taking place (Lamme & Roelfsema, 2000), as illustrated in Figure I-6. Last, but not least, a provoked disruption in area MT (or V5) of the cortex using

I.3.6 An early and local theory of consciousness: the Recurrent

Hypothesis

The Recurrent Hypothesis (F. [START_REF] Crick | A framework for consciousness[END_REF]Lamme & Roelfsema, 2000) posits that it is the establishment of recurrent feedback loops within the sensory cortex that are responsible (or correlated with) the emergence of conscious experience. The theory suggests, based on the work presented in section I.2, that sensory processing is divided in two steps: the feedforward sweep going up the hierarchy, and recurrent processes, which corresponds to the establishment of communication between high and low levels of stimulus processing, as well as horizontal connections between same-level modules or within modules (Lamme & Roelfsema, 2000).

What correlates with consciousness in this case is the second step: the recurrent processes [START_REF] Ffytche | The primary visual cortex, and feedback to it, are not necessary for conscious vision[END_REF][START_REF] Lamme | Masking Interrupts Figure-Ground Signals in V1[END_REF]. The first stage of processing, feedforward activation, does not lead to any conscious perception (Silvanto, Lavie, et al., 2005). What matters is that information is locally shared and sustained among the different levels in the hierarchy. In this case, conscious experience is fully supported by feedforward and feedback loops between local (sensory-specific) levels in the cortical hierarchy. This model works under the assumption that experience is pre-attentive. As such, reportability is not guaranteed when consciousness arises locally. A third step is required for reportability to occur, and it is the global sharing of the information, either in modules dealing with memory formation, or motor expression for report (Lamme, 2003, p. 2). Global sharing of the information is gated by attention, but mediates reportability rather than experience (F. [START_REF] Crick | A framework for consciousness[END_REF][START_REF] Lamme | Why visual attention and awareness are different[END_REF][START_REF] Lamme | Separate neural definitions of visual consciousness and visual attention; a case for phenomenal awareness[END_REF]Lamme & Roelfsema, 2000).

I.3.7 A late and global theory of consciousness: the Global Workspace

The Global Workspace Theory of consciousness [START_REF] Baars | Global workspace theory of consciousness : Toward a cognitive neuroscience of human experience[END_REF][START_REF] Dehaene | Conscious, preconscious, and subliminal processing : A testable taxonomy[END_REF][START_REF] Dehaene | Imaging unconscious semantic priming[END_REF]Dehaene & Naccache, 2001;[START_REF] Sergent | Imaging neural signatures of consciousness:'What','When','Where'and 'How'does it work?[END_REF]) posits that it is the involvement of a large, fronto-parietal, and distributed network of long-range neurons that is responsible for conscious access. Information sharing within the sensory cortices is crucial, but not sufficient [START_REF] Baars | Global workspace theory of consciousness : Toward a cognitive neuroscience of human experience[END_REF][START_REF] Dehaene | Conscious, preconscious, and subliminal processing : A testable taxonomy[END_REF][START_REF] Dehaene | Imaging unconscious semantic priming[END_REF]Dehaene & Naccache, 2001). In this theory, consciousness is construed as fame in the brain [START_REF] Dennett | Consciousness explained[END_REF], in the sense that different modules compete for the distributed global fronto-parietal workspace mentioned above. This workspace is responsible for the maintenance of the "famous" information within sensory system, as well as its export to other functional regions for further processing (including report).

In this case, consciousness emerges from the global ignition and sustainment in time of both sensory areas benefitting from fame, as well as the fronto-parietal network dedicated to information sharing. The Global Workspace theory of consciousness is post-attentive: access to the Workspace is mediated by attention.

The Recurrent Theory and the Global Workspace Theory appeal to a similar machinery, but they do not hypothesize they have the same function. In the Recurrent Hypothesis, simple feedforward activation is unconscious, while local recurrence suffices for experience.

Broadcasting through the fronto-parietal network allows for memorization and report.

Proponents of this proposition argue that it explains gist-perception, which would correspond to experience through local recurrence in the sensory cortex, without memorization and global sharing of its individual features [START_REF] Li | Rapid natural scene categorization in the near absence of attention[END_REF][START_REF] Reddy | Face identification in the near-absence of focal attention[END_REF]. The Global Workspace Theory also considers simple feedforward activation as unconscious, but argues that local recurrence is not sufficient for consciousness: rather, it would correspond to a "preconscious state" (Figure I-7 from [START_REF] Dehaene | Conscious, preconscious, and subliminal processing : A testable taxonomy[END_REF]. Consciousness itself only occurs when information is broadcasted through the Workspace [START_REF] Dehaene | Conscious, preconscious, and subliminal processing : A testable taxonomy[END_REF].

I.3.8 Finding neural correlates of consciousness

I.3.8.a

The neural dynamics of conscious access The theories described above all come from the careful examination of empirical data. Some of it has already been mentioned, however a more thorough description of these findings is necessary to understand how theories make different predictions about cerebral activity, and what currently remains indeterminate. Several experiments conducted in fMRI show that when presented with subliminal stimuli, participants display low, but relevant activity in expected sensory areas. When the same stimuli are visible and reported, it was found that they displayed stronger activity in the same places, as well as a myriad of supplementary regions spanning the parietal and frontal cortices (Carmel, Lavie, & Rees, 2006, p. 20;Dehaene & Naccache, 2001;[START_REF] Rees | Neural correlates of consciousness in humans[END_REF]. Many experiments have shown that the dynamics of the neural response to different types of stimuli, seen or unseen, differs across the cortical hierarchy and in supra-modal areas. Early studies using masking have shown that while very early-responding areas display the same activation regardless of consciousness, however higher-level areas have been shown to display activity directly related to stimulus strength during the feedforward sweep [START_REF] Dehaene | Cerebral mechanisms of word masking and unconscious repetition priming[END_REF][START_REF] Leopold | Activity changes in early visual cortex reflect monkeys' percepts during binocular rivalry[END_REF] at around 100ms after stimulus onset (P1/N1 wave). Later-onset activity however, at around 300ms (P3) appeared bimodally distributed, depending on whether participants were conscious of the stimulus or not [START_REF] Dehaene | A neuronal network model linking subjective reports and objective physiological data during conscious perception[END_REF][START_REF] Del Cul | Brain dynamics underlying the nonlinear threshold for access to consciousness[END_REF][START_REF] Kouider | A neural marker of perceptual consciousness in infants[END_REF][START_REF] Sergent | Timing of the brain events underlying access to consciousness during the attentional blink[END_REF][START_REF] Sergent | Is consciousness a gradual phenomenon? Evidence for an all-or-none bifurcation during the attentional blink[END_REF]), likely at the moment where higher level areas get involved in the processing of the stimulus (Figure I-8). These results suggested the selective involvement of a fronto-parietal network when stimuli become conscious, leading to a qualitatively different brain state within sensory areas [START_REF] Dehaene | Conscious, preconscious, and subliminal processing : A testable taxonomy[END_REF], in support of the Global Workspace Theory.

In response, it was argued that the activity elicited in amodal areas was not a correlate of conscious experience per se, but wholly related to the behavioral task: when participants could report a stimulus was present, they performed a motor act which elicited activity in areas outside the sensory cortex involved in the task. Whether participant have to report anything at all when they are not aware of the presented stimulus varies from one experiment to the next. However, they argued, theories like the Recurrent Hypothesis claim that experience can occur in cases when report is not possible. As such, what was evidenced in the studies mentioned above is not a correlate of conscious access, but a correlate of various cognitive processes such as attention, memory, or expectation [START_REF] Aru | Distilling the neural correlates of consciousness[END_REF]Koch & I.3.8.b No-report paradigms No report paradigms are experimental designs in which no explicit report of a participant's subjective experience is recorded, to avoid overestimating the neural correlates of consciousness by confounding them with the correlates of the reporting act. They can rely on strict behavioral techniques (optokinetic nystagmus: Frässle, Sommer, Jansen, Naber, & Einhäuser, 2014, or binocular rivalry: Maier, Panagiotaropoulos, [START_REF] Maier | Introduction to research topic-binocular rivalry : A gateway to studying consciousness[END_REF][START_REF] Tong | Neural bases of binocular rivalry[END_REF], or on machine-learning algorithms trained on neuroimaging data for decoding conscious perception in a "yes-report" condition, then generalized in a no-report condition [START_REF] Haynes | Decoding visual consciousness from human brain signals[END_REF][START_REF] Horikawa | Neural decoding of visual imagery during sleep[END_REF], which can be particularly useful when trying to assess whether non-communicating patients (e.g. postcomatose) consciously access external stimuli [START_REF] Monti | Willful modulation of brain activity in disorders of consciousness[END_REF].

It is important to note, however, that depending on the method chosen they can themselves suffer from different, yet very problematic confounds: behavioral techniques may underestimate the neural correlates of conscious perception (NCC) because performing a task on the relevant stimuli may drastically alter processing (e.g. categorization of A vs. B vs. passive listening). Likewise, introducing a large delay before asking for a report could introduce a memory bias. Conversely, machine learning algorithms are more efficient when trained on well-defined categories. In the absence of a report, we cannot inform the algorithm as to whether any given trial was conscious. As such, they may misconstrue unconconsious activity for a correlate of consciousness [START_REF] Tsuchiya | No-report paradigms : Extracting the true neural correlates of consciousness[END_REF]. We use and compare yes-report and no-report conditions in Chapter 4.

I.3.8.c

Disorders of Consciousness Looking for the NCC is not simply a matter of fundamental research: finding reliable signatures of conscious experience can be directly applied in clinical settings, in order to better assess patients who cannot provide explicit reports, such as patients suffering from disorders of consciousness [START_REF] Bekinschtein | Neural signature of the conscious processing of auditory regularities[END_REF][START_REF] Giacino | Disorders of consciousness after acquired brain injury : The state of the science[END_REF][START_REF] King | Information sharing in the brain indexes consciousness in noncommunicative patients[END_REF][START_REF] Monti | Willful modulation of brain activity in disorders of consciousness[END_REF][START_REF] Sergent | Multidimensional cognitive evaluation of patients with disorders of consciousness using EEG : A proof of concept study[END_REF].

While many patients recover some communicative abilities after a coma, some evolve in more ambiguous ways, in which they retain most of their physiological functions working, but appear as though they are awake yet unaware of their surroundings. These patients in "vegetative state" are non-communicative [START_REF] Jennett | Persistent vegetative state after brain damage : A syndrome in search of a name[END_REF], and as such it is difficult to assess how many cognitive abilities they still have. Currently, the diagnosis is mostly clinical, and evaluated in post-critical care using the 23-point Coma Recovery Scale (CRS-R) [START_REF] Giacino | The JFK Coma Recovery Scale-Revised : Measurement characteristics and diagnostic utility[END_REF]. Patients are considered to be in a vegetative state when they cannot follow instructions, communicate, verbally or otherwise, or demonstrate coherent responses to events happening in their environment (adaptive response to painful stimuli, visual fixation, emotional reaction to known faces, etc.).

Another disorder of consciousness is the minimally conscious state (MCS) [START_REF] Giacino | The minimally conscious state : Definition and diagnostic criteria[END_REF], which corresponds to patients being able to do at least one of the main activities in the CRS-R. MCS patients, because they can react appropriately to their environment, are considered to have some awareness of their surroundings, and as such, to be conscious, although potentially less than healthy people. It is important at this point to consider that until now, we have discussed transitive consciousness, or the ability to experience things. For these patients, the questions of consciousness shifts to the first part of the definition of consciousness from section I.1: "Being awake, as opposed to asleep -with the special case of dreams, or comatose states", which now relates to levels of consciousness [START_REF] Laureys | The neural correlate of (un) awareness : Lessons from the vegetative state[END_REF]. Of course, the two are interrelated: patients who can have transitive experiences have high levels of consciousness, just like the dead don't experience at all. The difficulty lies in the intermediate cases, where patients behaviors' are difficult to diagnose: is he/she conscious of anything [START_REF] Naccache | Is She Conscious?[END_REF]?

A clinical scale provides much information but remains dependent on many biases: patients' behavioral responses may be present but undetected due to their low intensity or the fact they can occur when no one is present to witness them. Similarly, health professionals may be more or less sensitive to patients' responses depending on their own expectations, and their demeanor may enhance or inhibit patients' behavioral responses. This can lead to poor test-retest performance of the scale, and overall low reliability. The CRS-R is quite robust to re-evaluation by the same or a different observer [START_REF] Giacino | The JFK Coma Recovery Scale-Revised : Measurement characteristics and diagnostic utility[END_REF], however, an objective measure derived from neuroimaging could be an important complementary tool [START_REF] Barttfeld | Signature of consciousness in the dynamics of resting-state brain activity[END_REF].

For this, no-report paradigms and informed theories on consciousness with testable predictions are of crucial importance. This will be a focus of Chapter 4.

I.3.9 Summary

The brain's response to sensory stimulation is called bottom-up processing. However, due to the multiplicity of stimuli arriving at our senses at all times, and the variability in quality of the information processed, we are not aware of everything going on at all times. Attention is a broad concept used to describe the processes that help us sift through this information and select relevant stimuli. Exogenous attention is mostly automatic, and works its way from the bottom-up: particularly salient or unique stimuli reorient attention rapidly, within 150ms.

Endogenous attention, however, is goal-oriented, and relies on processes we consider to be conscious. It takes about 200-300ms to be deployed [START_REF] Cheal | Does attention have different effects on line orientation and line arrangement discrimination[END_REF][START_REF] Hein | Visual attention and temporal discrimination : Differential effects of automatic and voluntary cueing[END_REF][START_REF] Ling | Sustained and transient covert attention enhance the signal via different contrast response functions[END_REF][START_REF] Posner | Attention and the detection of signals[END_REF][START_REF] Yantis | Abrupt visual onsets and selective attention : Voluntary versus automatic allocation[END_REF]. The cerebral mechanisms behind these two phenomena are thought to be partially shared. Whether attention is necessary for conscious access is currently still debated, and leads to different theoretical frameworks

with different yet testable predictions on behavior. Neural theories of consciousness are varied, and can consider consciousness to be the result of processes occurring in a specific location, or to a certain type of cerebral processing. Among the latter theories, which are more of interest for the work of the current thesis, two are of particular interest: the Recurrent Hypothesis posits that attention is not necessary for conscious access and that not all experiences can be reported.

Its cerebral mechanisms are thought to reside in the local entrainment of recurrent loops of information sharing. Global changes in brain activity elicited when participants become conscious are, in this case, the neural correlate of the processes leading up to a perceptual report and the NCC lie within local activity. The Global Workspace Theory supposes that attention is necessary for conscious access, and that there are no non-reportable experiences. In this case, recurrent processes are still necessary for consciousness, but the actual correlate of the phenomenon lies in the global activity evidenced over a fronto-parietal network of long-range neurons. Both theories make different predictions on whether the neural response to a stimulus is a graded or bimodal. Because testing the involvement of this global network may involve a confound when asking for a report, several experts are in favor of no-report paradigms, which are also of great help when it comes to developing diagnostic and prognostic tools for patients suffering from disorders of consciousness.

I.4 Present work

The question of whether consciousness is the result of an "early and local" or "late and global" phenomenon, as we call them in the preceding section, is currently still debated. One testable set of contradictory predictions between early and late models (pre-attentive and postattentive respectively) concerns the timing of perceptual awareness [START_REF] Lamme | Why visual attention and awareness are different[END_REF]: if consciousness arises before the input of attentional processes which help for memory and report, then it must occur within the first 100 -150ms (Lamme & Roelfsema, 2000). Later models of consciousness however, allow for access to occur at later times [START_REF] Dehaene | Conscious, preconscious, and subliminal processing : A testable taxonomy[END_REF][START_REF] Rees | Neural correlates of consciousness in humans[END_REF]: actually, they even predict that as long as the information is still present in the sensory cortex, it can be promoted to consciousness if it is broadcasted in the global workspace [START_REF] Sergent | The offline stream of conscious representations[END_REF]. This original prediction can be tested using post-cueing experiments, where attentional control is modulated after the stimulus has disappeared, and long after the 150ms time window [START_REF] Sergent | Cueing attention after the stimulus is gone can retrospectively trigger conscious perception[END_REF][START_REF] Thibault | Retrospective attention gates discrete conscious access to past sensory stimuli[END_REF].

Classically, post-cueing paradigms are supposed to study iconic memory, a subtype of sensory memory specific to vision, in which we examine how information about well-seen stimuli decays over time [START_REF] Sperling | The information available in brief visual presentations[END_REF]. A classical experiment among these was conducted with letter-arrays: when briefly presented with a 3 by 4 letter array, participants were spontaneously able to report as many as three or four from all items, regardless of row or column. However, it was shown that attentional cueing of a specific row after the disappearance of the letter array allowed participants to recall three or four letters from that specific row [START_REF] Sperling | The information available in brief visual presentations[END_REF]. Since these seminal experiments, many replications have been performed [START_REF] Landman | Set size effects in the macaque striate cortex[END_REF][START_REF] Sligte | Are there multiple visual short-term memory stores?[END_REF]. For our purposes, and within the context of early-versus-late debates on the emergence of conscious experience, a finding like this one is crucial: did the post-cue change something to experience, or did it change the participant's ability for report? Indeed, there are in this case two competing interpretations that cannot be disentangled within the classical iconic memory setting: on the one hand, we could argue that the entire array was consciously perceived, and led to experience, but that the number of items, because it exceeded working memory capacity (4-7 items in general [START_REF] Cowan | The magical number 4 in short-term memory : A reconsideration of mental storage capacity[END_REF][START_REF] Miller | The magical number seven, plus or minus two : Some limits on our capacity for processing information[END_REF], when the experiment presented 12 letters), was too high for all of them to be remembered and reported [START_REF] Sligte | Are there multiple visual short-term memory stores?[END_REF]. This interpretation of the iconic memory experiments seems to be the one that was initially favored: as Sperling put it in his introduction "more is seen than can be remembered"; the problem is one of memory, not of conscious perception. On the other hand, the post-cue may have introduced a bias during stimulus processing, which led to a different conscious experience of the array as a whole. Unfortunately, because the number of items presented exceeded working memory capacity, these two competing hypotheses could not be measured against the other.

The idea that a second stimulus occurring after a target could alter how we experience it, not how we remember it, seems off: if you haven't experienced it when it happened, then any later occurring experience of the target is an experience of a memory. But does it make any sense to experience the memory of something that yielded no subjective content in the first place?

A recent series of experiments investigated this intriguing possibility. Participants were presented with tilted Gabor patches at the perceptual threshold in one of two placeholders on the sides of a fixation circle (Figure I-9A). They were instructed to report its orientation, and how visible the stimulus was. Closely resembling previous studies, the appearance of the Gabor could be cued by the dimming of one placeholder a few hundred milliseconds before, which would indicate its location with 50% accuracy (exogenous attention orienting). This is a very classical procedure used to test for the effect of pre-cueing spatial attention on perception and consciousness [START_REF] Posner | Attention and the detection of signals[END_REF]. But the new feature in this study was that this Gabor patch could also be cued after its disappearance from the screen. In the case of this experiment, participants only have one item to process and report. The load on memory is thus minimal, and consequently any effect of the retro-cue should be attributed to (conscious) perception rather than memory. Interestingly, the presence of a congruent cue (same side as the target) helped participants become more accurate at the task and report higher visibility not only when it preceded the target, but also when it followed it, even with a delay as long as 400ms (Figure

I-9B

). This phenomenon was called "retro-perception", and showed that late reactivation of sensory information about a past stimulus could play a causal role in the emergence of conscious perception of this past stimulus. These observations validate a strong prediction of late and global theories of consciousness, where experience and access are gated by attention and depend on the involvement of an amodal fronto-parietal network. It also crucially highlights the flexibility in the timing of consciousness when compared to ongoing events in the external world [START_REF] Sergent | The offline stream of conscious representations[END_REF], in line with several other post-dictive phenomena [START_REF] Eagleman | Motion integration and postdiction in visual awareness[END_REF] or the psychological refractory period [START_REF] Levy | Central interference in driving : Is there any stopping the psychological refractory period[END_REF][START_REF] Marti | A shared cortical bottleneck underlying Attentional Blink and Psychological Refractory Period[END_REF]. The timing of consciousness may not entirely follow that of the events occurring "on the outside", because it doesn't need to, nor should it particularly do so: irrelevant stimuli occurring at one time may become highly important depending on later occurring inputs. As such, the stream of conscious representations may function in partial independence from the reality of sequential events or sensory processing, while keeping relative track of their order and time of occurrence [START_REF] Sergent | The offline stream of conscious representations[END_REF]. This means the timing of conscious processing is not necessarily time-locked to the onset of a stimulus. Rather, a more complex process is proposed: when a target stimulus is presented, information is rapidly processed during the feedforward sweep and following recurrent local interactions, however it is not conscious yet. In some cases, conditions may be favorable, such as having attention focused on it by chance, and initial sensory processing is enough in itself to trigger the processes leading to conscious perception. In other instances, however, conditions are not as favorable, and the information slowly decays within the relevant cortices without becoming conscious. It is in those instances that retro-perception can occur: making the stimulus salient after its offset, when information about it is still present in the brain, can trigger the processes leading up to conscious awareness, by reactivating the trace, and broadcasting it to the global workspace, for further processing and report [START_REF] Sergent | The offline stream of conscious representations[END_REF].

