Keywords: phenomenal consciousness, evolution, modularity, labelling, binding, epiphenomenon. _______

A number of concepts are included in the term "consciousness". We choose to concentrate here on phenomenal consciousness, the process through which we are able to experience aspects of our environment or of our physical state. We probably share this aspect of consciousness with many animals which, like us, feel pain or pleasure and experience colours, sounds, flavours, etc. Since phenomenal consciousness is a feature of some living species, we should be able to account for it in terms of natural selection. Does it have an adaptive function, or is it an epiphenomenon ? We shall give arguments to reject the second alternative. We propose that phenomenal properties of consciousness are involved in a labelling process that allows us to discriminate and to evaluate mental representations. We also discuss to what extent consciousness as such has been selected for this labelling function.

Le terme de "conscience" recouvre plusieurs concepts. Nous parlons ici de conscience phénoménale, cet ensemble de processus par lesquels nous avons une expérience de certains aspects de notre environnement et de notre état physiologique. Nous partageons probablement cet aspect de la conscience avec de nombreuses espèces animales qui, comme nous, ressentent de la douleur et du plaisir, et font l'expérience des couleurs, des sons, des odeurs, etc. Comme la conscience phénoménale est une caractéristique de beaucoup d'espèces vivantes, nous devons l'expliquer en invoquant la sélection naturelle. A-t-elle une fonction adaptative, ou estelle un simple épiphénomène ? Nous donnons des arguments en faveur de la première option. Nous suggérons le fait que les propriétés phénoménales de la conscience sont impliquées dans un processus d'étiquetage qui nous permet de discriminer et d'évaluer les représentations mentales. Nous discutons ensuite l'hypothèse selon laquelle la conscience en tant que telle aurait été sélectionnée pour cette fonction d'étiquetage.

On the Evolution of Phenomenal Consciousness

Evolution de la conscience phénoménale

Jean-Louis Dessalles * and Tiziana Zalla Paris -France dessalles@enst.fr

Introduction

The term "consciousness" is merely a label for many separate phenomena. Whatever consciousness is, it is something that, far from being an undifferentiated stream of inner events, is instead a composite phenomenon which corresponds to the activity of functionally differentiated modular systems. We need to distinguish here phenomenal consciousness from other cognitive processes, from conceptual knowledge and from higher-order conscious states [START_REF] Block | On a confusion about a function of consciousness[END_REF]. Phenomenal consciousness refers to qualitative properties of experience. The vividness of pain, pleasure, redness, the taste of red wine are examples of qualitative experiences 1 . We want to deal here with phenomenal consciousness for two main reasons. First, many non-human species may have qualitative experience (if we think of pain). We do not need to grant them other aspects of consciousness [START_REF] Griffin | The Question of Animal Awareness[END_REF]], just the ability to experience smell, colour or pleasure, even if their experience is qualitatively different from ours. An obvious question then arises : when, and why, did qualitative experience arise in phylogenetic history ? Our second motivation for considering phenomenal consciousness comes from the fact that its very existence, and the biological adaptive function it fulfils, remain highly mysterious. Is it a mere epiphenomenon, or worse : a non-scientific object, or does it play a precise, essential, biological role ?

In our view, in order for the concept of consciousness to be of scientific interest, one has to show that it is a natural kind, i.e. a phenomenon which is useful and convenient to isolate as explanandum for a more advanced scientific theory. In this presentation, we take the existence of phenomenal consciousness as granted and we attempt to make this existence compatible with evolutionary principles. We shall study first what properties characterise qualitative experience, before looking for a possible adaptive function.

We shall first consider the possibility that phenomenal consciousness is a mere fortuitous epiphenomenon, and that complex cognitive behaviour can take place in its absence. However we will reject this possibility. We will consider a possible account for phenomenal consciousness : it will be presented as a way to label experiences and mental states. Labelling is an essential feature of cognitive processing, though the most obvious way of labelling information, as used in computers, is not plausible in a neural implementation. Labelling through synchronous binding, and its qualitative correlate, will be presented as a solution which evolved to cope with environmental and behavioural complexity. Lastly, we will observe that this account, expressed first in terms of physical neural states, is insufficient to predict characteristic aspects of phenomenal consciousness. We will suggest that the ability to have experience is part of our phenotype and was retained as such by natural selection.