The present thesis investigates elements of the variability of conscious access mechanisms in time and content, in the continuity of these first studies on retro-perception. The second Chapter will investigate a cross-modal variation of retro-perception, involving speeded behavioral responses and response time measures, which are intimately linked to the timing of perceptual processing. The third Chapter will use retro-perception as means to elicit conscious access to visual words after their visual properties were degraded by masks: is wholly amodal perception possible, i.e. is it possible that participants perceive the meaning of word-stimuli without their visual features? Last, the fourth Chapter will interrogate the dynamics of conscious access, and report an iEEG investigation of the neural dynamics of auditory awareness, in the presence and absence of report.

Chapter II.

Audition can improve vision, even retrospectively. If retro-perception is the result of the engagement of late-onset amodal processes supporting conscious access, then it must not be restricted to vision. In this Chapter, we aim at replicating the retro-perception effect in a cross-modal setting, along with the use of speeded responses: contrary to accuracy or subjective reports, which are probed a fixed amount of time after stimuli were presented, response times are more directly related to the timings of perceptual processing, and may be differently affected by pre-and retro-cues because these occur at different times.

Participants were presented with Gabor patches at the perceptual threshold in one of two placeholders, and were instructed to press a response button as soon as they detected their presence. These patches could be preceded or followed by spatialized exogenous auditory cues outside the window for pre-attentive cross-modal interactions. The experiment included target absent and cue-absent trials. This protocol was a variation of previous studies in the literature which highlight the effect of cross-modal exogenous attention on visual perception (McDonald, [START_REF] Mcdonald | Cross-modal spatial cueing of attention influences visual perception[END_REF]McDonald et al., 2000). We found that pre-cueing with a short SOA (-150ms), regardless of cue spatialization, increased detection sensitivity to the stimuli, and decreased response time. However this was not true in retro-cueing. We concluded that cross-modal pre-cueing of visual stimuli using sounds increased participants' arousal as evidenced in several studies on visual search [START_REF] Kusnir | Phasic auditory alerting improves visual conscious perception[END_REF][START_REF] Pollack | Maintenance of alertness by a loud auditory signal[END_REF][START_REF] Van Der Burg | Pip and pop : Nonspatial auditory signals improve spatial visual search[END_REF]. Secondly, we found that spatial congruence between target and cue increased detection sensitivity and decreased response time in pre and retro-cueing, for short SOAs (-150ms and/or +150ms). This is a replication of previous experiments on cross-modal pre-cued attention (McDonald et al., 2000), and an extension of this effect to retro-cueing. We further analyzed response time data and fitted a variation of the drift-diffusion model, which distinguishes between perceptual processing/motor processes and perceptual decision-making [START_REF] Anders | The shifted Wald distribution for response time data analysis[END_REF]. We found that pre-cueing, regardless of congruence, increases the drift rate and diminishes the non-decision time. This is coherent with an effect of alertness, where the cue would give a "headstart" to motor preparation. Congruence affected the non-decision time alone, by lowering it in a similar fashion in pre and retro-cueing. One interpretation is that pre-and retro-cueing shortened the time preceding the onset of information accumulation by boosting the stimulus into awareness.

II.2 Introduction

What mechanisms lay the ground for conscious experience? As this question remains debated to this day, two radically different accounts aim at solving the issue. On the one hand it is argued that consciousness arises from local recurrent loops of information in the sensory cortices, and that top-down selection processes such as attention gate reportability, rather than perceptual awareness. [START_REF] Lamme | Why visual attention and awareness are different[END_REF][START_REF] Pitts | Gamma band activity and the P3 reflect post-perceptual processes, not visual awareness[END_REF][START_REF] Tsuchiya | No-report paradigms : Extracting the true neural correlates of consciousness[END_REF]. On the other hand, some state that conscious awareness is the result of information broadcasting and sharing over a broad network of regions beyond the sensory cortices [START_REF] Baars | Global workspace theory of consciousness : Toward a cognitive neuroscience of human experience[END_REF][START_REF] Dehaene | Conscious, preconscious, and subliminal processing : A testable taxonomy[END_REF]. A key difference between those two theories is that, in the first case, the conscious fate of attended sensory input is decided within sensory areas before any intervention of supra-modal areas. This means that whether a stimulus becomes conscious or not is determined within the first 100-150ms following its presentation (Pinto, van der Leij, Sligte, Lamme, & Scholte, 2013).

Recent studies [START_REF] Sergent | Cueing attention after the stimulus is gone can retrospectively trigger conscious perception[END_REF][START_REF] Thibault | Retrospective attention gates discrete conscious access to past sensory stimuli[END_REF]) used a Posner-like paradigm [START_REF] Posner | Attention and the detection of signals[END_REF], in order to test the different predictions from these two models: if conscious perception relates to the broadcast of sensory information via attentional top-down influence, then retrospectively reactivating sensory trace could potentially promote initially unseen stimuli into consciousness, whereas this possibility is excluded in the model where conscious perception is determined during initial sensory processing. In these studies, subjects were asked to judge the orientation of a Gabor patch while spatial visual cues, occurring after the offset of the target, attracted attention either to the target's past location or to the opposite location. These studies showed that such retrospective cueing of spatial attention still facilitated conscious perception of the past target. This effect was called "retro-perception" [START_REF] Sergent | The offline stream of conscious representations[END_REF], and highlighted the existence of temporal flexibility in the processes allowing for conscious access, while providing evidence for the importance of attention as a gating mechanism for conscious experience. However, since these studies used spatial cues within the same modality as the target, it cannot be excluded that this retroperception effect was at least partly due to local, visual interaction between the retrospective cue and the visual trace of the target, rather than to top-down attentional influences [START_REF] Xia | Retrospective triggering of conscious perception by an interstimulus interaction[END_REF].

In order to address this question we devised an experiment in which the target and the retrospective cues where presented in different sensory modalities, in order to minimize direct local interaction between experimental stimuli within sensory cortices. While direct interactions between sensory cortices exist during the feedforward sweep [START_REF] Kayser | Do early sensory cortices integrate cross-modal information?[END_REF], these can more easily be avoided by making the choice of using stimulus-onset asynchronies (SOAs) above 100ms, in order to fall out of the feedforward window. The effect of cross modal attention are well-known [START_REF] Spence | Crossmodal space and crossmodal attention[END_REF]: such experiments have notably evidenced effects of exogenous spatial pre-cueing in the auditory modality on visual perception of masked visual stimuli or stimuli at the perceptual threshold, both in behavioral and neuroimaging experiments [START_REF] Brang | Peripheral sounds rapidly activate visual cortex : Evidence from electrocorticography[END_REF]Hillyard, Störmer, Feng, Martinez, & McDonald, 2016a;McDonald et al., 2000;[START_REF] Mcdonald | Involuntary listening aids seeing : Evidence from human electrophysiology[END_REF]. In particular, pre-cueing the occurrence of a visual target with a spatialized sound can increase participants' discrimination of visual targets (W. [START_REF] Feng | Sounds Activate Visual Cortex and Improve Visual Discrimination[END_REF], or enhance perceived contrast of the target, which correlated with an increased neural response in the occipital cortex contralateral to the side of presentation [START_REF] Störmer | Cross-modal cueing of attention alters appearance and early cortical processing of visual stimuli[END_REF]. These results

suggest that cross-modal attention orienting can alter sensory processing in the visual cortex.

However, in all these studies, the auditory cue precedes the target. Here we wanted to test whether it is also possible to improve the perception of a visual stimulus with an auditory spatial cue that comes after this stimulus.

Additionally, we were interested in assessing the effect of cross-modal retrospective cues not only on perceptual accuracy but also on response times. While the influence of a precue on speeded responses has been studied extensively, showing acceleration of response times with valid pre-cueing [START_REF] Botwinick | Premotor and motor components of reaction time[END_REF][START_REF] Eimer | Effects of masked stimuli on motor activation : Behavioral and electrophysiological evidence[END_REF], 2003;[START_REF] Frith | Routes to action in reaction time tasks[END_REF][START_REF] Hackley | Response complexity and precue interval effects on the lateralized readiness potential[END_REF]Naccache & Dehaene, 2001; van Ede, de Lange, & Maris, 2012; [START_REF] Vorberg | Different time courses for visual perception and action priming[END_REF], retro-perception has not yet been examined in this manner. If auditory retro-cues can influence the perception of visual stimuli, we expect that this will also influence the time it takes to respond to the stimulus presence. However, predictions on the direction of the effect are not entirely straightforward: if retro-cueing improves perception it might speed up detection response time, in accordance with the effect observed for pre-cues. But we could also predict the contrary: if, as suggested by previous studies, retrocues retrospectively promote unseen stimuli into awareness, then response times on those trials should be delayed since response time might be then time-locked to the retro-cue rather than time-locked to the target presentation itself.

We thus designed an experiment aiming at extending retro-perception to a cross-modal setting in order to generalize the phenomenon beyond the visual modality, as well as examine its effect on perceptual decision-making. The experiment was a variation of the original paradigm by McDonald and colleagues showing the effect of exogenous auditory pre-cueing on visual perception (McDonald et al., 2000). Participants had to press a button as soon as they detected the presence of a visual Gabor patch to the left or to the right of fixation. We investigated how detection sensitivity and response time to this visual target was influenced by an auditory cue that could emanate from the same location as the target (congruent cue) or from the opposite location (incongruent cue) either before or, critically, after the stimulus.

We used 4 different SOAs: -600ms and -150ms in pre-cueing, and +150ms and +450ms in retro-cueing. We hypothesized that the two short SOAs (-150ms and +150ms) would be impacted by cue validity. Whether the -600ms SOAs should display inhibition of return was undetermined, as this effect is more context-dependent in cross-modal settings [START_REF] Mcdonald | Spatial relevance determines facilitatory and inhibitory effects of auditory covert spatial orienting[END_REF]. Finally, the original study on retro-perception showed that a positive cuecongruency effect was still present for retro-cues at 400ms, although it was reduced compared to effects observed for retro-cues at 100 or 200ms [START_REF] Sergent | Cueing attention after the stimulus is gone can retrospectively trigger conscious perception[END_REF][START_REF] Thibault | Retrospective attention gates discrete conscious access to past sensory stimuli[END_REF]..

Including a +450ms SOA in the present study allowed us to assess the time course of potential cross-modal retroperception effects.

II.3 Methods

II.3.1 Participants

Based on previous experiments on retro-perception, the number of participants was fixed to 20 prior to the experiment. 26 healthy participants were recruited, out of which 6 were excluded (failure to find the perceptual threshold (4), performance at chance (1) or the participant left before completing the experiment (1)). The remaining participants (18 females)

II.3.2 Materials

The experiment was a variation on the original paradigm by McDonald et al. (McDonald et al., 2000) and conducted in a dimly lit room. The background sound level in the room was 45 dB, which is expected for a quiet room with a running computer. Participants sat 110 cm away from a extra-large screen (OLED SAMSUNG Smart TV). Screen resolution was 1920x1080 px and refresh rate was 60 Hz. Luminance of the screen background was at around 18.2 cd/m² on average. Loudspeakers were mounted on the sides of the screen at 50° from the fixation point at the center of the screen. Stimuli were generated and responses recorded using the Psychophysics Toolbox for Matlab (Brainard & Vision, 1997a). Eye fixation was monitored using an Eyetribe tracking device (The Eye Tribe, Copenhagen).

II.3.3 Stimuli and procedure

Participants were asked to fixate a small dot (black, 0.2° diameter) at the center of the screen (Figure II-1A). A black concentric circle (black, 0.4° diameter) appeared around the dot at the beginning of a trial, and disappeared at the end of the trial. During each trial two placeholders were permanently present on both sides of the screen, indicating the two locations where a visual target could appear (26° from center). These placeholders were black circles with a 7.8° diameter. Visual targets were vertical Gabor patches with luminance variations of 1.5 cycle per degree modulated by a Gaussian envelop of 2.2° full width half maximum. These visual targets appeared randomly within either the left or right placeholder and lasted 53ms.

The auditory cue was a 78 dB burst of broadband (500 -5000 Hz) 'pink' noise delivered from one of the two loudspeakers for a duration of 83ms. Because the loudspeakers were on the sides of the screen, and thus not exactly at the same position as the Gabor patches, cues were delivered in stereo from both speakers with different amplitudes in each channel so that sounds appeared to be coming exactly from the targets' possible locations. Before the experiment was carried out, the audiovisual latency was measured using an oscilloscope. The thus measured 43ms audio-visual was taken into account in the stimulation program.

Participants were told that a Gabor patch with a 0° tilt could appear in one of two placeholders, and they were requested to press a button as fast as possible when they detected its appearance in either of the placeholders. Participants were warned that in 1/3 of trials, no visual stimulus would appear on screen, and that auditory stimuli were non-predictive of target location. Auditory cues were absent in 1/5 of all trials, balanced across conditions. Fixation lasted for 500 ms before each trial. The trial started with the appearance of a circle around the fixation dot. Then a jittered amount of time would elapse (1-1.5s) before target appearance. On 2/3 rd of the trials a 53ms Gabor patch was presented, either within the left or right placeholder.

This was followed by another jittered delay (1-1.5s), then a response window of 1.5s, during which the experiment waited for the participant to respond. If subject pressed the response button at any time during the trial, their response would be recorded, and the experiment would move on to the following trial. The sound cues could be presented during either of the jittered delays, at various SOAs from the target: -600 and -150 in precueing conditions, +150 and +450 in postcueing conditions.

Fixation was monitored online using the EyeTribe tracking device: if fixation was broken by more than 3° during the course of a trial, any response would be discarded and the trial be queued to be played again at the end of the block.

The experiment lasted for roughly 2h and was divided in three parts. The first was a 64trial long practice block with feedback after each trial. The second part was a psychophysical staircase to find the appropriate level of contrast for each participant (70% of "hit" trials in target present trials). This staircase was performed separately for both visual hemi-fields after a pilot experiment showed significant differences in performance depending on which side the target appeared, which did not replicate in the current experiment (1.1% ± 0.3 for right-sided targets, 0.9±0.3 for left-sided targets at the beginning of experiment). The staircase block was conducted using the Psychtoolbox-3 Quest procedure on 3 blocks of 40 trials without feedback or auditory stimuli. This procedure uses a fixed slope for estimating the psychometric function, it was fixed at 3.5. This staircase was followed by 16 experimental blocks of 40 trials. Even though the staircase procedure was calibrated for a 70% performance at the beginning of the experiment, participants were told to aim for 75% performance, and we tried maintaining this average throughout by adjusting Gabor patch contrast between blocks. This discrepancy was due to the fact that the staircase procedure was run without any auditory cues, and participants' overall performance usually increased by about 5 points between staircase and test session.

During the course of the experiment, the target contrast could be readjusted by steps of 0.1% contrast (applied on both left and right targets) when global performance on the preceding block was either too high (more than 80% hits) or too low (less than 70% hits).

II.3.4 Analysis

II.3.4.a Trial exclusion criteria

Trials were automatically discarded when the timings weren't respected during the experiments (hardware failure). We also discarded all trials where the response time was under 150ms after target onset on the assumption that these were artefactual: the minimum amount of time to produce a voluntary response is generally considered to be higher than this cutoff [START_REF] Luce | Response times : Their role in inferring elementary mental organization[END_REF]. This amounted to a loss of less than 3% ± 2 of trials on average. There was no upper RT boundary for trial exclusion because large RTs can drastically change the interpretation of data [START_REF] Ratcliff | Methods for dealing with reaction time outliers[END_REF][START_REF] Ulrich | Effects of truncation on reaction time analysis[END_REF]. Classical analyses were repeated with all trials, and led to the same conclusions.

II.3.4.b Task accuracy and signal detection theory

Task accuracy was evaluated using signal detection theory [START_REF] Green | Signal detection theory and psychophysics[END_REF]. We computed d', the measure of unbiased sensitivity to a stimulus' presence in all experimental conditions. All analyses performed on this data were classical inferential statistics.

II.3.4.c Response time data, classical analyses

We computed a first pass of classical inferential statistics, in which we examined the averaged median response time across participants.

II.3.5 Modeling

The analysis of response time data requires some amount of care: because RT values are always positive, they have a lower boundary, but no upper boundary. As such, RT distributions typically have a rightward skew, where the mean, mode and median differ from each other. As such, classical analyses ill-fitted to analyze RT data due to the skewed nature of RT distributions [START_REF] Balota | Moving beyond the mean in studies of mental chronometry : The power of response time distributional analyses[END_REF][START_REF] Heathcote | Fitting Wald and ex-Wald distributions to response time data : An example using functions for the S-PLUS package[END_REF][START_REF] Lo | To transform or not to transform : Using generalized linear mixed models to analyse reaction time data[END_REF]. Several alternative methods can be used to analyze distributions with a rightward skew. Some use appropriate parent distributions such as the shifted Wald [START_REF] Anders | The shifted Wald distribution for response time data analysis[END_REF], gamma [START_REF] Luce | Response times : Their role in inferring elementary mental organization[END_REF][START_REF] Ratcliff | Methods for dealing with reaction time outliers[END_REF], log-normal [START_REF] Ulrich | Information processing models generating lognormally distributed reaction times[END_REF], or ex-gaussian [START_REF] Dawson | Fitting the ex-Gaussian equation to reaction time distributions[END_REF] distributions, but the choice of model is rather arbitrary and the results should be interpreted with care [START_REF] Lo | To transform or not to transform : Using generalized linear mixed models to analyse reaction time data[END_REF][START_REF] Matzke | Psychological interpretation of the ex-Gaussian and shifted Wald parameters : A diffusion model analysis[END_REF]. More complex analysis methods use models thought to describe the underlying process leading to the production of the timed response, by modeling signal accumulation. Among the most well-known is the Drift Diffusion Model first proposed by Ratcliff et al [START_REF] Ratcliff | The diffusion decision model : Theory and data for twochoice decision tasks[END_REF][START_REF] Ratcliff | Diffusion Decision Model : Current Issues and History[END_REF], which supposes that evidence accumulates towards a response following a random walk with a drift until the amount of evidence accumulated crosses a boundary, at which point a perceptual decision is taken. It is widely used to model the process underlying 2 alternative force-choice tasks with speeded responses, where the drift rate can be either positive (towards response A) or negative (towards response B). Here, we use a variation of this framework, detailed in the Methods [START_REF] Anders | The shifted Wald distribution for response time data analysis[END_REF] that has 3 parameters of interest (Figure II-2A): information accumulates at a rate γ, which depends on the quality of information accumulated, until it reaches the decision boundary α which depends on cognitive control, and the overall time taken to produce a response includes an additional non-decision time θ, which corresponds to all the time processes unrelated to decision-making take.

For response time data analysis, only Hits (correct response in a target-present trial)

were analyzed. False alarm response times were not analyzed. Because the distribution of the data is positively skewed, but that significant information may be contained in the tail, it is generally recommended not to truncate the tail of the distribution in order to make it resemble a normal distribution [START_REF] Ulrich | Effects of truncation on reaction time analysis[END_REF]. We then modeled response time distributions using a shifted-Wald distribution and a maximum-likelihood estimation procedure from Anders et al 2016 [START_REF] Anders | The shifted Wald distribution for response time data analysis[END_REF]. Drift-diffusion models are generally used to describe 2-AFC tasks, where there exists two competing response options. In our case however, the task is a go/no-go, where participants opt-out of responding when they feel there was not target. In this case, the Wiener process is inappropriate for modeling: several of its parameters are poorly or non-applicable, such as the estimation of how much bias each participant has towards one possible answer, which cannot be estimated in our case: when participants choose to opt-out, there is no response time data to be modeled. A good substitute for go/no-go tasks is the shifted-Wald (SW) distribution, which has already been used to describe go/no-go drift-diffusion processes in several studies [START_REF] Anders | The shifted Wald distribution for response time data analysis[END_REF][START_REF] Faulkenberry | A single-boundary accumulator model of response times in an addition verification task[END_REF][START_REF] Gayet | Visual input that matches the content of visual working memory requires less (not faster) evidence sampling to reach conscious access[END_REF]. The shifted-Wald distribution, also called the shifted inverse-Gaussian is of the form:

(|, , ) = �2( -) 3 * exp �- [ -( -)] 2 2( -) �
Anders et al. [START_REF] Anders | The shifted Wald distribution for response time data analysis[END_REF] propose a Maximum Likelihood Estimation (MLE) fitting method fully described in their publication (The_Shifted_Wald_Distribution_for_Response_Time_Data_Analysis.pdf).