Properties and role of phenomenal consciousness

Is conscious experience nothing but an evolutionary epiphenomenon ?

What use is the ability to experience mental states or events in the outer world ? Most of the complex processes going on in our body are achieved without involving any conscious component. We are not conscious of our immune system, we do not feel each contraction of our stomach, we are not aware of maintaining our equilibrium at each moment. Many of our cognitive processes are performed without us being aware of them. We are even unable to monitor such processes. The way we analyse a visual scene, the way we recognise words in a complex acoustic signal, the way we adapt our walk on an uneven ground are good examples of such processes. In Fodor's terms [START_REF] Fodor | La modularité de l'esprit[END_REF]], these processes are achieved by domain specific modules which are characterised by their encapsulation and their relative inaccessibility. Only the output, in the case of sensory modules, is experienced by the living being. Such unconscious processes may be quite complex, they may be context sensitive (for instance priming may affect word recognition) even if, according to Fodor, modules are encapsulated and thus receive little influence from other processes. If we think of complex unconscious tasks like shape and object recognition, we may wonder why cognition involves consciousness at all. Why are we sentient beings, why are we not unconscious like robots ? From a Darwinian perspective, this is a crucial question : what is the adaptive value of consciousness, and if consciousness has no such value, why do we happen to be conscious ?

This question is even more vital from the functionalist perspective. Functionalism considers that what is relevant in cognition is the causal network of mental states which is involved in cognitive computations. Consciousness plays no role within these computations.

The role of consciousness is so obscure that many authors doubt it, considering phenomenal consciousness as an epiphenomenon. Epiphenomena are known in evolutionary biology. Features which were not selected for, but result from the selection of other characteristics, are evolutionary epiphenomena. The most often mentioned example is the human chin, which appeared as a consequence of face and jaw reduction. The chin is not an organ shaped by evolution in the first place. Similarly, if consciousness is considered as a mere property accompanying some neural mechanisms, it is nothing more than a fortuitous by-product of brain evolution. Any evolutionary epiphenomenon has two basic properties : it is fortuitous and neutral. It could have been different or non-existent, and it has no effect on the survival of individuals. Is consciousness such a fortuitous, neutral feature ? In our view, the fact that phenomenal consciousness is systematically associated to sensory input analysis indicates that it is not incidental.

Phenomenal consciousness may be considered as an epiphenomenon in another sense. It is sometimes said to be an emergent feature of complex functional organisations. This concept of consciousness emerging from complexity is however not operational. It does not explain why every brain region does not equally contribute to consciousness [START_REF] Edelman | The Remembered Present. A Biological Theory of consciousness[END_REF]]. It does not explain either why brain damage may alter phenomenal experience selectively.

A very serious claim is that phenomenal consciousness is systematically associated to a given physical neural state [START_REF] Edelman | The Remembered Present. A Biological Theory of consciousness[END_REF][START_REF] Damasio | The brain binds entities and events by multiregional activation from convergence zones[END_REF][START_REF] Crick | Towards a neurobiological theory of consciousness[END_REF]] :

Our basic hypothesis at the neural level is that it is useful to think of consciousness as being correlated with a special type of activity of perhaps a subset of neurones in the cortical system. Consciousness can undoubtedly take different forms, depending on which parts of the cortex are involved, but we hypothesize that there is one basic mechanism (or a few) underlying them all [Crick & Koch 1990, p. 266].

These authors consider consciousness as an authentic biological feature, but nothing prevents us from putting forward such a hypothesis to depict consciousness again as an epiphenomenon : what was retained by selection would not be consciousness itself, but the underlying neural mechanism. In this kind of description, consciousness plays no causal role by itself in cognitive activity. It is not supposed to be a mere fortuitous sideeffect, it is the mental correlate, experienced from a personal point of view [START_REF] Nagel | What is it like to be a bat ?[END_REF]], of a special kind of brain activity.