The basic rationale behind it is to minimize the difference between the observed RT data quantiles and the model-predicted quantiles using a single parameter β. From this β, all three parameters mentioned above can be directly calculated by closed-form maximum-likelihood estimators (Nagatsuka & Balakrishnan 2013) and used to compute the model-predicted quantiles for comparison and minimization in regards to the observed quantiles. We used their methods and adapted their analysis script to fit our needs.

In the analysis pipeline used, we fit a separate SW distribution to each experimental condition for each participant, and considered the global RT distribution to be a mixture of these. We only modeled response time distributions for experimental conditions where we observed an effect of the SOA or congruence, which were the short pre-and post-cued SOAs (-150ms and +150ms). With 20 participants, this led us to fit 2 (SOAs) x 2 (congruence) x 20

(participants) individual SW distributions, leading to as many estimations for each separate parameter. Classical statistical analyses were then performed for significance testing.

II.3.6 Data and script availability

Most analyses were performed using Matlab R2014b, except for modeling, which was performed in R. Data and analysis scripts are available at https://github.com/DaphneRR/Retro-Pink. particularly in visual search [START_REF] Kusnir | Phasic auditory alerting improves visual conscious perception[END_REF][START_REF] Pollack | Maintenance of alertness by a loud auditory signal[END_REF][START_REF] Van Der Burg | Pip and pop : Nonspatial auditory signals improve spatial visual search[END_REF]. Interestingly this effect is not symmetrical: it is either absent or strongly reduced when the cue comes after the stimulus.

II.4 Results

II.4.1 D prime

On top of this strong modulation by SOA, spatial congruency between target and cue also modulated detection sensitivity: visual detection sensitivity improved when the exogenous auditory cue was played on the same side as the target (congruent) compared to the opposite side (incongruent) both for pre and retro-cues at short SOAs. The ANOVA confirmed a significant main effect of congruency (F(1,19) = 4.53, p<0.05), but no interaction between congruency and SOA (F(2.59,49.15)=0.72, p=0.5). Bonferroni-corrected 10,000 bootstrapped confidence intervals of the mean difference between congruent and incongruent conditions This effect replicates what was evidenced in the literature (McDonald et al., 2000;[START_REF] Teder-Sälejärvi | Effects of spatial congruity on audio-visual multimodal integration[END_REF]. The effect of cross-modal cues are typically smaller than the effect of unimodal cues [START_REF] Koelewijn | Auditory and visual capture during focused visual attention[END_REF][START_REF] Spence | A new approach to the design of multimodal warning signals[END_REF], which can explain why post-cueing effects are smaller here than reported with visual targets and cues, effectively reducing the expected power of our statistical analyses [START_REF] Sergent | Cueing attention after the stimulus is gone can retrospectively trigger conscious perception[END_REF][START_REF] Thibault | Retrospective attention gates discrete conscious access to past sensory stimuli[END_REF].

II.4.2 Response time (average data analysis)

A 19.68]). This adds some precision to the evidenced effect of congruence, and localizes it to the shorter SOAs, symmetrically for pre-and retro-cueing.

As far as congruence is concerned, the effect of cueing are strongest at the short preand post-cued SOAs (-150ms and +150ms), which is coherent with the pattern of results observed from detection sensitivity, although for this task of speeded responses, response times seem to be more sensitive to congruence than our measure of detection. Since both measures go in the same direction, it suggests that our effects cannot be attributed to speed accuracy tradeoffs.

II.4.1 Model fitting diagnostics

Response time data being positively skewed, looking at summary statistics might actually lead to missing important aspects of the experimental effects on response times [START_REF] Lo | To transform or not to transform : Using generalized linear mixed models to analyse reaction time data[END_REF]. We thus fitted a shifted-Wald distribution using MLE as described in the Methods and illustrated in Figure II-2A. In brief the parameters of interest are , the drift rate, which is thought to reflect the quality of the information being processed, , the decision threshold, which reflects cognitive control (e.g. inhibitory control over the button press), and , the non-decision time, which is affected when motor demands for responding become increasingly difficult, or the course of perceptual processing or motor preparation is altered [START_REF] Anders | The shifted Wald distribution for response time data analysis[END_REF][START_REF] Voss | Interpreting the parameters of the diffusion model : An empirical validation[END_REF].

For this analysis, we only included SOAs where an effect of congruence on response times and accuracy could be observed or suggested in the data, i.e. the two short SOAs that were the focus of our initial hypotheses. This left us with S = 2 SOAs, C = 2 congruence levels, P = 20 participants, leading to 2 x 2 x 20 = 80 distributions (or experimental cells) to be fitted individually with an average number of trials of 55 ± 10, which is enough according to the simulations run in Anders 2016.

We ρ Δσ should be as close to zero as possible, a requirement that is reasonably met here for Δ � and ρ Δσ . Overall, we meet the requirements for these three goodness of fit checks, although they reveal some data-points are outlying.

II.4.2 Model fitting results

Parameters α, γ and θ were retrieved after model fitting and goodness of fit checks for each participant and condition (SOA, congruence). Each parameter was assessed for statistical significance using repeated measures ANOVA across participants for all conditions and their interactions. For the drift rate γ (Figure These results allow us to specify the origin of the effects observed on RTs.

II.4.2.a Effects of SOA

A main effect of the SOA was evidenced on information accumulation rate (γ),

indicating that the quality of the information used for the decision process was higher in the pre-cued condition. This may explain the large increase in accuracy in the short pre-cueing condition seen in Figure II-1B, and be the result of a general effect of well-timed sounds on visual perception, as was evidenced in several studies [START_REF] Kusnir | Phasic auditory alerting improves visual conscious perception[END_REF][START_REF] Pollack | Maintenance of alertness by a loud auditory signal[END_REF][START_REF] Van Der Burg | Pip and pop : Nonspatial auditory signals improve spatial visual search[END_REF]. The SOA also affected the non-decision time: pre-cueing reduced the non-decision time when compared to retro-cueing. Generally, it is considered that this delay covers perceptual encoding, presumably prior to evidence accumulation, as well as motor preparation and then response execution time following the decision process. Here, since the response requirements are identical in all conditions (same button press), it seems difficult to attribute the effects of our experimental conditions on response execution. This change in θ thus probably reflects a "head start" provided by the pre-cues compared to retro-cues, either by starting motor preparation earlier and/or preparing the sensory system for efficient processing.

II.4.2.b Effects of congruence

A significant effect of spatial congruency was only found on the non-decision time θ, independently of SOA. Motor execution is still not a good candidate process to explain this effect for the reasons presented above. The possible explanations are that congruence affected

II.5 Discussion

In this study we probed whether orienting spatial attention with a sound can not only help see future visual events at the same location, but also help revive past visual events that were missed. In other words, can exogenous cross-modal attention also work retrospectively?

To this aim we adapted an original study conducted by McDonald et al. which demonstrated the existence of cross-modal effects in the case of pre-cued attention (McDonald et al., 2000).

The present study replicated their original effect by showing that a non-informative but spatially congruent auditory pre-cue increases detection sensitivity to a visual stimulus. The use of several pre-and retro-cueing SOAs allowed us to show that there were two phenomena at play in cross-modal cueing effects. One was a general "arousal" effect, independent from the cue location, but very sensitive to timing: pre-cueing just before the target (-150ms) induced a general increase in sensitivity regardless of congruence and a response time reduction. This effect was absent when the pre-cue was presented too long before the target (-450ms) and was small or abolished for retro-cues, irrespective of the SOA. This effect is thus sensitive to the time order of stimulus presentation, and coherent with previous studies conducted in pre-cued visual search on cross-modal influences on perception [START_REF] Kusnir | Phasic auditory alerting improves visual conscious perception[END_REF][START_REF] Pollack | Maintenance of alertness by a loud auditory signal[END_REF][START_REF] Van Der Burg | Pip and pop : Nonspatial auditory signals improve spatial visual search[END_REF]. Another aspect of the cue effect is sensitive to the cue location:

cue spatial validity increased detection sensitivity and led to an identifiable decrease in RTs for short SOAs (-150ms and/or + 150ms). In contrast to the "alerting" effect, it did not appear to be influenced by whether the cue appeared before or after the target.

A more fine-grained analysis of response times focused on a distributional analysis of relevant SOAs revealed that the non-spatial effect of pre-cueing at short SOA described above corresponded both to an increase in the drift rate (reflecting an improved quality of the sensory information) and a shortening of peri-decisional processes compared to playing the sound after the visual target. These results can be interpreted in terms of alertness, whereby the occurrence of a pre-cue may have given a "headstart" to motor preparation and/or primed the sensory system for target appearance in the near-future. It should be noted that it is probably a general alerting effect rather than a specific temporal attention effect, since participants could not anticipate the exact time of occurrence of the target after the cue. In these cases, the more general term alertness is more appropriate to avoid confusing different phenomena [START_REF] Weinbach | Temporal orienting and alerting-the same or different[END_REF].

The spatial congruence of the cue affected the peri-decisional processes alone. The model used to analyze response times does not allow to definitely disentangle motor preparation from stimulus encoding. However, while the presence of a cue can be expected to influence motor preparation, it is difficult to imagine how its spatialization with regards to a visual stimulus would have an effect purely on motor planning when the elicited response does not depend on the cue's location (simple detection). According to previous literature on retroperception, spatially congruent cues lower the threshold for conscious access, even retrospectively, thus triggering conscious access to past target that would have been definitely missed otherwise [START_REF] Sergent | Cueing attention after the stimulus is gone can retrospectively trigger conscious perception[END_REF][START_REF] Thibault | Retrospective attention gates discrete conscious access to past sensory stimuli[END_REF]. It is thus possible that the effect of congruent retro-cues on non-decision time corresponds to earlier onset of conscious processing on those trials where initial processing of the target was not very efficient, and the retro-cue helped boost it into awareness, shortening the time it took to actually start the accumulation process. Indeed, in the context the accumulator model, entering conscious access could match with the onset of information accumulation.

These result helps settle one hypothesis we initially had: because the effect of retrocueing is to lead to conscious access of the stimulus, we thought the response process could be delayed, and that congruent cueing would lead to longer response times on average, by mixing "normally perceived" trials with "retro-perceived" trials, because conscious processing would have been initiated at a later time. What we evidenced here is actually the reverse: congruent cues shorten the non-decision time. One interpretation is that even when provided after target processing has begun, the cue acts as a facilitator if the delay is not too long, which in turn can lead to an increased chance of conscious perception. In fact, retro-cues have been shown to reduce response time in previous studies on retro-perception, although these studies did not focus on response times [START_REF] Sergent | Cueing attention after the stimulus is gone can retrospectively trigger conscious perception[END_REF].

A second conclusion can be drawn on the basis of our modeling results: while congruence has an effect on accuracy, it is not reflected by an increase in drift rate or a lowering of the decision threshold, meaning that the quality of the information and the criterion used for decision are unaffected. These results are compatible with previous studies on retro-perception, which showed that retro-cueing of this kind led to an increased chance of conscious perception without better information about the target stimulus [START_REF] Thibault | Retrospective attention gates discrete conscious access to past sensory stimuli[END_REF].

III.1 Introductory summary

An extreme prediction of the late and global theory of conscious access is that, because the putative mechanisms underlying access are amodal, they could in theory operate on any kind of information without particularly relying on sensory representations. In this chapter we investigate whether it is possible, using retro-perception, to tamper with the course of normal processing and induce truncated perception of semantic information of a stimulus without its sensory attributes.

Participants were presented with masked words around the perceptual threshold. As we have seen in the introduction, it is thought that masking disrupts the occurrence of recurrent processes, thought to be crucial for conscious access. This putatively occurs because the masks lead low levels in the visual system to process information relating to them, when higher levels try to exchange information pertaining to the preceding stimulus [START_REF] Di Lollo | Attention is a sterile concept; iterative reentry is a fertile substitute[END_REF]Lamme & Roelfsema, 2000). We hypothesized that, if the masks were not meaningful, they would only degrade low-level information of our target words and leave semantic representations intact.

We then retro-cued participants in the auditory modality using semantically related stimuli (e.g.

cueing "hedgehog" with "porcupine"), as there is evidence in the literature showing that related auditory words can unmask visual words [START_REF] Bernstein | Semantic priming : Subliminal perception or context?[END_REF][START_REF] Dark | Semantic priming, prime reportability, and retroactive priming are interdependent[END_REF]. Participants were asked to report the word's meaning, and a visual feature we hypothesized would be degraded by the masks (case or position), as well as different measures of subjective experience in three separate experiments. We found that congruent cueing increased participants' accuracy at reporting the word's meaning and their subjective ratings, but not its visual attribute. Bayesian hierarchical modeling of our data also suggests the same pattern, and predicts a strong decorrelation in congruent trials between participants' propensity to be accurate at the meaning and visual task, suggesting that whether they perceive one is less dependent on perception of the other in congruent trials. This unique finding favors the hypothesis that conscious access is an amodal process, and can operate on any kind of information, as well as showing that consciousness is not only flexible in time, but also in content.

III.2 Introduction

When we become conscious of a stimulus it usually means that we are aware of all its different attributes, both physical features and abstract meaning [START_REF] Baars | Global workspace theory of consciousness : Toward a cognitive neuroscience of human experience[END_REF][START_REF] Dehaene | Conscious, preconscious, and subliminal processing : A testable taxonomy[END_REF]Dehaene & Naccache, 2001;[START_REF] Kouider | How rich is consciousness? The partial awareness hypothesis[END_REF]Lamme & Roelfsema, 2000;Lamme et al., 2000). A much-debated issue is whether conscious perception gradually occurs during the initial phase of stimulus processing where these different representations are constructed within distinct brain areas [START_REF] Fisch | Neural "ignition" : Enhanced activation linked to perceptual awareness in human ventral stream visual cortex[END_REF]Lamme & Roelfsema, 2000;[START_REF] Pitts | Gamma band activity and the P3 reflect post-perceptual processes, not visual awareness[END_REF], or whether it corresponds to an all-or-none secondary amodal process that amplifies and broadcasts existing representations to a global workspace [START_REF] Baars | Global workspace theory of consciousness : Toward a cognitive neuroscience of human experience[END_REF]Dehaene & Naccache, 2001). A recent proposal suggested that the preceding views can be compatible: when a perceptual stimulus is processed, its different features may not all reach awareness together, so that we have "partial awareness" of the stimulus [START_REF] Kouider | Partial awareness and the illusion of phenomenal consciousness[END_REF]Kouider, De Gardelle, et al., 2010;[START_REF] Kouider | Partial awareness creates the "illusion" of subliminal semantic priming[END_REF].

In this case, individual features such as shape, color, etc. become conscious in an all-or-none fashion, but perception can be graded when considering how many of the whole set of attributes were accurately perceived. Partial awareness has been shown in cases where participants are aware of low-level, but not high-level properties of stimuli [START_REF] Kouider | Partial awareness creates the "illusion" of subliminal semantic priming[END_REF], which are then guessed based on what was seen, and prior information. If conscious perception is truly independent from sensory processing, then an extreme prediction is that the reverse could also occur: we could consciously perceive abstract representations untied to any sensory feature.

Here we tested this prediction using a phenomenon called retro-perception: conscious access to an initially unconscious stimulus can be triggered retrospectively after its disappearance [START_REF] Sergent | The offline stream of conscious representations[END_REF][START_REF] Sergent | Cueing attention after the stimulus is gone can retrospectively trigger conscious perception[END_REF][START_REF] Thibault | Retrospective attention gates discrete conscious access to past sensory stimuli[END_REF][START_REF] Xia | Retrospective triggering of conscious perception by an interstimulus interaction[END_REF], by using a retro-cue that reactivates the latent representations left by the stimulus. We reasoned that if the low-level properties of a stimulus were erased by a mask before the retro-cue, we might be able to induce "asensory" perception, comprising only an abstract representation of the stimulus. Here, visual words were rendered unconscious using visual masking [START_REF] Enns | What's new in visual masking?[END_REF]Lamme & Roelfsema, 2000;[START_REF] Weiskrantz | Parameters affecting conscious versus unconscious visual discrimination with damage to the visual cortex (V1)[END_REF]. They were followed by an auditory word, which served as a semantically congruent or incongruent retro-cue [START_REF] Bernstein | Semantic priming : Subliminal perception or context?[END_REF][START_REF] Briand | Retroactive semantic priming in a lexical decision task[END_REF][START_REF] Dark | Semantic priming, prime reportability, and retroactive priming are interdependent[END_REF][START_REF] Neely | Semantic priming effects in visual word recognition : A selective review of current findings and theories[END_REF][START_REF] Thomas | When word identification gets tough, retrospective semantic processing comes to the rescue[END_REF][START_REF] Vanvoorhis | Semantic matching, response mode, and response mapping as contributors to retroactive and proactive priming[END_REF] through associative priming (e.g. "porcupine" -"hedgehog"). If the auditory cue primed the relevant semantic links [START_REF] Collins | A spreading-activation theory of semantic processing[END_REF] after the occurrence of the target, it would specifically boost semantic processing and make this feature of the stimulus salient, whereas the visual masks would strongly degrade the target's visual information and make it inaccessible to consciousness [START_REF] Enns | What's new in visual masking?[END_REF]Lamme & Roelfsema, 2000;[START_REF] Weiskrantz | Parameters affecting conscious versus unconscious visual discrimination with damage to the visual cortex (V1)[END_REF].

Figure III-1 details our hypotheses and predictions. Once a visual word is presented, it is quickly processed through a hierarchy of analysis levels, from the extraction of local edges (V1), to the recognition of letter forms (V2-V4-V8), groups of letters (Occipito-Temporal Sulcus, OTS), and finally up to the activation of phonological and semantic representations [START_REF] Cohen | Specialization within the ventral stream : The case for the visual word form area[END_REF][START_REF] Dehaene | The neural code for written words : A proposal[END_REF][START_REF] Vinckier | Hierarchical Coding of Letter Strings in the Ventral Stream : Dissecting the Inner Organization of the Visual Word-Form System[END_REF]

(Figure III-1a).
The presentation of a pattern mask shortly after the stimulus degrades or replaces the visual representation of the stimulus at the low levels, presumably up to V8 [START_REF] Callaway | Local circuits in primary visual cortex of the macaque monkey[END_REF][START_REF] Enns | What's new in visual masking?[END_REF][START_REF] Felleman | Distributed hierarchical processing in the primate cerebral cortex[END_REF]Lamme & Roelfsema, 2000;[START_REF] Lamme | Masking Interrupts Figure-Ground Signals in V1[END_REF], while sparing the high level semantic and phonological representations, as demonstrated by experiments on subliminal priming [START_REF] Abrams | Unconscious semantic priming in the absence of partial awareness[END_REF][START_REF] Fowler | Lexical access with and without awareness[END_REF][START_REF] Humphreys | Automatic phonological priming in visual word recognition[END_REF][START_REF] Kouider | Levels of processing during non-conscious perception : A critical review of visual masking[END_REF]Naccache & Dehaene, 2001)

(Figure III-1b).
Therefore, after the word has been masked, the brain retains a trace of the semantic and phonological attributes of the stimulus, but no trace of its physical attributes. The subsequent presentation of a semantic context, for example an auditory word that is semantically congruent, should reactivate the semantic trace left by the masked word, as suggested by previous studies [START_REF] Bernstein | Semantic priming : Subliminal perception or context?[END_REF][START_REF] Briand | Retroactive semantic priming in a lexical decision task[END_REF][START_REF] Dark | Semantic priming, prime reportability, and retroactive priming are interdependent[END_REF][START_REF] Neely | Semantic priming effects in visual word recognition : A selective review of current findings and theories[END_REF][START_REF] Thomas | When word identification gets tough, retrospective semantic processing comes to the rescue[END_REF][START_REF] Vanvoorhis | Semantic matching, response mode, and response mapping as contributors to retroactive and proactive priming[END_REF] (Figure III-1c). Our prediction is that such reactivation of a latent semantic representation could trigger conscious access to this representation. Since, at this stage, there is a mismatch in the information present at high and low levels of processing, the first containing information about the word, and the second information about the mask, reentrant processes, which are thought to be strongly related to conscious access (Dehaene et al., 2006;Lamme & Roelfsema, 2000), would fail to emerge between them (Di Lollo, 2018), leaving the semantic representation stranded, and broadcasted on its own to a global workspace of areas including fronto-parieto-cingulate hubs of consciousness (Figure III-1d).

III.3 Methods

III.3.1 Participants

For Experiment 1, 23 healthy participants underwent a pre-selection session prior to the experiment that allowed excluding subjects whose overall performance was at chance (or at ceiling) with our masked stimuli. Fifteen participants (10 females 5 males) matched our selection criterion, with an overall case discrimination performance comprised between 65% and 90% and were included in the main experiment. They were aged 23 ± 3.3. For Experiment 2, 23 participants were recruited, out of which 5 were excluded prior to analysis because their performance was at chance or they did not follow the instructions. The remaining 18 participants (9 females and 9 males) had an average age of 27.5 ± 7.1. In Experiment 3, 26 participants were recruited, out of which 8 were excluded prior to analysis because their overall case discrimination performance was at chance or they did not follow the instructions.

Remaining participants (8 males) had an average age of 24.5 ± 3.

III.3.2 Participants' selection procedure

Given the difficulty of the task, we chose in Experiment 1 to run a pre-selection session that was designed to exclude participants unable to perform the task from the get-go, as well as provide others with some training. We took this opportunity to test a variety of target durations in order to choose the best suited for the testing session. This session lasted 1 hour and included 140 trials preceded by a short, 18 trial training block with progressively decreasing target duration, from 120ms to 12ms. During the preselection block, participants were presented with stimuli matching the criteria of the main session without being the same and no auditory cue.

Tested target durations during the preselection session always included 24ms and 36ms, and the 3 rd and 4 th duration tested ranged from 48ms to 94ms. Participants were asked to identify the word aloud in a microphone, report its case and visibility. The criterion for inclusion in our experiment was that performance at the case discrimination task should, for at least one target duration, be within the bounds [65% -90%].

In Experiments 2 & 3, we used a different selection procedure. The experiment was performed over a single session, which started with a 72 trial-long training session with progressively decreasing target duration without any auditory cueing. If participants reported being unable to do the task, and performance suggested as much, we ran a second 72 trial-long training session, at the end of which we evaluated whether a given participant could perform the task.

III.3.3 Stimulus-list and experimental design

Our stimulus list was composed of 432 unique words (targets) from the French language. These words ranged from 3 to 8 letters long (+ one 2-letter word), and did not carry accents of cedillas.

They were selected for being strongly associated to another word from the French language less than three-syllables-long, which could then be used as a congruent cue for that word. These associations were determined through 5 different free-association experiments (two experiments conducted in laboratory settings [START_REF] Ferrand | Normes d'associations verbales pour 260 mots «abstraits[END_REF][START_REF] Ferrand | Normes d'associations verbales pour 366 noms d'objets concrets[END_REF], the online database dictionnaire des associations verbales du français [START_REF] Debrenne | Le dictionnaire des associations verbales du français et ses applications. Variétés, variations and formes du français[END_REF] (www.dictaverf.nsu.ru), and our own online experiments which mimicked Debrenne et al.'s methodology for additional words described in the Supplementary Note 2). Cue-target pairs were considered "strongly associated" when, in one free-association experiment, >= 25% of participants first responded the target when presented with the cue. This experiment followed a Participants knew that the target was absent on some trials. Stimulus-lists for all participants were designed prior to the experiments in order to properly counter-balance all factors: each target word would be absent the same number of times overall, each target word would be presented once per participant (except absent trials), and across all participants, each target would appear in all conditions the same number of times. Priming from one trial to the next by auditory cues was controlled by having pairs of participants receive the cues in the same order, with one receiving the congruent target, the other the incongruent one. Cues could appear from zero to twice (in a congruent and an incongruent trial in this case) for each participant.

Incongruent pairs were designed by associating a cue to another target which shared no obvious association to it, and never changed throughout the experiments. No homophones were used as cues, 4 words and their homonyms were used as targets with differing cues (saint/sein, sang/sans, haut/eau, fin/faim in French).

III.3.4 Materials

Materials slightly differed between Experiment 1 and the other two. Differences are indicated in parentheses throughout. Stimuli were generated and responses recorded using the Psychophysics Toolbox for Matlab [START_REF] Brainard | The psychophysics toolbox[END_REF]. Stimuli were presented on a CRT monitor Sony Trinitron GDM-F520 (Sony MULTISCAN E400 in Exp.2&3). Refresh rate was 85 Hz and screen resolution was 1280 by 1024 pixels (1152 by 864 in Exp.2&3).

Participants were seated 70cm away from the monitor (57cm in Exp.2&3), in a dimly lit room.

Eye fixation was monitored and recorded using an Eye Tribe tracking device (an EyeLink 1000+ for Exp.2&3). Stimuli were presented on a gray background (52.5 cd/m² in Exp.1; 44.5 cd/m² in Exp.2&3), and participants were told to fixate a dot inside a small black circle of 1.6° in diameter (0.4° in Exp.2&3) at the onset of each trial. The start of the stimulus sequence was conditioned on appropriate fixation (less than 2° from center for more than 200ms; criterion was 1.2° for Exp.2&3). Targets were words written in a mono-spaced font (Courier New), either in uppercase or lowercase. They were approximately 1° high and 2° to 7.4° large depending on word-length in Exp.1 (approximately 0.6° high by 1.6° to 5° large in Exp.2&3). Note that the entire display was smaller in Experiment 2 to ensure that all stimuli would still be presented within reading range even though they were displaced from the center of the screen by 0.7°.

These changes in size were kept for Experiment 3. A HyperX Cloud headset with microphone (Kingston Technology Company Inc.) was used to play the auditory words (retro-cues) and record participants' verbal responses. Audio-visual delay was measured using a photodiode applied to the computer screen and the computer's audio output, together with an oscilloscope for visualization. The delay was within a range of 10ms with no identifiable jitter. This delay was taken into account in the stimulation scripts.

III.3.5 Procedure

In Experiment 1, each trial began with the onset of a circle around the central point, when fixation was successfully maintained for 200ms. Following a 1-2s jittered delay, 6 random strings of non-alphanumerical characters (picked randomly without replacement among the following: %#?&/!|± ¶) with the same font and size as target words would rapidly flash on screen. These strings would vary both in length (3 to 8 items, all six possible length being displayed once in each trial) and duration (two masks of each possible duration: 12, 96 or 192ms). A 7 th non-alphanumerical mask would follow suit, be 9-characters long and last for 200ms immediately before target onset. Targets would be displayed for 12, 24, 36 or 48ms.

There were 11% of "target absent" trials where a mask was presented instead of a target, with the same length and duration probability. The target (or mask in "target absent" trials) was then replaced with an 8 th and last mask that was again 9-characters long and lasted for 200ms. Fifteen milliseconds after the offset of the last mask, i.e. 215ms after the offset of the visual word target, an auditory cue was presented to the participant. The auditory cue lasted less than 800ms and was either semantically related to the target (congruent trials), or completely unrelated (incongruent trials). Participants waited until the offset of the cue before they could respond.

They first named the target visual word aloud and their response was recorded via a microphone. If they had not seen any word, they were instructed to provide a random word to the best of their ability. Participants were then prompted to report whether the word was written in upper or lower case (and guess if they did not know) by selecting an option on the screen using the keyboard. Finally they had to rate the word's visibility on a 9-point scale (details available in Supplementary Note 1).

In Experiment 2, the procedure was identical to Experiment 1 except for the following:

two synchronous streams of masks were presented just above and below a small fixation circle.

On each trial a single target word appeared, in one of the streams, while on the other stream another mask of the same length would be displayed. Note that while there is a known preference for reading in the right visual hemifield [START_REF] Ellis | Length, formats, neighbours, hemispheres, and the processing of words presented laterally or at fixation[END_REF]) compared to the left, no such strong preference seems to exist between the upper and lower hemifields [START_REF] Hagenbeek | Left-right and upper-lower visual field asymmetries for face matching, letter naming, and lexical decision[END_REF]. In 11% of trials, no target word was presented and the trial only consisted in two streams of masks. While in Experiment 1 the interval between the target offset and the cue onset (ISI) was kept constant at 215ms, in Experiment 2 and 3 it was the interval between target onset and cue onset (SOA) that was kept constant at 248ms, with the introduction of a blank period between last mask offset and cue onset with a duration adjusted according to target duration (between 0 and 36ms). Immediately after the offset of the auditory cue, participants were prompted by a response screen to report where the target had appeared (top or bottom). They then had to provide a hybrid visibility/confidence rating on this judgment using a cursor on a 9-point scale (see detailed instructions in Supplementary Note 1). Then they were required to report the target word verbally (recorded via a microphone), and then provide a hybrid visibility/confidence rating on this judgment. Participants were asked to use the lowest level when they were entirely not-confident about their decision because they had not seen anything, the first half of the scale if some letters were perceived or a glimpse of the target without much certainty of their response, and the second half if they were fairly to definitely certain of their response. Because of our instructions, this scale was a hybrid between visibility (lowest rating for unseen trials) and confidence.

Experiment 3 differed from Experiment 2 in the following ways: only one stream of letter-strings was presented, and it was centered, mimicking the setup of Experiment 1.

Participants were first prompted to report whether the target was uppercase or lowercase, and how confident they were about this decision on a 9-point scale. Then they were required to report the target word verbally, and rate their confidence on their word identification response on a 9-point scale. This time, the scales were used to measure only the confidence on the previous decision; there was no mention of visibility. Participants were asked to use the lowest level when they were entirely not-confident about their decision i.e. response at random, the first half of the scale if their confidence was low to medium, and the second half if they were fairly to definitely certain of their response.

III.3.6 Analysis

Fixation control was checked as described in Supplementary Note 3. Detection sensitivity was derived from visibility or confidence ratings using Signal Detection Theory [START_REF] Mcmillan | Detection theory[END_REF]: areas under the receiver operating characteristic curve (AROC) were computed by comparing distributions of ratings on target-present trials in each considered condition to ratings provided in target-absent trials, using the following relationship [START_REF] Hanley | The meaning and use of the area under a receiver operating characteristic (ROC) curve[END_REF]:

= 1 * 2 (1)
Where U is the result of a Mann-Whitney U-test on two independent samples of respective sizes n1 and n2.

One issue with the word identification task is that participants may, in congruent trials, guess the correct word simply by randomly producing a word semantically associated with the cue, whereas this is not true in incongruent trials. To correct for these guesses in congruent trials, we calculated for each participant and at each stimulus duration the percentage of incongruent trials where they produced the matching semantically associated word (resulting in a special type of error), and subtracted these estimates from word identification performance in congruent trials.

Conditional performance described in the last panel of all figures requires two methodological precisions. Performance at the case or position task was calculated for trials where the word was correctly identified. In the congruent condition this required discarding the "correct-metric ranges from 0.5 (ratings distributed similarly whether trials contained a stimulus or not)

to 1 (perfect discrimination between stimulus-present and stimulus-absent trials) (Figure

III-2c).

A repeated measures two-way ANOVA revealed a significant increase in detection sensitivity with target duration (F(3.00,42.00)=92.00, p<0.001), and congruence (F(1.00,14.00)=15.79, p<0.005) with an interaction between these two factors (F(3.00,42.00)=3.62, p<0.05). Bootstrapped 95% confidence intervals of the mean difference between congruent and incongruent conditions revealed a significant difference in performance for the two longer durations, after correction for multiple comparisons (target dur. (95%BCI [-0.14, 0.15]). This result demonstrate that, in agreement with our prediction, participants were not able to discriminate the visual properties of the masked stimulus any better with the help of a semantic retro-cue, contrasting with a clear improvement in detecting and identifying the word. The classical frequentist analysis gave the same conclusion.

The apparent discrepancy between the effect of cue congruence on the identification of the word and the discrimination of its features was further examined by means of our Bayesian model, which was fitted on the joint distributions of word identification and visual discrimination responses across all target durations (see Methods). This allowed us to estimate a measure of the association between the expected accuracies of the two responses (Figure 9) of the model). This measure describes how likely participants are to either be correct/incorrect at both tasks, rather than be correct at one and wrong at the other. Reported values in Figure 2e can be expressed as how many times more likely participants are to have coherent information about the stimulus, rather than incoherent. This measure of correlation significantly varied with congruence, in favor of higher exp() values in the incongruent condition (95%BCI [-1.29, -0.61]). In incongruent trials, participants were up to 3 times more likely to have a similar performance in word identification and case discrimination than dissimilar performances, whereas in the congruent condition this likelihood did not significantly differ from one, meaning that they were as likely to be correct at both tasks than they were at only one of them. These results suggest a decorrelation between participants' ability to report visual feature and word meaning specifically when the cue was congruent.

III-2e, see exp () in equation (
Our last analysis further investigated the origin of this decorrelation. According to our hypothesis, congruent retro-cues trigger conscious access to the latent semantic representation of a previously unconscious word, while conscious access to its visual features is no longer possible due to masking. In this hypothesis, the decorrelation between the two tasks in congruent trials is due to these "retro-perceived" trials. We computed the conditional probability of correctly reporting the word's case when its meaning was correctly identified.

We used a correction for guesses in the congruent condition further described in the Methods section: we used an individual visibility threshold above which we considered most guesses to be excluded, so that they were all "conscious trials" (see details in Methods) (Figure III-2f). A 95% bootstrapped confidence interval of the mean difference revealed for these word-correct trials a significant decrease in performance for case discrimination in the congruent condition (CI = [3.33,10.00]). Since the estimated performance in congruent is a mixture of trials that would have been perceived with no help of the cue (as in incongruent trials) and retroperceived trials, we also computed an estimate of performance in retroperceived trials only (see Methods).

This estimate goes down to 69.2% average correct case discrimination, with a confidence interval including 50% (CI = [48.85,78.42]), compared to 86.3% in all congruent and 92.9% in incongruent trials. This suggests that, on those retroperceived trials where the visual word was identified thanks to the retrospective influence of the congruent cue, participants were almost at chance at reporting the word's case. In conclusion, this first experiment validates the counterintuitive prediction that a retrospective semantic context can yield awareness of a "freefloating" word, devoid of visual attributes.

III.4.2 Experiment 2

In order to test the generality of this proposition we conducted a second experiment testing another fundamental visual property of the stimulus: its location in space. In experiment 2, case discrimination was replaced by position on-screen, with two streams of masks appearing above and below the central fixation point. Across trials the target appeared randomly in one or the other stream (Figure III-3a). Participants were instructed to report the position of the target word, then read the target out loud into a microphone, and provide hybrid visibility/confidence ratings after each response. They were instructed to respond "0" when they had not seen anything besides the masks, and otherwise to rate their confidence in the response. More information on visibility ratings is provided in the Methods section (see Supplementary Note 1 for the full instructions). Note that the order of the tasks was switched compared to experiment 1, in order to control for an effect of task order on performance. Eighteen participants took part in this experiment. [67.21, 81.98]), compared to 86.8% in all congruent and 97.8% in incongruent trials.

Overall, Experiment 2 showed the same pattern of results, even when tasks were switched and position was used instead of case. When provided with separate visibility/confidence ratings for both dimensions of the stimulus, participants were more confident for congruent stimuli than incongruent ones, even when asked to rate specifically their confidence of the word's position, which they did not discriminate any better. This might reflect "overconfidence" in position discrimination performance in retro-perceived trials, a form of illusion of knowing where the word had been presented in these trials. Alternatively this could be due to a confidence leak between the identification and the position tasks [START_REF] Rahnev | Confidence leak in perceptual decision making[END_REF], but task-order is counter-intuitive for this explanation, as confidence about position is provided before confidence about the word. Finally it could also be explained by the hybrid nature of our visibility/confidence scale.

III.4.3 Experiment 3

In order to determine whether or not there is an illusion of having seen the visual properties of the stimulus in retro-perceived trials, we conducted a third experiment with the following changes: case discrimination was used again, with two separate confidence ratings The conditional probability of correctly reporting the word's case when it was identified and participant were confident on their identification (see details in Methods)

(Figure III-4g)
was also in favor of a decrease in performance for case discrimination on those "conscious" trials in the congruent condition (CI = [2.44,7.52]), with an estimated performance in retroperceived trials of 60.1% that was not statistically different from 50% (CI = [39.7,71.6]), compared to 80.1% in all congruent and 85.2% in incongruent trials.

Overall, Experiment 3 replicated our main finding, which is that retrospective context allows for delayed conscious access to a visual word's meaning, but not its visual features.

Changing the instructions from hybrid visibility/confidence to confidence proper allowed to clarify our interpretation: while Experiment 1 showed an increase in participants' ability to detect the presence of the stimulus with congruence, Experiment 2 further confirmed that a participant's ability to detect the stimulus was not dimension-dependent, so that they reported high visibility for all of a stimulus' features when it was present, regardless of average performance. Experiment 3 gave the added information that this is not true for confidence:

while congruence increases performance at the word identification task along with improved confidence judgments when compared to incongruent trials, it is different for case discrimination: performance doesn't increase, and neither do confidence ratings when compared to incongruent trials.

III.4.4 Partial awareness control

One criticism that can be addressed at this point concerns whether participants, because the stimuli are at the perceptual threshold, perceived one or a few letters of the target stimuli and were able to guess the correct answer in congruent trials based on this information. In this case, the effect evidenced here would be partial awareness of sensory information of stimuli with the help of which participants inferred the correct meaning, rather than isolated perception of amodal features. We will discuss the different arguments for partial awareness of sensory information in the following paragraphs. We will rely on an example word "hedgehog" with varying case. We will also distinguish between lucky guesses, where participants saw nothing, guessed and succeeded, and informed guesses, where participants used partially perceived information together with the cue to venture a guess.

III.4.4.a Argument 1: the retro-cue does not affect sensory perception but informs judgement.

One possibility is that participants could have perceived all visual features of parts of the stimuli (individual letters or recurring substrings) down to V1, including case and location ("?D??HOG" -> cue -> guess) before cue onset, and made an informed guess thanks to the cue.

In this case we would expect performance at case discrimination task to be the same in congruent and incongruent, and informed guesses to increase in congruent trials. This would entail that letter and case perception always precede semantics in informed guesses. Our model however predicts a higher probability of getting only one answer correct in congruent trials after accounting for lucky guesses (Figures f in the main text). Likewise, the probability of having the visual feature correct in word-correct trials is lower in congruent trials than in incongruent trials after accounting for lucky guesses, when this number should not be different if the cue did not affect perception of low-level features (Figures g in the main text). As such, we believe we can reject this hypothesis.

Another possibility is that participants perceived the "letter form" or "word form" of the stimulus, which is case and position insensitive ("?D??HOG/?d??hog" -> cue -> guess) before cue onset, and used this information to make an informed guess. In this case, we would expect the same performance at the case discrimination task in congruent and incongruent trials, but an increase in informed guesses in congruent ones. We would also expect, in word-incorrect trials (where informed guesses were incorrect), letter or word-fragment perception to be: independent of congruence because the cue does not affect perception, and independent of case perception because partial awareness only concerns letter or word-form. We calculated the relative orthographic similarity between response and target for each trial by using the Levenshtein distance (see Methods) and used this metric to assess word fragment or letter perception.

Figure

III.4.4.b Argument 2: the retro-cue affects perception of sensory features

Another duo of possibilities concerns a possible retrospective effect of the cue on sensory perception. The first possibility, which is that the retro-cue would allow for retroperception of all features down to V1 (???? -> cue -> "?D??HOG" -> guess) is easily discarded by the fact that participants should in this case be generally better at case discrimination in congruent trials, which is not evidenced here.

The second possibility is that participants retro-perceived the letter or word-form of the stimulus (???? -> cue -> "?D??HOG/?d??hog" -> guess), and used this information to make an informed guess. In this case, we would expect letter or word-fragment perception to be

III.5 Discussion

We have shown that retrospectively cueing a masked word using a semantically related auditory cue increases participants' ability to detect and report the target's meaning. However, this was not true for the visual features of our target words: the cue had no effect on the ability to report the target's case or location. Retrospective cueing thus resulted in conscious access to high-level semantic properties of the stimulus with no access to its basic sensory features. This suggests that a retrospective semantic context can bring into awareness the semantic trace left be a previous masked stimulus even if the masks have destroyed the low-level representations of this stimulus (Figure III-1). This validates a strong prediction of the proposition that conscious access mechanisms operate independently from the build-up of sensory representations of a stimulus.

An important line of research on visual masking has shown that, when visual words are masked, participants can sometimes retain partial awareness of these words: on some trials they may consciously see a few letters of the word, and combine this information with a following related word to guess the identity of the masked word (Kouider et al., 2007;[START_REF] Kouider | Partial awareness creates the "illusion" of subliminal semantic priming[END_REF]. This partial awareness effect, although present at longer target durations cannot account for the present findings, as argued in the relevant section.

Several previous studies have shown that a retrospective context can influence the lexical processing or identification of a preceding masked prime word [START_REF] Bernstein | Semantic priming : Subliminal perception or context?[END_REF][START_REF] Briand | Retroactive semantic priming in a lexical decision task[END_REF][START_REF] Dark | Semantic priming, prime reportability, and retroactive priming are interdependent[END_REF][START_REF] Neely | Semantic priming effects in visual word recognition : A selective review of current findings and theories[END_REF][START_REF] Thomas | When word identification gets tough, retrospective semantic processing comes to the rescue[END_REF][START_REF] Vanvoorhis | Semantic matching, response mode, and response mapping as contributors to retroactive and proactive priming[END_REF] but none of them has investigated whether conscious perception per se was modified by the retrospective context word, or tested sensory attributes of the prime word. Here, we directly addressed conscious perception by collecting subjective visibility on each trial. Both mean subjective visibility, and the objective detection sensitivity measure that derives from it, demonstrate that congruent retro-cue directly improve conscious perception of the masked word: it increases the number of trials where participants report that they have seen the word.