Our claim about the modular properties of qualitative experience will allow us to argue against epiphenomenon hypotheses and to put forward a possible role for phenomenal consciousness in evolution.

Modularity of qualitative experience

The existence of conscious experience, which has recently become the object of many scientific and philosophical investigations, seems to deserve closer examination. The quality of sensory states at the phenomenal level -how things look, sound, how we feel them -appears to be modality-dependant. Mental disorders occurring after brain injuries sometimes reveal that some particular aspect of consciousness may be selectively impaired. For instance blind-sight patients declare to be blind in a certain area of their visual field. These patients suffer from brain damage, and their blind area corresponds precisely to the location of lesions in the primary visual cortex. However, it has been shown that they are still able to perform visual processing like localising simple visual stimuli, elementary patterns or movements [START_REF] Weiskrantz | Varieties of residual experience[END_REF][START_REF] Weiskrantz | Residual vision in a scotoma. A follow-up study of 'form' discrimination[END_REF]. These patients are totally unaware of their residual visual capacity. They just claim they are "guessing" during visual tests. Their phenomenal experience is selectively impaired in the visual modality.

Different types of neuropsychological syndromes (like amnesia, hemineglect, agnosia) that alter or suppress aspects of qualitative experience suggest the existence of dissociations within the sensory domain of information processing. As far as we can conclude from such neural deficits, each property of a given experience seems to be produced by a fixed and specialised neural architecture. These highly selective syndromes suggest that phenomenal consciousness is not globally distributed, but modular and that its modular properties mirror the organisation of sensory input modules.

Qualitative aspects of experience originate at the output of sensory modules 2 . They are and remain separate (we never confuse the redness of an apple with its taste). Memory and perception are never experienced as a mixture of indistinct sensations. Qualitative experience is also mandatory : you can't avoid experiencing redness when you look at a red screen, seeing a visual array as a three-dimensional objet, or hearing an utterance of a sentence (in a language one knows) as a sentence.

These modular properties suggest that an adaptive role for phenomenal consciousness is to improve the ability to discriminate perceptual and mental states.

Qualitative labelling of experience

Cognitive labelling through phenomenal properties

One of the most basic and important tasks a living creature has to perform in order to eat, move, mate and avoid predation is to extract relevant information from its sensory inputs and from its memory. This is what information processing is all about. The task is indeed not a trivial one. Biologically relevant information is indirectly defined by genes, possibly through learning, though genes can only give a rough indication. This is sometimes sufficient. For a frog, any small flying object is a priori edible. For learning to take place, however, situations must be distinguished. A frog is unable to learn anything about flies, since all flies look the same. With a specialised device for labelling experienced situations and a simple feed-back like edible / non-edible, a creature can learn a lot about things relevant to food.

There are thus two basic labelling functions : (1) evaluation, in order to mark situations as positive or negative according to various scales (edible, dangerous, attractive, etc.) ; (2) perceptual labelling, which aims at individualising contexts for complex representational processing. Our claim is that phenomenal consciousness performs both functions, and that this is its main biological purpose, the reason for which it has been selected during phylogenetic history.

The requirements are different for each label type. Perceptual labels are necessarily the result of a highly combinatory device, so that many distinct labels can be generated, while a " value " label must have a wide dynamic range, so that such labels can be accurately compared (the two requirements are not mutually exclusive). Our ability to simultaneously experience colour, shape, temperature, weight, sound features, distance and so on meets the first requirement. Any combination of all such parameters is likely to be unique. On the other hand, our experience of physical pain or pleasure, of sadness or joy, of pride, of nostalgia, etc. ranges from slight feeling to extreme intensity, and is thus suitable for comparative assessments of situation significance.