Since this effect is observable in better objective detection d', this cannot be due to a bias in responding that one has seen: it corresponds to genuine conscious perception.

This improved conscious perception of the word identity contrasts with an absence of information on visual features: on those trials where a congruent retro-cue allowed conscious perception of the word identity, participants were unable to report its case or position on the screen, validating the hypothesis that conscious access can occur after the visual properties of Such asensory conscious perception raises fascinating questions for future research, and notably how do participants know that this asensory perception actually comes from the external environment? How come they are confident, and rightfully so, that the stimulus was there although they have no associated visual representation of this stimulus? According to our schematic model, the latent semantic trace left by the masked word is reactivated by contextual influence and this reactivation allows broadcasting this representation to a global workspace of areas associated with conscious access. But in principle spontaneous fluctuations in the activity of semantic areas independently of any external input could also take the form of a "mock trace" that could be mistaken for a trace left by an actual external stimulus. One possibility is that a genuine trace left by a stimulus might be more precise in its encoding than spontaneous fluctuations and that this difference in encoding is the index used to produce a confidence judgment about the presence of an external stimulus, even in the absence of any associated visual information.

The present results also interrogate the relationship between awareness and sensory integration. It shows that an external stimulus can be consciously perceived even when its different features are not bound together, and even if some of its basic features are actually missing. However it does not allow concluding definitively on the role of top-down attention and conscious access mechanisms on feature integration [START_REF] Treisman | A feature-integration theory of attention[END_REF]. Whether feature binding occurs pre-experientially or not is still very much debated [START_REF] Bouvier | Visual Feature Binding Requires Reentry[END_REF][START_REF] Chen | Color-motion feature binding occurs unconsciously[END_REF][START_REF] Keizer | Consciousness is not necessary for visual feature binding[END_REF][START_REF] Koivisto | Relationship between visual binding, reentry and awareness[END_REF].

One could argue that current results provide some evidence in favor of the idea that representations are not bound pre-experientially [START_REF] Treisman | A feature-integration theory of attention[END_REF], otherwise all features should reach consciousness together at the time of retrospective cueing. However, it can also be argued that because of the short delay between target and pattern mask, there was not enough time for recurrent processing to occur before low level information was erased (Di Lollo, 2018; V. A. F. Lamme & Roelfsema, 2000), and that if recurrent processing had occurred normally, it would have bound the features pre-experientially.

This experiment, in line with others on retro-perception [START_REF] Sergent | Cueing attention after the stimulus is gone can retrospectively trigger conscious perception[END_REF][START_REF] Thibault | Retrospective attention gates discrete conscious access to past sensory stimuli[END_REF][START_REF] Xia | Retrospective triggering of conscious perception by an interstimulus interaction[END_REF], shows that it is possible to influence whether a stimulus reaches awareness or not, even several hundred milliseconds after its disappearance. This highlights high flexibility in the way external events are perceived, not only in terms of whether an event is accessed or not, but goes to show that what is accessed is also subject to change according to the current context. Our experimental findings can be viewed as evidence that consciousness is an intrinsically dynamic process. Perceived events are not sequentially processed, bound and propelled through experience after waiting in line: rather it seems that different features can become conscious if and when they become relevant according to the ongoing context.

In conclusion, this study demonstrates the extreme flexibility of the form that can take conscious perception of an external stimulus, both in terms of timing and in terms of content.

Conscious perception of a word can be reduced to its most essential property: its meaning. This can be seen as a form of quintessential conscious access: we know that a stimulus has been presented; we know what it is; we don't know what it looks like. These results therefore challenge our understanding of the relationship that conscious experience entertains with external events, and interrogate the very nature of what we call a conscious percept.

IV.2 Introduction

The neural mechanisms underlying conscious access have been extensively studied during the last few decades. Conscious perception, whether visual, auditory or tactile is associated with increased activity in sensory areas (Dehaene, Naccache, Cohen, Bihan, et al., 2001;[START_REF] Sadaghiani | Distributed and antagonistic contributions of ongoing activity fluctuations to auditory stimulus detection[END_REF][START_REF] Super | Internal state of monkey primary visual cortex (V1) predicts figure-ground perception[END_REF], a widening of the network involved in stimulus processing, including frontal, parietal and cingulate areas [START_REF] Boly | Baseline brain activity fluctuations predict somatosensory perception in humans[END_REF]Dehaene, Naccache, Cohen, Bihan, et al., 2001;[START_REF] Sadaghiani | Distributed and antagonistic contributions of ongoing activity fluctuations to auditory stimulus detection[END_REF], as well as increased connectivity [START_REF] Gaillard | Converging intracranial markers of conscious access[END_REF][START_REF] Melloni | Synchronization of neural activity across cortical areas correlates with conscious perception[END_REF] and long-term maintenance of the information [START_REF] Del Cul | Brain dynamics underlying the nonlinear threshold for access to consciousness[END_REF][START_REF] Gaillard | Converging intracranial markers of conscious access[END_REF][START_REF] Sergent | Timing of the brain events underlying access to consciousness during the attentional blink[END_REF].

Most of these studies have been conducted with healthy participants who reported whether they were aware of the presented stimulus [START_REF] Aru | Distilling the neural correlates of consciousness[END_REF][START_REF] Sergent | Imaging neural signatures of consciousness:'What','When','Where'and 'How'does it work?[END_REF], and this metric was used to contrast conscious and unconscious conditions. Although this methodology has allowed a breakthrough in research on the neural underpinnings of consciousness, it is now criticized for two reasons: first, although we are aiming at understanding the neural mechanisms of conscious access in general, the fact that the participants have to report their perception confounds our correlates of conscious perception with attentional and decisional processes linked to the task of reporting; this is highlighted by recent evidence shows that providing a behavioral report has an important effect on neural activity during stimulus processing [START_REF] Pitts | Gamma band activity and the P3 reflect post-perceptual processes, not visual awareness[END_REF][START_REF] Tsuchiya | No-report paradigms : Extracting the true neural correlates of consciousness[END_REF]. The second, more practical reason is that one of the aims for finding neural correlates of conscious access is to improve diagnostic and prognostic tools for the evaluation of patients with disorders of consciousness (DOC), who cannot provide explicit reports [START_REF] Giacino | Disorders of consciousness after acquired brain injury : The state of the science[END_REF][START_REF] Giacino | The JFK Coma Recovery Scale-Revised : Measurement characteristics and diagnostic utility[END_REF][START_REF] Laureys | The neural correlate of (un) awareness : Lessons from the vegetative state[END_REF][START_REF] Sergent | Multidimensional cognitive evaluation of patients with disorders of consciousness using EEG : A proof of concept study[END_REF][START_REF] Sergent | Imaging neural signatures of consciousness:'What','When','Where'and 'How'does it work?[END_REF]. Our current challenge is to go beyond correlates in the presence of a report, and look for signatures which do not involve any behavioral output from the participant, but still accurately reflect whether a stimulus was consciously perceived or not.

A good candidate may be found in the investigation of cerebral dynamics in response to stimuli of increasing intensity. Indeed, several studies have identified a sharp threshold separating conscious vs. unconscious perception at late stages of processing (beyond 200-250ms post-stimulus). This has been observed first in attentional blink experiments [START_REF] Dehaene | A neuronal network model linking subjective reports and objective physiological data during conscious perception[END_REF][START_REF] Marti | Discrete and continuous mechanisms of temporal selection in rapid visual streams[END_REF][START_REF] Sergent | Timing of the brain events underlying access to consciousness during the attentional blink[END_REF][START_REF] Sergent | Is consciousness a gradual phenomenon? Evidence for an all-or-none bifurcation during the attentional blink[END_REF]. In one of these studies, for example, the relationship between activity at different epochs was examined as a function of subjective reports. They found that, while early evoked activity in EEG in response to a stimulus was unrelated to trial-by-trial subjective reports, the visual N2 wave (270ms after stimulus onset) displayed a positive linear relationship with subjective ratings, and the visual P3b wave (300ms) displayed a bimodal pattern: it was only present in trials where participants reported visibility above 50% [START_REF] Sergent | Timing of the brain events underlying access to consciousness during the attentional blink[END_REF]. Indications of bimodal dynamics have also been found in masking experiments, where the amplitude of the first EEG waves increased linearly with the stimulus strength with no specific disruption around the masking threshold, while later activity (> 250ms) showed a non-linear increase with stimulus strength, with an inflection point around threshold [START_REF] Del Cul | Brain dynamics underlying the nonlinear threshold for access to consciousness[END_REF][START_REF] Kouider | A neural marker of perceptual consciousness in infants[END_REF] suggesting that the underlying activity split bimodally according to whether the stimulus was above threshold (conscious) or below (unconscious). Interestingly this property was also observed in EEG recordings of preverbal infants, who did not report the stimuli they were seeing [START_REF] Kouider | A neural marker of perceptual consciousness in infants[END_REF].

To test whether bimodal dynamics might constitute a reliable signature of consciousness, it is important to generalize it to sensory modalities other than vision, and validate the fact that it does not disappear in the absence of explicit report [START_REF] Tsuchiya | No-report paradigms : Extracting the true neural correlates of consciousness[END_REF]. Furthermore, previous studies on masking inferred bimodal dynamics based on non-linear increase in signal magnitude with stimulus strength, but whether this inference is correct remains to be formally investigated, as we will show below.

To address these topics we developed a modeling framework and designed a new experimental protocol that was first conducted in electroencephalography (EEG) in healthy participants, and replicated in DOC patients as well as in intracranial stereotaxic EEG (sEEG).

The present chapter presents the experiments conducted in sEEG. In this experimental protocol, The stimuli were French vowels ("a" or "e") at six different intensities around the perceptual threshold (expressed in signal-to-noise ratio SNR): they either ranged from -3dB (highest intensity) to -11dB (lowest intensity) by steps of 2dB, or from -5dB to -13dB steps of 2dB, depending on patients' threshold, evaluated during the training block. Fourteen patients underwent the Active session, where they had to report the identity and audibility of the target stimulus on each trial. 12 of them agreed to continue on with the Passive session, in which they passively listened to the same sounds, and had to perform a distractor task on visual stimuli (see Materials and Methods).

This investigation has two aims: replicating the initial results obtained in scalp-EEG in healthy participants, and better understand the cerebral origin of this bimodal dynamic, initially detected in scalp EEG, as well as potentially refine our method for detecting the wanted pattern.

Indeed, in order to identify indicators of a bimodal dynamic between conscious and unconscious states, we relied on a modeling framework, initially developed for investigating scalp-EEG recordings, which contrasted two types of dynamics (Figure IV-1). First, the strength of some cerebral activity may increase non-linearly with SNR level, but in a unimodal manner (Figure IV-1a). Indeed, there is no guarantee that the physical measure we use for expressing stimulus intensity (in this instance SNR of the stimulus over the noise in dB scale) maps linearly onto our "internal" sensitivity scale [START_REF] Green | Signal detection theory and psychophysics[END_REF]. In this case, different levels of stimulation would lead to as many unimodal distributions whose means would increase non-linearly with stimulus strength. A second possibility is that activity could display a bimodal pattern, split between conscious and unconscious trials (Figure IV-1b). In this case, the SNR level would act on the proportion of trials in one or the other state. Simulations of these models show that both models can lead to similar pattern in mean activity with stimulus strength, making it a poor diagnostic for differentiating the two models. In contrast, the predicted variability across trials is highly different in both cases. In the unimodal situation we expect that signal variability across trials (standard deviation SD) should either remain constant or increase monotonically with stimulus strength. In the bimodal model, in contrast, because trials are split between two distributions of different means when stimulation is at threshold, the standard deviation across trials will increase if trials don't belong to only one of these distributions. Away from threshold SNR, variability declines as activity across trials becomes more homogenous towards "non-conscious" or "conscious" activity levels. Therefore, we make the original prediction that the specific signature of bimodal or "step" dynamics should be found

IV.3.2 Stimuli

The stimuli were two vowels, used in a previous scalp EEG experiment conducted within the lab, "a" and "e", of the same duration (150ms). They were synthesized using the MBROLA software (Dutoit, Pagel, Pierret, Bataille, & van der Vrecken, 1996) with the following parameters: allophones 'a' and 'e' from the FR4 database (French female voice), duration of 150ms, 200Hz pitch at 50% of the duration.

Vowels were superimposed on a background noise that equalizes the masking of all frequencies for human ears (Threshold Equalizing Noise or TEN;Moore, Huss, Vickers, Glasberg, & Alcántara, 2000). The background noise was present without interruption for the whole duration of a block of trials. We generated enough noise to avoid any repetition during the experiment.

The stimuli were presented at five sound levels in terms of signal-to-noise ratio (SNR) ([-13dB, -11dB, -9dB, -7dB, -5dB] in the active session, or [-11dB, -9dB, -7dB, -5dB, -3dB] in the passive session). For three participants, the SNRs chosen in the active session were [-11dB, -9dB, -7dB, -5dB, -3dB] for reasons described in the Task section of the Methods (see also Supplementary Table VI-3). The behavioral threshold for detecting these stimuli was experimentally determined to be around 9dB in healthy participants. Some trials were composed of noise alone, without superimposed stimuli.

IV.3.5 Electrode localization

To assess the electrodes location, the following procedure was applied (Perez-Garcia 2015):

Patient's DICOM files were converted to NIfTI, after which the posterior and anterior commisures (AC, PC), as well as the interhemispheric point (IH) were interactively localized using BrainVisa Anatomist [START_REF] Rivière | Anatomist : A python framework for interactive 3D visualization of neuroimaging data[END_REF]. Data from Site 1 were then processed as follows: structural MRI pre-and post-implantation (pre-MRI and post-MRI respectively) were realigned. Electrode coordinates were measured in the post-MRI, and exported to the pre-MRI where electrode artifacts are absent, and precise localization can be achieved. In Site 2, BrainVisa Morphologist Pipeline was used to automatically segment brain structures from the pre-MRI, after which the post-MRI and post-CT (computed tomography) were realigned to the pre-MRI using BALADIN [START_REF] Isambert | A phantom study of the accuracy of CT, MR and PET image registrations with a block matching-based algorithm[END_REF]. Electrode contacts were determined on the post-CT.

IV.3.6 Data analysis

IV.3.6.a Behavior

Analyses were performed on mean % correct trials at the vowel identification task, mean audibility ratings, and standard deviation of audibility ratings.

IV.3.6.b sEEG pre-processing

Intracranial data were acquired with an audio-video-EEG monitoring system, all with Micromed with a sampling rate of either 1024Hz (site 1) or 4096Hz (site 2), except for one participant who was recorded at 512Hz (Patient 2, site 1, see Supplementary Table VI-3). The continuous data were first filtered using a 0.1Hz to 250Hz bandpass and [50 100 150] Hz bandstop fourth-order Butterworth filters in forward and reverse directions to avoid phase-shift (for data from Site 2, the band-stop filter was 3 rd order to avoid instability). These were then resampled to a common sampling rate of 1024Hz. Epochs were extracted from -800 to 1200 ms relative to the onset of the auditory stimulus. For trials with no vowel, a fictitious sound start was chosen with the same constraints as for sound-present trials. Data from each contact were re-referenced to their nearest neighbor on the same electrode. These bipolar montages will be referred to as "electrodes". Visual inspection was then performed, to discard trials with epileptic activity, noisy or pathological channels. Secondly, signal variance across all trials and time-points was assessed for each electrode, to verify they did not cross the threshold of 10 5 of variance overall, which would have indicated the presence of obvious yet missed artifacts (either a damaged electrode or strong epileptic activity). In this case, visual inspection was repeated to verify this was not due to the presence of a few artefactual trials that were missed during the first inspection. If so, these trials were discarded, otherwise these electrodes were discarded. When trials were discarded, they were discarded for all electrodes (mean = 8±5, range = 0 to 19 discarded trials, mean = 9±9, range = 0 to 34 discarded electrodes). These preprocessing steps were performed using the FieldTrip toolbox [START_REF] Oostenveld | FieldTrip : Open source software for advanced analysis of MEG, EEG, and invasive electrophysiological data[END_REF].

IV.3.6.c Electrode selection

Event-related potentials (ERPs) were calculated by averaging epochs in each condition (SNRlevel). A baseline correction was applied using the window [-0.8s 0s] and subtracting its average from data. To assess which electrodes displayed evoked activity in response to our auditory stimuli, we compared the SNR-max condition to the no-sound-condition by performing a cluster-based non parametric test (Monte-Carlo method) using the FieldTrip toolbox [START_REF] Oostenveld | FieldTrip : Open source software for advanced analysis of MEG, EEG, and invasive electrophysiological data[END_REF]. It included temporal neighbors, but no spatial neighboring was applied, meaning that correction for multiple comparisons was performed across time for each electrodes, but not across electrodes. This test performed 1000 permutations and computed sums of t for each cluster. Effects were considered significant if they were observed in less than 5% of surrogate permutations.

IV.3.6.d 3-D visualization

3-D images of electrodes are represented in MNI-space in the TopographX software, in development within the lab in collaboration of Antoine Dieulesaint, in Unity 3D. The MNI ICBM-152 template used was derived from FreeSurfer (http://surfer.nmr.mgh.harvard.edu/), and is the same as the one used in BrainNet Viewer (https://www.nitrc.org/projects/bnv/). As TopographX was developed in Unity 3D, we were able to make dynamic visualization of our dataset available online for the reported analyses (https://daphnerr.github.io/), which is particularly useful for sEEG data examination. This visualization tool will be made available online for download in a standalone version, and will also be open to external contributions and suggestions at a later time, along with a dedicated tutorial. The current (beta) version of

TopographX is available at https://github.com/DaphneRR/TopographX as a standalone version.

IV.3.6.e Decoding

Multivariate pattern analysis (MVPA) across electrodes was conducted with the MNE-Python toolbox [START_REF] Gramfort | MEG and EEG data analysis with MNE-Python[END_REF][START_REF] Gramfort | MNE software for processing MEG and EEG data[END_REF], using specific tools for temporal decoding and generalization in time (Dehaene & King, 2016). This analysis was performed by training a linear classifier to decode the presence vs. the absence of a sound, by contrasting the no-sound condition with the SNR-max condition. We wished to assess how well the decoded pattern across electrodes would generalize to other SNR levels, in time, and to the passive session. The procedure was the following, repeated for each patient: in a 6-fold cross-validation setup, the classifier was trained on 5/6 th of no-sound and SNR-max dataset using a logistic regression.

The remaining 1/6 th of no-sound and SNR-max trials were used for testing the quality of decoding. We then generalized to other SNRs by testing the trained classifier to 1/6 th of data from other SNRs (generalization across conditions). This procedure was repeated 6 times (cross-validation steps) by interchanging which subset of data was used for training and testing.

From this analysis, we could extract for each trial a prediction value equal to the distance from the classifier's decision plan. This value was all the more positive when it was predicted there would be a vowel in the examined trial, and negative when the reverse was predicted. This metric was used to estimate classification sensitivity at each cross validation step by comparing the distributions of prediction values for target-present for each SNR and target-absent trials, out of which we could derive the area under the receiver operating curve (AROC) by using a signed-rank test. These measures were averaged across cross-validation steps to calculate the global classification sensitivity for each SNR. Prediction values were kept to test our different dynamical models and predictions. This procedure was applied to each time step independently.

IV.3.6.f Generalization in time Temporal generalization of the classifier pattern was performed by testing at all other time steps the pattern derived from one of them, and repeating this for each time step. Temporal generalization was performed on the reference contrast: no-sound vs. SNR-max. To reduce computation time, we applied a decimation factor of 5. The procedure was otherwise the same as described above (Dehaene & King, 2016).

IV.4 Results

IV.4.1 Behavioral reports during the active session

In the active session, participants reported on their experience. They were asked to identify the stimulus ("a" or "e") and rate its subjective audibility on an 11-point scale (0 to 10). Vowel identification performance followed a logistic profile, with an inflection point around -11dB

(Figure IV-3a). This corresponds to the perceptual threshold. Mean subjective ratings followed a similar curve in all patients regardless of the SNR range used (-13 to -5dB for 10 patients, For the three patients who were presented with higher SNRs, the profiles of variability were less consistent, with peak variability at -11dB, -5 dB or -3dB.

IV.4.2 General pattern of evoked response to sounds in the

Active and Passive sessions

In order to detect the electrodes that were responsive to our stimuli, we contrasted stimulus absent trials to the loudest stimulus-present condition, for each electrode and each participant (total number of electrodes tested after exclusion from preprocessing: 1128; for an initial number of electrodes of 1291). An electrode was considered to be responsive to our stimuli if it showed a significant cluster of at least one sample over the considered time period -38.4, 20.6, 13.1] and [-34.7, 20.8, 13.3]). See Online Visualizations 1&2 at https://daphnerr.github.io/ for interactive displays and exploration of the data shown in Figure IV-4.

In the active session, processing started as expected in Heschl's gyrus at around 100ms, the locus of the primary auditory cortex (A1), and gradually extended to other locations in the superior temporal gyrus before reaching frontal and occipital areas. Activity in many areas including frontal and temporal electrodes were strongly sustained in time, until the end of the recorded time period (1.2s) (Figure IV-4b, left panel). A large hemispheric asymmetry was evidenced, in favor of more activity in the left hemisphere. It is known that auditory processing is not equivalent in both hemispheres [START_REF] Belin | Lateralization of Speech and Auditory Temporal Processing[END_REF][START_REF] Gage | Auditory evoked M100 reflects onset acoustics of speech sounds[END_REF][START_REF] Hickok | Dorsal and ventral streams : A framework for understanding aspects of the functional anatomy of language[END_REF][START_REF] Robin | Auditory perception of temporal and spectral events in patients with focal left and right cerebral lesions[END_REF]. In fact, the left auditory cortex is known to respond more specifically to speech-related sounds [START_REF] Morillon | Neurophysiological origin of human brain asymmetry for speech and language[END_REF], which could explain the evidenced pattern. See Online Visualization 3.

In the passive session, processing displayed a similar pattern with early involvement of electrodes in Heschl's gyrus around 100ms, and an extension to other temporal areas.

Processing involved fewer regions of the cerebral cortex, but was surprisingly sustained in time, with most responding electrodes still active after 500ms, and some up to the end of the recorded period. In this session, recruited electrodes were mostly located in the temporal cortex, with a smaller involvement of frontal electrodes in Broca's area ( Sixteen percent of all electrodes (180 over 1135) displayed significant activity in the Active session. In the passive session, this number lowered to 49/1135 (4%). Individual metrics are available in Supplementary Table VI-3. Note that implantations were relatively balanced over the two hemispheres, with 55% of contacts located in the left hemisphere. Nonetheless, the cerebral response to a stimulus seems to be strongly left-lateralized.

IV.4.3 Classification and temporal generalization

We performed a multivariate classification analysis, which helped characterize the different stages of processing by using the combined information of all electrodes (Figure IV-5).

Although this analysis is not commonly used in sEEG, because location information about the source of activity is lost, we used it in this case to assess the dynamics of activity patterns across all electrodes. Indeed, one of our assumptions is that bimodal dynamics should arise from the interaction of different sites across long-distance connectivity, and thus should be detectable locally in each cortical site taking part in this interaction, but also globally as a cross-site pattern [START_REF] Marti | Time-resolved decoding of two processing chains during dual-task interference[END_REF]. This matches the pattern evidenced from the previous analysis

IV.4.4 Mean and variability profiles over decoded patterns

The classification prediction values obtained for each trial in this multivariate classification analysis provide a general index of stimulus-evoked brain activity trial by trial.

For the two selected patients we derived these prediction values for all SNRs (generalization across conditions, see Methods) and studied how mean prediction values but also variability of prediction values across trials varied with SNR, in order to infer the underlying dynamics (see process. This pattern is sustained throughout the tested period (600ms). In the passive session, this patient also displays the bimodal pattern, although with a higher threshold SNR at around -9dB, as could be expected: indeed, when stimuli are not voluntarily attended, one expects the conscious access threshold to occur for higher stimulation levels [START_REF] Fritz | Auditory attention-focusing the searchlight on sound[END_REF]. This pattern is visible between 200ms and 300ms, and is less clear for later time windows. These observations remain to be formally tested by developing adequate statistical tests for non-monotony of variability pattern within a single subject.

In patient 6 (Figure IV-6b), the non-monotonous pattern can be visually identified starting from the 200ms time window, with a peak of variability observed around -9dB. The passive session however is far less clear, although the bimodal pattern may be presented between 300 and 500ms. Definite conclusions cannot be drawn from simple inspection, and require additional testing.

Overall, the two selected patients appear to display some evidence of the hypothesized bimodal process in the decoded prediction values. an increase in response delays as we moved further from the primary cortex.

IV.4.5 Evoked potentials at selected cortical sites

The electrode located in the left Heschl gyrus (green) showed a sharp transient response at 80ms, followed by a sharp response with a sign inversion, suggesting that the cortical source moved relative to the contact point. No sustained activity was observable beyond 400ms.

The electrode located in the left middle temporal gyrus (lighter blue) showed a sharp transient response starting at 100ms, also followed by an inversion, and then a third phase of activity, beyond 400ms, that was sustained in time. Whereas the magnitude of activity in the initial two phases seemed to scale with stimulus strength (just as in Heschl's gyrus), in this last phase beyond 400ms is more ambiguous and could be the result of a bimodal process.

The electrode located in the left insula showed a sharp response at 140ms followed by a sustained response, first graded then seemingly bimodal between low SNRs and high SNRs, with a clear distinction from the no-sound condition.

Finally frontal areas started responding later, after 200ms, with no sharp transient but rather a sustained response up to 800ms post-stimulus. It is difficult to assess from ERPs alone whether the underlying dynamics across trials are unimodal or bimodal, since it is poorly reflected in mean activity. We thus moved on to more informative metrics, and computed the mean and standard deviation profiles for each electrode over time windows of interest.

IV.4.6

Mean and variability profiles at selected cortical sites As for the variability profiles, during the first response (around 100ms) variability increased with SNR, but there was no clear indication of a peak in variability at intermediate threshold that would have signed bimodal dynamics. This suggests that this very early phase of sensory processing followed unimodal linear dynamics, as suggested in previous studies (Kouider et al). During the second phase of response in Heschl's gyrus, the variability across trials is ambiguous, but points towards the conclusion that the underlying dynamics were unimodal.

At the temporal superior site (Figure IV-7 second line) our time windows of interest capture a unimodal initial response between 150 and 250ms as in Heschl's gyrus, followed by a bimodal pattern between 250ms to 400ms. The latest phase of processing, from 400ms to 600ms seems to return to a unimodal pattern.

The middle temporal gyrus site does not appear to particularly exhibit a bimodal pattern of activity at any of the time windows tested, although there appears to be a slight increase in the midpoint SNR level, which would require additional analyses to draw formal conclusions.

The recording sites in the insula and Broca's area display clear indications of bimodal dynamics for prolonged periods of time, starting from 200ms on. In this case, we have good evidence that the "inverted v" shape found in participants' behavioral profiles is found in cerebral activity, within frontal areas.

IV.5 Discussion

In the current experiment, we aimed at investigating the dynamics of neural activity associated with conscious access, directly recorded from different cortical sites using stereotaxic EEG. The behavioral patterns showed a gradual increase in identification accuracy and subjective audibility, with a sharp increase in variability in the subjective reports around perceptual threshold. As in a previous scalp-EEG experiment using the same stimulation protocol, this was found for most patients using a single range of SNRs and no psychophysical staircase, which is very convenient for future use at the patient's bedside. We identified electrodes which responded to the presented stimuli in Active and Passive sessions. Electrodes implanted in the temporal cortices responded in both in the active and passive session. However, in the active session, recruited electrodes spanned most cerebral lobes, whereas in the passive session, they were primarily confined to the temporal cortex, with some more modest insular and frontal activations. Interestingly, elicited activity was detectable from around 100ms until the end of the recorded epoch, at 1.2s, in both sessions. Hemispheric asymmetries were evidenced: this activity was largely present in the left hemisphere. Asymmetries in auditory processing are well-known, and may in our case be due to the phonological nature of our stimuli [START_REF] Belin | Lateralization of Speech and Auditory Temporal Processing[END_REF][START_REF] Gage | Auditory evoked M100 reflects onset acoustics of speech sounds[END_REF][START_REF] Hickok | Dorsal and ventral streams : A framework for understanding aspects of the functional anatomy of language[END_REF][START_REF] Robin | Auditory perception of temporal and spectral events in patients with focal left and right cerebral lesions[END_REF].

We used a linear classifier to decode the contrast "sound present at max SNR" vs. "sound absent" in order to study brain dynamics over "global" activity patterns. We found that the classifier was most effective when patients were implanted in the left hemisphere, especially when their implantation included frontal and temporal electrodes. As the ultimate goal is to find appropriate models to discriminate unimodal and bimodal variations of activity according to SNR on the classifier prediction values, we sub-selected 6 electrodes from 2 patients with the best decoded patterns, from the frontal, insular, and temporal cortices. Visual inspection of the evoked potentials in time, mean and standard deviation of activity across trials at several intervals allowed us to evidence several patterns in the data which can help design appropriate models. In variability profiles, we found that the standard deviation of activity could vary in different ways, including the "inverted v shape" which characterizes bimodal dynamics.

Interestingly standard deviation could also increase or decrease with SNR, possibly indicating a process akin to variance quenching, which has recently been related to efficient perceptual processing [START_REF] Arazi | Neural variability quenching predicts individual perceptual abilities[END_REF][START_REF] Churchland | Stimulus onset quenches neural variability : A widespread cortical phenomenon[END_REF]. The evidenced results may indicate that activity in response to a stimulus start as a unimodal process: cerebral activity increases gradually with stimulus strength. Processing moves on to different cerebral areas, to cross-modal stimuli. We replicated the known effect of cross-modal exogenous attention in pre-cueing, and showed that attention also influenced perception in retro-cueing. This effect was accompanied by faster response times, which we examined in depth by using a variation of the drift-diffusion model [START_REF] Anders | The shifted Wald distribution for response time data analysis[END_REF]. Pre-cueing the target led to generally better performance regardless of whether the cue was informative, suggesting auditory cues had a general effect of alert on perceptual processing, which had already been demonstrated [START_REF] Kusnir | Phasic auditory alerting improves visual conscious perception[END_REF][START_REF] Pollack | Maintenance of alertness by a loud auditory signal[END_REF][START_REF] Van Der Burg | Pip and pop : Nonspatial auditory signals improve spatial visual search[END_REF]. Interestingly, this effect was absent in retro-cueing. Spatial congruence of the cue also improved performance, in a similar fashion in pre and retro-cueing. Analysis of RTs using the model revealed that the alert effect acts on the perceptual decision by increasing the information accumulation rate, and by reducing the non-decision time, potentially through priming the motor response. The effect of spatial congruence, in contrast, reduced the non-decision processes only, which may indicate shortening of the time preceding the onset of information accumulation by boosting the stimulus into awareness.

V.2 Chapter 2: Limits and perspectives

While this study provides an additional replication of the original studies on retro-perception, the evidenced effects are quite small. It is not unexpected, as we know that cross-modal attentional experiments lead to smaller effect sizes than unimodal stimulation. This can also be the result of our way of locating sounds: cues are played at different intensities from both locations to mimic a source coming from the target location, which is less accurate than having the visual target and auditory cue actually come from the same location. Given that spatial resolution in audition and vision may not be exactly similar, this experiment may benefit from reversing the modalities used, in which case the cue may have a stronger effect on perception.

One interesting piece of evidence in our study is that this cross-modal effect is only present for short SOAs: we do not have any evidence of inhibition-of-return [START_REF] Klein | Inhibition of return[END_REF] in pre-cueing (-600ms), or any type of influence on later retro-cued SOAs, even though we used the +450ms SOA, at which an effect of cue congruence was evidenced in the preceding studies. Whether this is due to the size of our effect, or to a mechanism specific to cross-modal stimulation remains to be determined.

V.3 Chapter 3: Review

Our second study used the retro-perception for probing the flexibility of conscious access not only in terms of timing but also in terms of type of content. This study had two aims: extending retro-perception to high-level cueing, i.e. using semantic links between target and cue, and, crucially, probing the possibility of conscious access to high-level representations detached from low-level features by testing what happens when the low-level features of retro-cued targets are degraded. Participants were presented with visual masked words in uppercase or lowercase. Crucially, the masks were not meaningful, and we hypothesized they would leave semantic representations intact in short-term memory. We then retro-cued these targets using associated auditory cues (hedgehog -"porcupine"). In three different replications, participants were able to report the word's meaning and detect it with better accuracy with semantically associated retro-cues, however they were unable to report its visual features any better. We evaluated whether this was due to partial awareness (Kouider et al., 2007;Kouider, De Gardelle, et al., 2010;[START_REF] Kouider | Partial awareness creates the "illusion" of subliminal semantic priming[END_REF], in which participants would have seen a few visual elements of the word and consistently guessed the correct answer thanks to the congruent cue.

We examined orthographic distance, and concluded that although there is partial awareness for longer target durations, it cannot account for the evidenced effect. Finally, we found that the effect of congruent retro-cues was accompanied by a decrease in participants' propensity to be correct or incorrect at both tasks at the same time in congruent trials, suggesting partial independence in perceiving one, the other, or both features.

V.4 Chapter 3: Limits and perspectives

One lasting criticism in our study is that congruent cues can help the participants guess the correct answer, whereas incongruent cues cannot. While we estimated individually participants' tendency to guess, it remains possible that our correction for guessing overestimates the evidenced effect: we computed this correction in incongruent trials, where visual information is not helpful, and used it to congruent trials but in these, visual information is relevant for guessing, thus the guess rate may be higher. We believe we have taken the necessary precautions to claim we have controlled for this, but this experiment would benefit from finding a way to avoid correct for guesses in a definitive manner. This could be achieved by using a less associated cue to the target [START_REF] Bernstein | Semantic priming : Subliminal perception or context?[END_REF]: if the cue dog is first associated to cat at 70%, then puppy at 15%, etc., we could use the second most associated word as the target to dramatically reduce the guess rate. How much this would reduce the evidenced effect however remains to be determined.

Another interesting avenue for future research would be to replicate this finding in different experimental settings: would this work in an auditory paradigm using informational masking?

Would participants be able to identify spoken words, but unable to tell whether the speaker was male or female? This is an ongoing project within the lab, conducted by Claire Sergent and Axel Garnier-Allain.

Another protocol that could possibly circumvent the main criticism to our initial study would use masked target images of animals of differently colored hair/feathers, and cue them using their names (black cow -"cow"), associated words (black cow -"milk"), or animal calls (black cow -"moo"), as experiments show conscious access can be triggered using this type of stimuli [START_REF] Lupyan | Language can boost otherwise unseen objects into visual awareness[END_REF]. Would participants be able to retrieve the pictures' meanings, but not physical attributes of the animals, such as their color?

V.5 Chapter 4: Review

Our third study was focused on the cerebral mechanisms underlying conscious access, in the context of a global project involving scalp-EEG in healthy participants and DOC patients, as well as the present investigation in sEEG [START_REF] Giacino | Disorders of consciousness after acquired brain injury : The state of the science[END_REF][START_REF] Laureys | The neural correlate of (un) awareness : Lessons from the vegetative state[END_REF][START_REF] Sergent | Imaging neural signatures of consciousness:'What','When','Where'and 'How'does it work?[END_REF]. We aimed at finding dynamics markers of conscious awareness around the perceptual threshold in audition, in the presence or absence of report [START_REF] Tsuchiya | No-report paradigms : Extracting the true neural correlates of consciousness[END_REF].

Participants were presented with auditory vowels embedded in continuous noise, at various signal-to-noise ratios ranging from inaudible to clearly identifiable. They underwent two sessions using the same stimuli: in the first they were instructed to report their perception, and in the second, they were given irrelevant tasks in order to distract them. We found that the activations related to the presence vs. the absence of a sound were vastly different in the active and passive session, the first involving electrodes spanning all cerebral lobes, whereas the second only involved the auditory cortex and a few frontal electrodes, as far as we could tell within the restrictions of our implantation schemes. Decoding the contrast "SNR max" vs.

"sound absent" revealed that the linear classifier decoded best when participants had left implantations, particularly when these included both frontal and temporal electrodes. This was true in the active and the passive sessions. Exploratory analyses of evoked activity in several electrodes, by direct visualization and computing the mean and variability profiles at different time points showed that the expected burst of variability around the perceptual threshold could be evidenced in frontal and insular regions, but this was less clear or absent in temporal regions.

Interestingly, standard deviation could also monotonically increase or decrease with SNR.

Appropriate modeling of these different scenarios across all electrodes and time-points could prove to be very informative, and help refine models used for patient evaluation.

V.6 Chapter 4: Limits and perspectives

This study is currently unfinished, as we are still developing analysis tools to describe the rich dynamics observed in our recordings. As such, we have concentrated only on a subset of the electrodes that responded to the stimuli. Although this was informative and will help us conduct appropriate analyses in the future, it is not definitive evidence in favor or against the formulated hypothesis, which is that variability in cerebral activity would largely increase around the perceptual threshold, reflecting a bimodal pattern of activity between a "conscious" and an "unconscious" state. The analyses presented here are replications or in-depth investigation of results obtained in scalp EEG. However, intracranial EEG data can be analyzed in several other ways, which have not been performed yet. Among these, connectivity and oscillatory power analyses are first contenders. More specifically, moving from raw data to oscillatory power in a relevant frequency band may provide us with useful information about the cerebral dynamics around the perceptual threshold.

Some patients were also implanted with "micro-electrodes", which record multi-unit data at a very fine scale. These electrodes could be investigated on their own, after assessing that their locations are of interest. This could bring finer information not only on the neural dynamics of conscious access in general but also shed light on specific mechanisms underlying information coding (vowel "a" versus "e" etc.).

V.7 Conclusion

The behavioral experiments presented in the current thesis generalize retro-perception to crossmodal and high-level stimuli, and favor the theory of a "late and global" occurrence of conscious access. Indeed, we have shown that retro-perception is not restricted to unimodal stimuli, but also occurs in an audio-visual setting, with a strong effect on response time. Retroperception is possible using high-level features of stimuli such as semantics, and can be used to bring to awareness the available information pertaining to degraded stimuli. In this case, we evidenced that participants could retrieve a word's meaning, apparently detached from its sensory attributes. We believe this brings evidence that consciousness is not only flexible with regards to the actual timing of events in the external world, but also in content, in the sense that different parts of a stimulus can reach awareness, even supra-modal attributes, depending on the relative relevance of ongoing events, and available information about them. This finding brings strong support to the notion that conscious access is an amodal process. Secondly, our results seem to suggest that high-level information doesn't need to be bound to its lower-level counterpart, and we know low-level information can be accessed alone and used for inferring higher-level features (Kouider et al., 2007;Kouider, De Gardelle, et al., 2010;[START_REF] Kouider | Partial awareness creates the "illusion" of subliminal semantic priming[END_REF]. Experientially, do these percepts differ dramatically from "bound" ones, beyond metacognitive judgements? Although this was not formally probed, participants did not appear particularly surprised, or metacognitively inaccurate when they reported perceiving word meaning but not its physical attributes. Should we consider that the available information was always "misbound" to mask information, or a probabilistic estimation of its accompanying features? Or that unbound perception is a common occurrence and does not elicit surprise? This could be an interesting next step in the context of this protocol.

The last chapter presented in this manuscript investigated the neural predictions made by the Global Workspace Theory of consciousness. The results obtained strongly support the notion that no-report paradigms are of crucial importance to assess the neural correlates of consciousness, given the large differences we have evidenced between sessions. This investigation is an important first step in the process of finding NCCs reliant on cerebral dynamics in response to stimuli of different intensities rather than on the presence of a single type of response. This would provide a marker grounded in theoretical assumptions and help pave the way for more refined analyses of pathological activity in DOC patients in the future.

you

  Hamdi Habacha for coming by and saying hi every morning by the end! Thank you Stéphane Roussakovsky, Aboubacar Souane and Sabine Filiu for your administrative, technical, and personal support. Thanks to the Paris and Lyon sEEG platforms, Katia Lehongre, Benoît Chatard and Pierre Bourdillon. I thank the Fondation pour la Recherche Médicale and the Fondation Bettencourt for their financial support.

  Figure I-1: Schematic functional anatomy of the brain. Early anatomical maps used anatomical criteria to define sub-regions. From these, the brain is divided in four cortical lobes and several sub-areas. The cerebellum is a different structure thought not to have a key role in consciousness. More recent maps such as this one include functional anatomy indicated in text, that can correspond or not to well-defined anatomical regions in the brain. Function is not necessarily defined by location. (Arizona State University, 2011)

Figure I- 3 :

 3 Figure I-3: Visual word processing. Visual word processing occurs in the ventral visual stream. From the detection of oriented bars in V1, to the recognition of small words and recurring substrings in the left occipito-temporal sulcus of the infero-temporal cortex, the visual word form area. Along the hierarchy of processing, populations of neurons increase their receptive field sizes and respond to more global and complex features of stimuli, all the while losing precision in locating the stimuli they are detecting. (From Vinckier 2005).