The perceptual labelling role we give to phenomenal consciousness can be inferred from psychological studies on memory source monitoring. [START_REF] Johnson | Phenomenal characteristics of memories for perceived and imagined autobiographical events[END_REF][START_REF] Singer | Synchronization of cortical activity and its putative role in information processing and learning[END_REF] claim that the phenomenal qualitative properties of mental experiences are the very source of a more general process of discrimination, judgement and attribution of mental events. They suggest that phenomenal properties of experience play a critical role in discriminating knowing from remembering, and thus, create a source for one's sense of personal past. When memory information without qualitative characteristics is recalled, it is experienced as mere knowledge or belief. Hence, phenomenal properties relating to perceptual and contextual information appear as important cues for discriminating representations, which is essential for assessing the reliability of information. Confusion about the nature and the source of different mental representations is likely to be the cause of misattribution in the confabulation syndromes where amnesiac patients actually mix up the imagined, perceptual and memory representations.

From an engineering perspective, a modular labelling system appears to be rather odd. Labelling information is indeed a simple task in computer science and data transmission. All you have to do is to add unambiguous labels, e.g. as headers, to messages. When you receive data on your network navigator or when you open a file with a word processor, objects received or read identify themselves as text, picture, table or whatever because they contain heading information giving their type. Labelling on a computer is thus easily performed by adding information to information. However, there is an obvious difference between computers and neural networks, so obvious that it remains implicit and is sometimes overlooked. This difference presumably prevented evolution from using headers as labels. To put it crudely, information is never merely transmitted in a neural network. Let us briefly clarify this point.

By definition, in a digital communication context, a message becomes information only when appropriate operations have picked out its features. This definition fits in with what we know of sensory analysis in the mammal brain. Take the example of an image. When received on a retina, it is a mere matrix of pixels, bearing no information in terms of boundaries or outlines. This latter information appears after it has been detected by edge detectors. What is transmitted to a further processing level, e.g. an object recognition level, is expressed in terms of lines or edges, no longer in terms of pixels. In such a processing sequence, information is never transmitted as such, because at each stage, the symbol set changes. Things are different on a computer : you may mark out a piece of text to indicate the make-up and still have a text, mostly composed of the same characters with a few additional marks. In neural networks, this is impossible. Any processing changes the nature of information 3 . An edge detector is fed with pixels, but its output is of a different kind : it indicates the presence or absence of an edge with a given orientation at a given location. The engineering solution which consists in adding headers to a message in order to identify it unambiguously, as for instance in electronic mail, does not work with neural circuitry 4 , since such headers would be lost at each processing stage.

One possibility is to consider that perceptive details have to be forgotten at higher levels of a hierarchical cognitive architecture. Detailed features play a role at the first stages of recognition, but are of no use afterwards when abstract features are processed. C. von der [START_REF] Der Malsburg | Am I Thinking Assemblies ?[END_REF] shows in detail why such an organisation is not convincing at all, because of its lack of flexibility and parsimony. A purely hierarchical system necessitates the existence of dedicated units to represent high-level patterns. But whereas the number of combinations that have to be distinguished is virtually infinite, the number of such dedicated units in the brain are certainly limited. In other words, a purely hierarchical organisation lacks combinatorial power. Also, such dedicated units being separate, they cannot be the basis for generalisation :

When I consider a particular scene, I absorb knowledge about the objects involved, by modifying the interactions within and between the corresponding mental symbols. I want to be able to have this knowledge at my disposal in other situations if they involve partly the same objects or aspects. This, however, is possible only through physical overlap between mental symbols. Avoiding this overlap destroys the basis for generalization. [von der [START_REF] Der Malsburg | Am I Thinking Assemblies ?[END_REF] Von der Malsburg suggests that "mental symbols" are not limited to a given hierarchical processing level. What he calls the "natural representation" of an object simultaneously involves all its constituent elements. As a consequence, no information is lost in the integration process.