  , inhibition (Van Gaal, Ridderinkhof, Scholte, & Lamme, 2010; van Gaal, Ridderinkhof, van den Wildenberg, & Lamme, 2009), conflict adaptation (Van Gaal, Lamme, & Ridderinkhof, 2010) etc. Unconscious processes span most cognitive functions, raising the question of what is the function consciousness. We will not speculate here, but rather ask questions that can point us in the right direction: what determines whether a stimulus becomes conscious or not, and what processes are specific to conscious processing? Answering these questions might help understand better what consciousness is, and does.

  without attention is thus very possible (Figure I-5b). (Figure inspired and adapted from[START_REF] Lamme | Why visual attention and awareness are different[END_REF]

Figure I- 9 :

 9 Figure I-9: Retro-perception. Participants had to detect tilted Gabors at the perceptual threshold. These could be cued by the dimming of a placeholder before (400ms or 200ms) or after (200ms or 400ms) the presentation of the target stimulus. They had to report its orientation and subjective visibility (A). Accuracy at the Gabor orientation task is displayed in blue for congruent trials, where cue and target appeared on the same side of fixation, and red for incongruent trials. Congruent cues enhanced accuracy when they were presented before, or after the target stimulus. From Sergent 2013.

  Daphné Rimsky-Robert 1 , Viola Störmer 2 , Jérôme Sackur 3 and Claire Sergent 1 Integrative Neuroscience and Cognition Center -UMR 8002 CNRS/Université Paris Descartes University of California, San Diego LSCP, École des Hautes Études en Sciences Sociales II.1 Introductory summary

  had a mean age of 25 y-o ± 3.86. All participants gave informed consent in writing prior to participation, and the Université Paris Descartes Review Board CERES approved the protocols for the study in accordance with French regulations and the Declaration of Helsinki. Participants received a compensation of 10€ per hour of their time.

Figure

  Figure II-1Bshows the results on detection d'. Detection d' increased sharply when the sound cue occurred just before the visual stimulus, irrespective of whether the cue was spatially congruent or incongruent with the target. This was confirmed by a 2(congruency)x4(SOAs) repeated measures ANOVA which revealed a significant main effect of SOA on detection sensitivity (F(1.82,34.67)=14.03, p<0.001 with Greenhouse-Geisser correction). Furthermore, Bonferroni-corrected 10,000 bootstrapped confidence intervals of the mean difference between the cued conditions (either congruent or incongruent) and no-cue conditions revealed a significant difference in mean sensitivity for the short pre-cue SOA (-150ms), but no other (-600ms: CI = [-0.34, 0.28], -150ms: CI = [0.18, 0.80], +150ms: CI = [-0.23, 0.36], +450ms: CI = [-0.33, 0.23]), suggesting that this effect was indeed specific of this short-SOA pre-cue. This observation is in line with previous research showing that pre-cueing a visual stimulus with an auditory cue globally improves participants' performance regardless of spatialization,

  revealed no significant difference at any SOA (-600ms: CI = [-0.09, 0.11], -150ms: CI = [-0.07, 0.25], +150ms: CI = [-0.04, 0.21], +450ms: CI = [-0.10, 0.09]).

  first pass of classical analyses on averaged median response time data (Figure II-1C) across participants was performed using a 2x4 repeated measures ANOVA on Hit trials (correct responses in target present trials). The profile of response time observed in the different conditions seemed to mirror what was found for detection sensitivity. Bonferroni-corrected 10,000 bootstrapped confidence intervals of the mean difference between cued and uncued conditions revealed a significant difference in response time for the short pre-and post-cued (-150ms and +150ms) SOAs (-600ms: CI = [-31.33, 7.20], -150ms: CI = [-77.47, -41.93],+150ms: CI = [-24.50, -1.13], +450ms: CI =[-10.71, 12.51]). This first, simple analysis of response time data showed that when comparing averaged median response time, pre-and postcueing within a short time-window, regardless of congruence, induced a shortening of response time. The ANOVA revealed a significant effect of SOA on response time in favor of shorter response times (F(2.44,46.41)=22.91, p<0.001 with Greenhouse-Geisser correction) as well as a significant decrease in response time with congruence (F(1,19)=7.79,p=0.01), and no interaction between these factors (F(2.72,51.67)=1.82, p=0.16 with Greenhouse-Geisser correction). Interestingly, the effect of congruence here is evidenced more clearly here, suggesting that cue spatialization affects behavior and reduces response time. Bonferronicorrected 10,000 bootstrapped confidence intervals of the mean difference between congruent and incongruent conditions revealed a significant decrease in mean response time for the short pre-cued condition and a marginal effect for the short post-cued condition 12.50], -150ms: CI = [-27.48, -0.01], +150ms: CI =[-40.39, 2.40], +450ms: CI =[-14.12,

  followed the 3-step model-fitting goodness of fit checks recommended in Anders 2016. The QQ plot in Supplementary Figure VI-1A shows no systematic misfit of the predicted values against the actual RT data, but highlights the presence of several outlying data points that have very large values. The decile residual distribution plot is shown in Supplementary Figure VI-1 B. Because the data is positive and right-skewed, we should expect the variance in the residuals to increase with variance in the data and decile number. To partially account for this bias when performing goodness of fits checks, the residual standard deviations were standardized by the shifted-Wald standard deviation (() = � 3 ⁄ ). Here, the plot shows a good ordering of the deciles, apart from a few outlying data-points in extreme RT values. The last goodness of fit plot (Supplementary Figure VI-1C) shows the sum of standardized residuals by experimental cell Δ, and denotes the 95% quantile range of these values as well as its mean Δ � , and standard deviation σ. The reported value ρ Δσ is the Pearson correlation coefficient between Δ and SD, which measures how efficient SD is as a standardization statistic. Values of Δ � and

  7. All participants gave informed consent in writing prior to participation, and the Université Paris Descartes Review Board CERES approved the protocols for the study in accordance with French regulations and the Declaration of Helsinki. Participants received a compensation of 10€ per hour of their time.

  2x4 factorial design: congruence (2 levels) and target duration (4 levels), plus an extra level of stimulus absent trials. Each of the 9 sub-conditions included 48 individual trials, split in 15 blocks (12 in experiments 2&3) lasting around 10 minutes, with short breaks in between.

  12ms: CI = [-0.03, 0.04], 24ms: CI = [-0.004 0.08], 36ms: CI = [0.04, 0.1], 48ms: CI = [0.01 0.05]). This suggests that beyond performance at the word identification task, congruent retrospective cueing increased the probability of conscious access to the masked word. Finally, we assessed the effect of the retro-cue on discriminating a basic visual feature of the target, namely its case (uppercase or lowercase). Responses at the case discrimination task (Figure III-2d) were analyzed as described in the modeling section in the Methods. Model fitting revealed a positive effect of target duration on performance (95%BCI [0.45, 0.76]), but no effect of congruence (95%BCI [-0.19, 0.46]), nor any interaction between these factors

  Figure 3b-f shows the results of Experiment 2. Modeling performance for the word identification task (Figure III-3b) revealed a positive effect of congruence (95%BCI[2.01, 4.12]), and target duration (95%BCI [1.05, 1.57]). Their interaction was significant but negative, indicating a decrease in the effect of congruence with target duration (95%BCI[-0.67, -0.11]). The expected accuracy of word identification was systematically different across congruent and incongruent trials for all durations in favor of higher performance in the congruent condition as indicated by the black stars at the bottom of Figure3b. A classical repeated measures ANOVA and corrected t-tests conducted on guess-corrected data revealed a similar pattern (repeated measures two-way ANOVA: target duration: F(1.56,26.44)=57.07, p<0.001, congruence: F(1.00,17.00)=5.94, p=0.026, no interaction: F(3.00,51.00)=2.27, p=0.09. Bonferroni-corrected paired one-tailed ttests conducted for each duration: target dur. 12ms: CI=[-6.07, 2.64], 24ms: CI=[-3.47, 7.15], 36ms: CI=[-3.59, 8.12], 48ms:CI=[1.26, 10.95]). These results replicate those of Experiment 1, although with slightly decreased performance in general and a smaller effect of congruence, which may be the result of an overall increase in task difficulty because participants had to split their attention between two locations.A repeated measures two-way ANOVA on the hybrid visibility/confidence ratings (Supplementary Figure VI-3) for word identification revealed a significant increase with target duration (F(1.19,20.28)=41.70, p<0.001) and congruence (F(1.00,17.00)=9.57, p<0.001), with an interaction between these two factors (F(3.00,51.00)=24.05, p<0.001). Analysis of the AROC for detection sensitivity (FigureIII-3c) with a repeated measures two-way ANOVA revealed a significant increase in detection sensitivity with target duration (F(3.00,51.00)=38.84, p<0.001), and congruence (F(1.00,17.00)=9.24, p<0.01) with an interaction between these two factors (F(3.00,51.00)=8.51, p<0.001). Bootstrapped 95% confidence intervals of the mean difference between congruent and incongruent conditions revealed a significant difference in performance for the two easiest conditions, after correction for multiple comparisons (target dur. 12ms: CI = [-0.05, 0.01], 24ms: CI = [-0.02, 0.04], 36ms: CI = [0.02, 0.09], 48ms: CI = [0.03 0.09]). As in Experiment 1, congruent retro-cues improved the ability of participants to report the identity of the masked word as well as detect whether it had been presented. We next examined the effect of retro-cues on participant's ability to report a basic physical feature of the word: its position on screen. Modeling performance for position discrimination (Figure III-3d) revealed a positive effect of target duration on performance (95%BCI [0.44, 0.65]), but no effect of congruence (95%BCI [-0.15, 0.42]), nor any interaction between these factors (95%BCI [-0.14, 0.10]), as in Experiment 1. The classical frequentist analysis gave the same conclusion. A repeated measures two-way ANOVA on the hybrid visibility/confidence ratings (Supplementary Figure VI-3) for stimulus position revealed a significant increase with target duration (F(1.35,22.94)=64.10, p<0.001) and congruence (F(1.00,17.00)=30.30, p<0.001), with an interaction between these two factors (F(3.00,51.00)=6.72, p<0.001). Analysis of the AROC for detection sensitivity (Figure III-3e) with a repeated measures two-way ANOVA revealed a main effect target duration: F(3.00,51.00)=48.61, p<0.001, and congruence (F(1.00,17.00)=7.54, p=0.01) but no interaction between these two factors (F(3.00,51.00)=2.35, p=0.08). Bootstrapped 95% confidence intervals of the mean difference between congruent and incongruent conditions revealed a significant difference in performance for the two easiest conditions, after correction for multiple comparisons (target dur. 12ms: CI = [-0.03, 0.02], 24ms: CI = [-0.04 0.04], 36ms: CI = [0.006, 0.07], 48ms: CI = [0.008 0.05]). The analysis of response coherence exp() across tasks (Figure III-3f) replicated our previous observations, namely that the correlation between word identification and position discrimination sharply decreased in congruent trials relative to incongruent trials (95%BCI [-2.06, -1.16]).The conditional probability of correctly reporting the word's position when it was identified and participant were confident in their identification (see details in Methods) (Figure III-3g) was also in favor of more errors in the congruent condition(CI = [7.85, 14.11]), with an estimated performance at position discrimination in retro-perceived trials of 75.4% (CI =

(

  see the Methods section for more information) (Figure III-4a) that were unambiguously targeting confidence at each response. No mention of visibility was made in the instructions."0" became an absence of confidence in the participant's response, regardless of whether they perceived the stimulus. Task order was the same as Experiment 2. All analyses were identical to Experiment 2.Modeling performance at the word identification task (FigureIII-4b) revealed a positive effect of congruence (95%BCI [1.24, 2.61]), and target duration (95%BCI [1.57, 1.99]). Their interaction was significant but negative, indicating a decrease in the effect of congruence with target duration (95%BCI[-0.47, -0.06]). The expected accuracy of word identification was systematically different across congruent and incongruent trials for all durations in favor of higher performance in the congruent condition as indicated by the black stars at the bottom of Figure4b. A classical repeated measures ANOVA and corrected t-tests conducted on data corrected for guesses revealed the same pattern (repeated measures two-way ANOVA: target duration: F(1.64,27.89)=207.51, p<0.001, congruence: F(1.00,17.00)=28.17, p<0.001, interaction: F(3.00,51.00)=10.34, p<0.001. Bonferroni-corrected paired one-tailed ttests conducted for each duration: target dur. 12ms: CI=[-6.80, 2.29], 24ms: CI= 8.91], 36ms: CI=[5.18, 15.70], 48ms: CI=[5.35, 17.75]). These results replicate those of Experiments1 & 2.A repeated measures two-way ANOVA on confidence ratings (Supplementary FigureVI-4) for the word identification revealed a significant increase in confidence with target duration (F(1.24,21.16)=132.04, p<0.001) and congruence (F(1.00,17.00)=65.54, p<0.001), with an interaction between these two factors (F(1.24,21.16)=28.22, p<0.001). Analysis of the AROC for detection sensitivity (FigureIII-4c) with a repeated measures two-way ANOVA revealed a significant increase in detection sensitivity with target duration (F(1.93,39.89)=121.19, p<0.001), and congruence (F(1.00,17.00)=14.90, p=0.001) with an interaction between these two factors (F(3.00,51.00)=6.26, p=0.001). Bootstrapped 95% confidence intervals of the mean difference between congruent and incongruent conditions revealed a significant difference in performance for the two easiest conditions, after correction for multiple comparisons (target dur. 12ms: CI = [-0.03, 0.008], 24ms: CI = [-0.002 0.06], 36ms: CI = [0.01 0.1], 48ms: CI = [0.03 0.1]). These results replicate those of Experiment 2, suggesting that distinguishing between confidence and visibility for the word identification task has no effect on participants' behavior in the context of our experiment. As such, participants displayed increased confidence in their response in the congruent condition, which is coherent with increased task performance. Modeling performance at the case discrimination (Figure III-4d) task revealed a positive effect of target duration on performance (95%BCI [0.22, 0.44]), no effect of congruence (95%BCI [-0.34, 0.18]), and no interaction between these factors (95%BCI [-0.09, 0.14]). This result replicates those of Experiments 1 & 2. The classical frequentist analysis gave the same conclusion.A repeated measures two-way ANOVA on confidence ratings for case discrimination (Supplementary FigureVI-4) revealed a significant increase in confidence with target duration (F(1.31,22.31)=69.95, p<0.001) and congruence (F(1.00,17.00)=9.07, p<0.01), with an interaction between these two factors (F(1.31,22.31)=7.05, p<0.001). Analysis of the AROC for detection sensitivity based on these confidence ratings (FigureIII-4e) with a repeated measures two-way ANOVA revealed a significant increase in detection sensitivity with target duration (F(3.00,51.00)=96.94, p<0.001), but no effect of congruence (F(1.00,17.00)=2.84, p=0.11) with, nonetheless, an interaction between these two factors (F(2.45,41.69)=4.45, p=0.02). Bootstrapped 95% confidence intervals of the mean difference between congruent and incongruent conditions revealed a significant difference in performance for the easiest condition only, after correction for multiple comparisons (target dur. 12ms: CI = [-0.02, 0.03], 24ms: CI = [-0.05 0.2], 36ms: CI = [-0.03, 0.5], 48ms: CI = [0.02 0.08]). Interestingly, switching from visibility to confidence ratings was non-trivial for the visual feature task. While participants had increased detection sensitivity with congruence at this task in Experiment 2, which used hybrid ratings, this is not the case with confidence-only ratings except for the most visible trials, suggesting that when visibility is no longer a factor, participants have some knowledge of their inability to report the word's visual feature when it was very difficult to perceive. The analysis of response coherence exp() across tasks (Figure III-4f) replicated previous observations, showing decreased correlation between both tasks for congruent relative to incongruent trials (95%BCI [-0.64, -0.13]).

a

  meaningful stimulus have been erased by a mask (Figure III-1). This asensory perception is however not accompanied by any sort of "visual illusion": detailed analysis of confidence ratings show that participants are aware of their lack of information about the visual properties of the stimulus (experiment 3, Figure III-4e).

  participants listened to simple auditory stimuli, embedded in continuous noise and played at different intensities around the auditory threshold, while performing a task on the stimuli (active condition) or listening passively to the stimuli (passive condition). The patients investigated in sEEG are patients suffering from severe and drug-resistant epilepsy who go under pre-surgical evaluation of their disease by having depth electrodes implanted in areas chosen according to clinical criteria. These patients remain in the hospital for recordings during a week or more, and can choose to participate in experiments during this time. Intracranial EEG recordings have the specific advantage of having very high resolution in time, as well as being localized very precisely, since the electrodes' localization in the brain is determined with the help of anatomic MRI scans.

Figure

  Figure IV-3a, or -11 to -3dB for 3 patients who found it more difficult to hear the targets during training, Figure IV-3b). The standard deviation of audibility ratings across trials, however, showed a non-monotonic pattern for most patients, with a sharp increase around the perceptual threshold, as predicted by the bimodal model (Figure IV-3a). This corresponded to a general spreading of ratings over all possibilities, possibly due to a bimodal distribution of ratings (Figure IV-3a lower panel; individual distributions are available in Supplementary Figure VI-5). For the three patients who were presented with higher SNRs, the profiles of variability

(

  0ms to 1200ms, see Methods). In the active session, the responsive electrodes spanned the frontal, temporal, and occipital lobes (Figure IV-4a, left panel), whereas significant electrodes in the Passive session were only located in the temporal lobe, with 2 contacts detected in the frontal lobe, both located in Broca's area (Figure IV-4a, right panel, MNI coordinates [

Figure

  IV-4b, right panel). SeeOnline Visualization 4.

(

  Figure IV-4b), where we can see that activity extends massively beyond the temporal cortex around this epoch in the active session. Based on this analysis, two participants with left frontotemporal implantations were selected due to the quality of their decoded pattern, and were used for detailed analyses (circled in orange in Figure IV-5).

Figure IV- 1 )

 1 Figure IV-1). Figure IV-6 shows these profiles for the two selected patients (Patient 3 (Figure IV-6a) and Patient 6 (Figure IV-6b)), in the active and the passive conditions, for mean predictive values over selected time windows of interest. Mean and variability for the "no sound" condition was considered as a baseline and subtracted from the other values. As expected, mean prediction values increases with SNR in both sessions and both patients.

For

  these two patients (Patient 3 and Patient 6), we inspected the Event-Related potentials (ERPs) or Local Field Potentials (LFPs) evoked by the stimuli (Buzsáki et al., 2012) at all SNRs, in several regions of interest spanning the temporal and frontal lobes (responding electrodes, see Figure IV-4). Figure IV-7 shows the activity evoked by stimuli at different SNRs for the Active session. Color references in this paragraph are shown on Figure IV-7. Electrodes shown in indigo (Temporal superior gyrus of the left hemisphere) and mauve (Inferior frontal gyrus, pars triangularis) belong to Patient 6, whereas all other belong to Patient 3, low-pass filtered at 30Hz for better readability. The corresponding figure for the Passive session is available in Supplementary Figure VI-8. As expected the evoked potentials showed

Figure IV- 8

 8 Figure IV-8 shows the mean and across-trial variability profiles for the six selected electrodes shown in Figure IV-7, located in Heschl's gyrus, the superior and middle temporal gyri the insula, and Broca's area. The signal recorded at each trial was averaged across eight

  Salpêtrière Hospital (Comité Consultatif de Protection des Personnes participant à une Recherche Biomédicale) and the « Comité de Protection des Personnes du CPP Ile de France XI » within the "Imagerie Spectrale Dynamique" (ISD) research program.