The symbols of communication [e.g. written words] are mere parsimonious tokens for the images they are to evoke in the reader's mind. In contrast, the symbols of mind have to fully represent all aspects of our imaginations. [von der [START_REF] Der Malsburg | Am I Thinking Assemblies ?[END_REF] If we accept this kind of description, phenomenal qualities, which appear at the output of modular sensory systems, are available for higher-level processes. In this context, the labelling role played by phenomenal consciousness becomes manifest. Processes like the justification and the revision of beliefs, especially perceptual beliefs, are sensitive to qualitative aspects that are only present at non-conceptual levels. For instance, in order for such operations to be accomplished, one should preserve the origin (e.g. external vs. internal) of the representations which are poised for use in reasoning and in the rational control of speech and action. The perceptual origin of the representation seems to be assessed from the abundance of phenomenal details. [START_REF] Kelly | The Construction of Subjective Experience : Memory Attribution[END_REF] argue that the feeling of familiarity arises from attributions based on internal cues, such as the ease or relative fluency of perceptual operation, the quality of memories and the vividness of visual images. The experience of remembering is not the result of some intrinsic qualities of "memory trace", but rather reflects the operation of a decision process that assigns ongoing mental events to particular sources. People normally use the presence of perceptual details in a mental state as a cue to infer that they are recalling, rather than imagining, and perceiving rather than remembering. Phenomenal and qualitative properties accompanying some kinds of mental states, e.g. perceptual or proprioceptual states and some episodic memory states, are important cues that enable us to ascribe them to ourselves 5,6 .

Phenomenal qualities seem thus to strongly interfere with higher-order cognitive processes. As a consequence, we are always in hybrid mental states, partly conceptual and partly made of contextual qualitative information. The labelling of conceptual representations by qualitative properties is only possible if the latter may enter as constituents in cognitive representations, as suggested by von der Malsburg. Recent advances in brain modelling makes this requirement plausible, as described below.

Neural labelling implementation

With phenomenal consciousness, natural selection seems to have discovered a way of labelling inputs which is compatible with neural implementation. But how is it implemented ? [START_REF] Edelman | The Remembered Present. A Biological Theory of consciousness[END_REF] suggests that conscious perception relies on active categorisation. He explains that a set of several neural maps is responsible for the integrated conscious perception of scenes. This set of maps has been selected among other possible combinations of groups of neurones during ontogenesis. Perception itself results from the selection of a neural circuit among all possible combinations of connections between maps, through a process called reentry, which is a recurrent exchange of signals between maps. This is supposed to explain how sensory input analysis can be distributed over several locations in the brain and still produce a unified perception that is rich enough to be categorised. Thanks to reentry, perception is compared with memory traces through an active process that modifies both perception and memory. Primary consciousness results from these categorisation processes. Edelman, using the same principle, explains how such a unified, conscious, perception of a scene is connected to what he calls "values". Reentry is supposed to occur between cortical maps and specific locations in the limbic system that implement values. The latter connection accounts for the evaluation of the perceived situation. Areas responsible for evaluation (esp. limbic system, hypothalamus, brain stem) are phylogenetically older than those performing categorisation (thalamus and cortex). Both systems are necessary for consciousness.

This account by Edelman is attractive, but it is far from being fully developed. For instance, Edelman's theory does not help understand why some complex cognitive processes are performed unconsciously. Also, Edelman's description is a purely neuronal account. There is no indication of any specific role that qualitative properties of experience could play, even if the author claims that consciousness is cognitively efficient and increases evolutionary adaptation of individuals. We shall now consider another neural account of phenomenal consciousness that may allow us to avoid these drawbacks.

Our hypothesis is that phenomenal consciousness has an adaptive function which is to allow discrimination and labelling of perceptual and mental states. The issue of knowing how labelling is achieved is connected to a problem concerning perception itself, known as the binding problem. As Damasio puts it :

It is not enough for the brain to analyze the world into its components parts : the brain must bind together those parts that make whole entities and events, both for recognition and recall. Consciousness must necessarily be based on the mechanisms that perform the binding. [START_REF] Damasio | The brain binds entities and events by multiregional activation from convergence zones[END_REF] In the brain, contrary to what happens in computers, different kinds of processing occur in different locations. For instance, colour analysis, shape recognition, movement and several other characteristics of visual scenes are detected in separate parts of the visual cortex. However, our brain constructs a single and global view of the scene. This integration requires a binding mechanism, so that we are able to simultaneously assign red colour, direction and form to a single object of the visual scene, that object moving toward us over there that we identified as a car. Objects exists as complex representations in our mind because we are able to link several phenomenal characteristics we could extract from our sensory processing and correlate them together as single objects. As we said, qualitative experience is not a general property of our mental states and mental processes. We claimed that different aspects of experience depend on different sensory modalities. However, qualitative properties experienced in a given situation are bound together across modalities and are unified into a single representation.

Synchronous neural activity, since von der Malsburg [1986] and others, is often invoked to account for binding. It has been experimentally observed that neurones located in different cortical areas may function synchronously [START_REF] Singer | Synchronization of cortical activity and its putative role in information processing and learning[END_REF]]. Evidence from neurophysiology and from connectionist studies [von der [START_REF] Der Malsburg | A Neural Cocktail-Party Processor[END_REF] suggested that frequency locking between neurone groups could account for the integration of different features of a given perceived situation.

Binding through synchronous neural activity is temporary. This explains why its combinatory power is virtually infinite. As [START_REF] Singer | Synchronization of cortical activity and its putative role in information processing and learning[END_REF] puts it, "the essential advantage of assembly coding is that individual cells can participate at different times in the representation of different objects". Hence every combination of extracted characteristics can be integrated into a single representation and possibly memorised as such. This combinatory power is what is needed for a perceptual labelling device. Our suggestion is thus that (1) dynamic feature binding allows labelling of situations ; creatures with this ability can cope with much more complex environments ;

(2) phenomenal consciousness was selected as a way to perform labelling through binding.

At this point, we have an idea about the kind of adaptive role played by phenomenal consciousness. We also have plausible models of the way the labelling function may be implemented. We still need accounts for the role phenomenal consciousness played in its own evolutionary emergence. Was it directly selected, or is it an evolutionary epiphenomenon ?

An evolutionary role for phenomenal consciousness

Phenomenal variety and signal discrimination

The claim that qualitative experience directly contributed to the ability of individuals to adapt to their environment during phylogenesis is equivalent to saying that qualitative experience is part of the phenotype. In evolutionary systems, we call phenotype the set of characteristics which are directly evaluated in the selection process [Dessalles 1992[START_REF] Dessalles | L'ordinateur génétique[END_REF]. Let us consider an analogy. Ethologists consider bird songs as adaptive : a mute song bird would not perform well, being unable to signal its territory properly. The ancestors of song birds were selected for their ability to sing. Should we consider that singing itself was selected, or rather that the syrinx (bird pharynx) was selected in order to allow territory signalling ? Perhaps we should look at the neural processes that are involved in singing and say they were also selected for territorial signalling purposes. What did selection retain after all, if not the genetic changes that make the difference between song birds and their non-singing ancestors ? From genes to neural processes, syrinx and song, there is a long chain of embryological events. Each of them is necessary for singing to occur. However, when ethologists study song birds, they are more prone to consider that the song itself was shaped by evolution to perform territory signalling, rather than syrinx or neural states. There are two reasons for this : first, actual songs seem to be optimal according to the way "fitness" (here efficient territory signalling) is assessed 7 ; second, the fitness of the song can be assessed directly, whereas the fitness of syrinx is indirect and we must refer to the singing ability 8 .

For the same reasons, we claim that from an evolutionary perspective we should include phenomenal consciousness into the phenotype of conscious beings rather than the neural states that underlie qualitative experiences. We indicated how phenomenal consciousness, through its labelling ability, could be assigned a fitness value. Now we want to show that qualitative properties of experience are, in a sense, optimal for the labelling ability. We should however be aware of two difficulties. Bird song can in no way be considered as an evolutionary epiphenomenon as phenomenal consciousness can. Also, even if song is a more abstract entity than physiological organs, it can be objectively measured, whereas qualitative experiences are not accessible : they are private to a single, subjective perspective [START_REF] Nagel | What is it like to be a bat ?[END_REF]].