		IV.3	Material and Methods
	IV.3.1	Participants
	14 epileptic patients from two medical centers (Site 1 = Hôpital Neurologique et
	Neurochirugical Pierre Wertheimer, Hospices Civils de Lyon, Bron, France with the help of
	Benoît Chatard, Site 2 = Hôpital Universitaire Pitié Salpêtrière, Paris, France with the help of
	Katia Lehongre) were included (age mean =31.8, sd =10.8, range = 19 -47 years old; 5 males,
	8 patients from Site 1). These patients suffered from drug-refractory focal epilepsy and were
	implanted stereotactically with depth electrodes as part of a presurgical evaluation. Implantation
	sites were selected on clinical criteria. This experiment was approved by the Ethical Committee
	of Pitié-

  of activity (global workspaceDehaene & Changeux 2015; dehaene et al TICS 2006). We trained a linear classifier on the contrast "max-SNR -no-sound" and tested how this classifier generalized to other points in time (see Methods). This generalization in time helps understand what processes are sustained in time(Dehaene & King, 2016). Here the objective was two-fold: on the one hand, the differences in implantation schemes may lead to different classification accuracies and help locate what regions of the cortex inform the classifier, on the other hand,

different schemes may lead to different patterns of temporal generalization. We found that classification was successful for all patients in the Active session up to late epochs, with better accuracy and generalization in time in left implantations, particularly when they included frontal and temporal electrodes. In the passive session, classification was successful still, but less so, and generalized in time for shorter periods (Supplementary Figure

VI-7

). Overall these results show that decoded patterns generalize in time from 300ms, suggesting a period of sustained processes, already evidenced in the literature

[START_REF] Baria | Initial-state-dependent, robust, transient neural dynamics encode conscious visual perception[END_REF][START_REF] King | Two distinct dynamic modes subtend the detection of unexpected sounds[END_REF][START_REF] King | Brain mechanisms underlying the brief maintenance of seen and unseen sensory information[END_REF] Marti,

but not the pineal gland, which actually regulates day-night cycles(Widmaier, Raff, Strang, & Vander,

2008)

Although blindsight is now known to also involves cortical structures[START_REF] Goebel | Sustained extrastriate cortical activation without visual awareness revealed by fMRI studies of hemianopic patients[END_REF][START_REF] Silvanto | A re-evaluation of blindsight and the role of striate cortex (V1) in visual awareness[END_REF]

[START_REF] Kanwisher | The fusiform face area : A module in human extrastriate cortex specialized for face perception[END_REF].

Acknowledgements ................................................................................................................................. Table of contents .....................................................................................................................................

guesses" trials mentioned earlier. In order to identify those trials we used each participant's trial-by-trial visibility ratings. We calculated an individual, per target duration, visibility cutoff below which a correct trial could safely be classified as guess (since the response was correct but the subject themselves stated that they did not know it was correct with their rating). If a participant's guess was n% of trials as estimated above, we excluded all trials with visibility equal or below that of the n th percentile. This cut-off was most of the time between 0 and 1 (0.27±0.46 for 12ms, 0.47±0.64 for 24ms, 0.74±0.88 for 36ms, 0.87±0.64 for 48ms for Experiment 1). The last panel of all figures also includes an estimate of conditional performance for "retroperceived" trials, i.e. the additional "word-correct" trials brought about by congruent retro-cues. If performance on case or position discrimination on those trials was not changed compared to the control word-correct incongruent cue condition, adding these new word-correct trials should not change the conditional performance rate for congruent versus incongruent trials.

The rationale behind this analysis is that if all incongruent trial errors are caused by a variety of factors grouped in X (such as wrong button presses, attentional lapses, or effect of the task excluding congruence), then all errors in congruent trials are due to the sum of errors caused by X and those caused by congruence of the cue. We aimed at dissociating these two phenomena and estimating performance in the congruent condition given that one has retroperceived (e.g. removing errors shared with the incongruent condition). We hypothesized that retro-perception could only occur in congruent trials and always entailed a correct word-answer. We also hypothesized that congruent cues would affect the probability of retro-perceiving, but not the probabilities of having a correct word or visual-feature answer outside of retro-perception, as we excluded low-visibility trials containing most guesses. With the help of Bayes' theorem and the law of total probability, we can provide an estimate of the conditional probability of having a correct answer at the visual feature task, provided that retro-perception occurred. See Supplementary Note 4 for the full demonstration. Trial exclusion criteria and word identification task analysis are available in Supplementary Note 5.

III.3.7 Modeling

In parallel we also analyzed participants' accuracies at identifying the masked word and reporting its visual features (e.g., case or position) with a multivariate logistic model [START_REF] Cox | The analysis of multivariate binary data[END_REF]. Given that our dataset is grouped according to 'observational units' (participants) which should be considered as random sample from a population, we adopted a hierarchical Bayesian approach. One issue in the modelling of word identification responses was correcting for

III.4 Results

III.4.1 Experiment 1

In order to test our hypothesis, in Experiment 1 we presented 15 participants with sequences of masks among which was inserted a target word that could last 12, 24, 36 or 48ms, thus varying the strength of masking across trials (Figure III-2a). Target-absent trials were included in the experiment (11% of the trials) in order to test participants' ability to simply detect the presence of a word. Each stimulus sequence was followed by an auditory cue, which could be semantically associated to the target word (congruent trials) or not (incongruent trials)

with equal probability (Figure III-2a). After each trial, participants had to report in order: the target word (by speaking into a microphone), the word's case (upper or lower) using the keyboard, and rate the overall visibility of the stimulus on a 9-points scale with 0 being "I have not seen anything but the masks", and 8 "I have perfectly seen the target word" (see Supplementary Note 1 for the full instructions). In total each participant was presented with 432 different visual words with no repetition (see Supplementary In all 3 experiments, task performance results were analyzed both using a classical frequentist approach (ANOVAs) and a hierarchical Bayesian modeling approach (see Methods). Both led to similar conclusions.

Given that our task involves semantically related retro-cues, a non-negligible fraction of correct identifications of the target word in congruent trials might actually correspond to 'guesses': participants will tend to report words from the same semantic field as the cue and thus could name the target correctly just by chance. In order to estimate these guessing probabilities we analyzed the errors made in incongruent trials. Indeed, in incongruent trials when participants report the semantically associated word in the list instead of the target word, this corresponds to an error, and it is thus not confounded with correct responses (see Methods for details). The rate of "semantically associated errors" in incongruent trials thus provides an estimate of the participant's tendency to name the semantically associated word even when this word was not presented. This rate was estimated for each stimulus duration, since guess rate predictably declined as the target became more visible.

These guessing probabilities were taken into account in our two independent analyses (frequentist and hierarchical Bayesian) which converged in demonstrating the same pattern of results in the word identification task These results suggest that participants subjectively felt that they were seeing the target word better when it was followed by a congruent cue. We further examined this measurement by studying participants' objective ability to report whether a word has been presented or not (detection sensitivity). Detection sensitivity was derived from the participants' visibility rating by computing the area under the Receiver Operating Characteristics curve (AROC) i.e. by comparing ratings for stimulus-present versus stimulus-absent trials for each condition. This

IV.1 Introductory summary

The neural mechanisms underlying conscious access are currently still under investigation.

Conscious perception is known to relate to increased activity in sensory areas (Dehaene, Naccache, Cohen, Bihan, et al., 2001;[START_REF] Sadaghiani | Distributed and antagonistic contributions of ongoing activity fluctuations to auditory stimulus detection[END_REF][START_REF] Super | Internal state of monkey primary visual cortex (V1) predicts figure-ground perception[END_REF], a widening of the network involved in stimulus processing, including frontal, parietal and cingulate areas [START_REF] Boly | Baseline brain activity fluctuations predict somatosensory perception in humans[END_REF]Dehaene, Naccache, Cohen, Bihan, et al., 2001;[START_REF] Sadaghiani | Distributed and antagonistic contributions of ongoing activity fluctuations to auditory stimulus detection[END_REF], as well as increased connectivity [START_REF] Gaillard | Converging intracranial markers of conscious access[END_REF][START_REF] Melloni | Synchronization of neural activity across cortical areas correlates with conscious perception[END_REF] and long-term maintenance of the information [START_REF] Del Cul | Brain dynamics underlying the nonlinear threshold for access to consciousness[END_REF][START_REF] Gaillard | Converging intracranial markers of conscious access[END_REF][START_REF] Sergent | Timing of the brain events underlying access to consciousness during the attentional blink[END_REF]. One clinical application of finding appropriate markers of conscious access is to better diagnose patients suffering from disorders of consciousness (DOC), who cannot explicitly communicate their potential awareness of their surroundings [START_REF] Giacino | Disorders of consciousness after acquired brain injury : The state of the science[END_REF][START_REF] Laureys | The neural correlate of (un) awareness : Lessons from the vegetative state[END_REF][START_REF] Sergent | Imaging neural signatures of consciousness:'What','When','Where'and 'How'does it work?[END_REF]. Here, we use an auditory protocol in patients implanted with stereotaxic electrodes (sEEG), in which they first actively listen to target stimuli in ambient noise at various signal-to-noise ratios ranging from nearly inaudible to easily identifiable, and provide behavioral reports (stimulus identification and subjective audibility). In a second session, these same patients passively listen to the same stimuli while performing an unrelated task. The use of no-report paradigms have proven to lead to different results than "yes-report" paradigms [START_REF] Tsuchiya | No-report paradigms : Extracting the true neural correlates of consciousness[END_REF]. We first evidenced that neural activity differs between these two sessions: in the active session, evoked activity can be found in all cerebral lobes, whereas the passive session elicits activity mostly confined to the temporal lobe, and some frontal electrodes. Interestingly, sustained activity can be found until 1.2s in both sessions. Secondly, a potential marker of conscious access is the onset of a late pattern of activity (around 300ms) which is strongly related to positive subjective reports [START_REF] Sergent | Timing of the brain events underlying access to consciousness during the attentional blink[END_REF] and presentation of stimuli above the perceptual threshold [START_REF] Del Cul | Brain dynamics underlying the nonlinear threshold for access to consciousness[END_REF][START_REF] Kouider | A neural marker of perceptual consciousness in infants[END_REF]. Because this pattern appears to occur when participants report being aware of a stimulus, we hypothesized that around the perceptual threshold, neural activity across trials would be split between consciously perceived (displaying the pattern) and not consciously perceived (not displaying the pattern). In this case, stimuli well below the threshold would all remain at a "low" state, and stimuli well above would reach a "high" state, whereas stimuli at-threshold would be split 50/50. In this case, we should observe a burst in variability across trials at the perceptual threshold. In this chapter, we present preliminary evidence that this burst in variability at threshold can be evidenced in behavior and neural activity using the high resolution in time and space of sEEG.

IV.3.3 Material

The experiment was coded using Matlab and Psychtoolbox (Brainard & Vision, 1997a), and ran on a different laptop in each site, both had a 60Hz LCD screen, with resolution 1920x1080.

Precise timing of auditory stimuli was controlled by using the ASIO4all virtual sound card on both computers, and trigger-to-audio delays were verified to be millisecond correct with an oscilloscope, Due to the nature of our protocol, audio-visual delays were not corrected for. The audio stimuli were presented using a single loudspeaker (BOSE SoundLink Mini II) mounted above the screen and centered (Figure IV-2b). Some small deviations in materials are reported in Supplementary Table VI-3, along with the reason why we deviated from the main protocol.

Participants were seated in their hospital bed or chair, and asked to stay in whatever position felt most comfortable for the duration of the experiment.

IV.3.4 Task

The experiment included two sessions. While stimulation was the same, participants' task and instructions varied. In the first session (active), participants were prompted to attentively listen to the auditory stimuli, and to report which vowel was presented, as well as rate their subjective audibility on an 11-point scale; the second session (passive) involved distracted listening, with no instruction relating to the stimuli, but tasks allowing to estimate mind wandering [START_REF] Bastian | Mind wandering at the fingertips : Automatic parsing of subjective states based on response time variability[END_REF][START_REF] Mrazek | Mindfulness training improves working memory capacity and GRE performance while reducing mind wandering[END_REF].

Written instructions were provided to the participants, and the experimenter verified that they were well understood. The experiment started with a staircase-like procedure, to verify that the thickness of the patient's head-bandage did not prevent them from hearing the stimuli:

participants were presented with random stimuli from the experiment in the background noise, with gradually increasing SNR, from -13dB to -3dB with an increment of 1dB each step. They were asked to report whether they had heard something, not heard anything, or if they weren't sure. The experimenter then assessed whether the global sound level of all stimuli (noise included) should be increased. This was followed by a training block using the standard SNR range (-13dB to -5dB), at the end of which the experimenter checked that participants' performance at the vowel identification task was above chance across all trials. If not, the range of SNRs was adjusted to make the task easier (see Supplementary During the ensuing experimental phase, each trial followed the same sequence: a jittered time interval passed before the presentation of the vowel (0.8 -1.2s), followed by the target vowel (0.15s) and a post-target delay (1.2 -1.5s including target duration), after which a response screen appeared (Figure IV-2a). A fixation point was displayed from the start of the test until the response screen. The background noise was present continuously throughout each block; it stopped only during the breaks between blocks. There was no timeout for responding in the active session, however in the passive session, where behavior was unrelated to the auditory stimuli, a 10s timeout was used, after which the experiment moved on to the next trial.

In order to limit the total amount of time of our experiment to accommodate clinical constraints, in active sessions we provided patients with a dual response screen (Figure IV-2b).

They were presented with a vertical 11-point scale to report how well they had heard the sound.

Participants could use two sets of keys on the keyboard to move the cursor on the audibility scale and report the identity of the vowel at the same time. The left sided set of keys was labeled in yellow, whereas the right-sided set was blue, and each corresponded to one vowel, which was chosen randomly in each experimental block. Initially, the audibility cursor in the presented scale appeared gray at a random level, until participants pressed a colored button. In this case, the cursor would turn the appropriate color, and display which vowel it corresponded to.

Participants were instructed to choose both their audibility rating, and the type of vowel they wanted to report using this setup, with the possibility to change at will until pressing the space bar. For audibility ratings, participants were instructed to use the lowest level if and only if they had not heard anything. Otherwise, they were told to rate how well they had heard the stimulus.

They used the bottom half of the scale for cases where they had the impression that a vowel might have been present but were unsure, or the upper-half when they were certain a vowel was present, all regardless of their ability to identify it. They were told to try and use the full extent of the scale. In order to help participants calibrate their ratings, the loudest stimuli were presented at the beginning of each block.

The active session always preceded the passive one, because there was always a risk that we could not perform both, depending on the patient's fatigue and hospital constraints (see

Supplementary Table VI-3).

During the passive listening session, participants heard the same stimuli, with the same number of trials, but did not have to identify them or report their from the temporal to the frontal lobes. In these later steps, some areas, in this case evidenced in the insula and Broca's area, display a bimodal pattern of activity dependent on stimulus strength. What activity exactly may be elicited in areas which do not display this bimodal pattern, whether a unimodal or bimodal reactivation, or no secondary response, remains to be investigated.

Model fitting is the current goal for this study, where we would define and use appropriate definitions to discriminate between a unimodal and a bimodal pattern of activity on individual electrodes as well as on the decoded pattern in both sessions, in patients with appropriate implantation schemes. Here, we presented a first investigation of our intracranial data, which will be further examined in the hope to better understand the neural dynamics underlying conscious access around the perceptual threshold. The task instructions for Experiment 3 were:

You will answer 4 questions:

1) Was this word in uppercase or lowercase?

2) How confident are you in this uppercase/lowercase response? You have a colored scale ranging from red to green, with 9 points. The lowest rating (completely red) corresponds to no confidence at all, and the highest means you are fully confident. 0: not confident at all (reported at chance).

1-4: I am not very confident.

5-8: I am more and more confident that I correctly reported the word's case.

3 0: not confident at all (reported at chance).

1-4: I am not very confident.

5-8: I am more and more confident that I correctly reported the word.

The two confidence measures are distinct: you may be confident about the word's case because you caught a single letter, but not know what the word was. The reverse is also possible. Freely reply to each rating separately.

VI.2.2

Supplementary Note 2: Free association experiments and word selection.

We conducted two online experiments, in which participants were given a single word and prompted to type in the first word that came to mind in response. They were instructed that they had 15 minutes to respond to roughly 100 words. The first experiment tested 203 words and had between 40 and 67 responders to each unit. Data was pre-processed with a semi-automatic spellcheck using the Office suite for Windows. The software detected anomalies, and the experimenter chose the appropriate correction depending on whether the anomaly was an obvious spelling error or not (unintelligible responses were kept as is). Out of these, 137 item pairs fared above the threshold for use on our experiments, which was that >= 25% of responders gave the same answer to one item. The second experiment tested 95 words and had 70 to 79 responders to each of them. Out of these, 73 item pairs were kept according to our criteria. One obvious internet troll was excluded from these experiments, as he responded a single profanity to all proposed items. We otherwise followed the methods of Debrenne et al.

Word selection from the online database (Debrenne et al.) was performed with our participant demographic in mind: we filtered French native speakers, 18 to 25 years old, and selected words matching our criteria. Extra words were then selected from existing articles on free association [START_REF] Ferrand | Normes d'associations verbales pour 366 noms d'objets concrets[END_REF]2001) with the same criteria to complete our list. Words were of all grammatical classes, but kept in dictionary form if possible.

VI.2.3

Supplementary Note 3: Fixation control.

Fixation was initially checked for 200ms before the onset of any stimulus in all experiments.

Experiments 1 & 3, given that all stimuli were displayed in the same location, was not subjected to further investigation for fixation control. For Experiment 2, in which stimuli were displaced and participants instructed to fixate in the center, checking that instructions were followed is more crucial. The one strategy participants could have used to increase their overall chance to report the correct position of the stimuli was to fixate one of the locations of interest rather than the center, and report that the stimulus was there if they had seen it, or in the other location if they hadn't. This strategy would clearly render the experiment null, as they would probably be unable to read words from the other location, and negate the effect of post-cueing. A simple way to check that participants did not use this strategy was to look at stimulus-absent trials, which should all be reported as being in one location (the unattended one) in this case.

Participants responded that the target was "up" on average on 43.8± 16.4% of catch trials.

Individual values ranged from 29.2% to 77% which did not, in our opinion, reflect more than individual bias toward one key or location in choosing a response.

VI.2.4

Supplementary Note 4: Conditional probability analysis.

This analysis relies on several hypotheses displayed in bold throughout the demonstration, and displayed for more intuitive understanding in Supplementary Figure V-3.

We will denote W the event "word correct", F "feature correct", "R" the event "retroperception". Indices c and i correspond to congruent and incongruent trials, index w indicates word-correct trials. We would like to know how likely participants are to accurately report the visual feature in word-correct and retro-perceived trials.

By definition of conditional probabilities, we know that:

We aim at calculating p c (F | W, R). From Equation 1we can deduce:

Where p cw (P), p cw (F | P � ) are unknown.

Estimating ():

By definition of conditional probabilities, we know that:

We hypothesize that, in retro-perceived trials, word are always correctly identified:

From the law of total probability we know that:

We hypothesize that retro-perception does not occur in incongruent trials, and since all retro-perceived trials involve a correct word answer:

We hypothesize that congruent cues will induce retro-perception, but not affect perception in non-retroperceived trials (i.e. probabilities of success). This means that probabilities are equal in congruent and incongruent conditions, outside of retroperception. From equation (10), we can now formalize this hypothesis:

Which, when applied to equation ( 10) gives:

By combining equations ( 5), ( 6), and ( 11), we find that:

We hypothesized that participants cannot retro-perceive in incongruent trials. As such:

Estimating ( | � ):

From the previous hypothesis, we can state that when the word was correct but was not retro-perceived, the probability of identifying the word's visual feature is equal in congruent and incongruent trials. Since we hypothesized earlier that participants cannot retro-perceive in incongruent trials, this is equivalent to the rightmost term.

Note that contrary to equations ( 7) and ( 8), this ensures that that performance when the cue was not effective does not depend on the probability of the cue being effective. This is so because we are now working with word-correct trials.

By combining equations ( 14) and ( 15) we can calculate:

VI.2.5 Supplementary Note 5: Preprocessing.

Trial exclusion criteria: For Experiments 1, 2 & 3, trials were excluded when the verbal response was null or inconsistent with instructions as described in the following section, or if presentation timings were not respected. Word identification was performed orally to avoid issues related to spelling mistakes had they used the keyboard, and guessing with the help of the auditory cue had they been presented with multiple choices. Audio tracks from each participant were transcribed manually. Trials were rejected when participants were absolutely unintelligible (no transcription of any word-like structure was possible, such as groaning), didn't provide a verbal response, or responded a variation of "I don't know", which they were prompted never to use, but instead to say the first word that came to mind. Responses provided in foreign languages or pseudo-words were kept as valid responses because they could contain information about what was perceived. The average proportion of rejected trials in total was 4 ± 8% (including one outlying participant with 31% rejected trials because they did not try guessing at all, and never responded when they did not know what to say. All analyses were also conducted without said outlier, but led to the same conclusions. As such, he was kept for analyses), 3.7± 2.8 % in Experiment 2, and 1.1 ± 0.9 % in Experiment 3, mostly due to participants not responding on some trials .

VI.2.6

Supplementary Note 6: Stimuli, data and analyses.

The stimulus-list and their respective association rates are available in the supplementary spreadsheet available online together with the preprocessed data and scripts for analyses (using R) on GitHub (https://github.com/mattelisi/ghost-word-analysis). For the stimuli, cues and their associated targets are listed together with their reported % of association according to 5 different studies [START_REF] Debrenne | Le dictionnaire des associations verbales du français et ses applications. Variétés, variations and formes du français[END_REF][START_REF] Ferrand | Normes d'associations verbales pour 260 mots «abstraits[END_REF][START_REF] Ferrand | Normes d'associations verbales pour 366 noms d'objets concrets[END_REF].

One line contains two alternative pairs of words because in Experiment 2 where we tested position on-screen, we replaced the high/low pair with decision/choice.