We assume that phenomenal consciousness is a biological characteristic of living species, so we should be able to account for it in terms of natural selection. Any observed complex characteristic of living beings which is not a side-effect must have (or have had) an adaptive value 9 . We suggested that phenomenal consciousness is associated with an adaptive function, which is to label experience at the output of perceptual systems, in such a way that representations do not necessarily become purely abstract when they reach central systems. However, we have no direct evidence showing that phenomenal consciousness was itself selected to perform this labelling function. We still have to discard the possibility that it is an evolutionary epiphenomenon : neural processes could have been selected directly to perform the labelling function, and they would happen to have phenomenal correlates. The question is thus to know whether qualitative experiences are phenotypic or not. Can we assess the optimality of neural processes performing labelling without making reference to qualitative experiences ?

We want to suggest that there is a "mapping" between the physical input space and the qualitative space, and that such a mapping is not predicted by the epiphenomenon hypothesis. Consider an example from phonology. The three vowels [a], [i] and [u], present in words like apple, see, and fool 10 , are basic phonemes present in virtually all natural languages [START_REF] Maddieson | Patterns of sounds[END_REF]]. Being able to distinguish them is thus essential for any human being. [a], [i] and [u] look indeed very different to a human ear. This qualitative contrasted appearance is consistent with the fact that the discrimination performance is maximum for these vocalic phonemes [START_REF] Lindblom | Phonetic universals in vowel systems[END_REF]]. It can be shown though spectral analysis that these three phonemes are objectively "distant" : by measuring basic spectral characteristics called "formants", acousticians show that [a],

[i] and [u] are located in opposite corners of the accessible space. These studies by acoustic engineers are generally considered as relevant because they establish an objective link between our intuition (the three vowels look different) and the requirements of robust communication (symbols used for communication should be maximally different to be easily distinguished). From another perspective, however, such an apparently plausible result should be regarded as quite unlikely. Why should our qualitative feeling about the dissimilarity of these phonemes be correlated with communication requirements ? If qualitative experience is nothing but an evolutionary epiphenomenon, we would expect no such dissimilarity between qualitative states corresponding to the perception of [a], [i] and [u].

This example reminds us that for some discrimination tasks 11 , it seems that we are fully aware of all the differences we are able to detect. In other words, in such cases, our discriminatory power is entirely due to the grain of qualitative conscious aspects of our experience. Our performance relies on the fact that all the qualities we are able to experience in a given modality are different and separate. We can take other examples involving colour or flavour discrimination. We are aware of all colour shades that we can discriminate. This good performance, compared to other mammal species, is due to the fact that normal human beings 12 experience different wavelengths in a contrasted way. For instance, colours usually distinguished in English have quite contrasting qualitative appearances. We can even assess subjective distances by saying that blue is closer to violet than to yellow. Similarly, pineapple taste is not so far from lemon, but not at all like tomato. All the stimuli which are biologically relevant and that we effortlessly discriminate induce clearly distinct qualitative experiences. This is hard to explain if phenomenal consciousness was not involved in the evolutionary process. Why aren't there colours (or tastes or sounds) that we would experience as identical but that we would still be able to discriminate ? If phenomenal experience was a mere by-product of neural evolution, we could suppose that only neural processes are needed for detecting physical information without calling for the corresponding qualitative states.

Phenomenal variety, the fact that qualitative experiences in a given modality are differentiated, may be given a technical explanation. It is well-known, from an engineering perspective, that signal discrimination is easier if signals are spread over a wide energy range and compared to maximally distinct patterns 13 . [a], [i] and [u] are acoustically the most distinctive vocalic sounds our vocal tract is able to emit. The fact that we experience phonemes like [a], [i] and [u] as clearly distinct suggest that phenomenal properties are involved in the discrimination process and that they carry information.

The only possibility which is consistent with phenomenal variety is that qualitative experience is not an evolutionary epiphenomenon : it plays a direct role in discrimination and as a consequence was selected for its own sake. In other words, we perform discriminations on the basis of phenomenal qualities. First conscious species were selected according to this ability which requires a rich repertoire of phenomenal qualities in each modality. The fact that qualitative experience has a modular structure that systematically mirrors the organisation of perceptual systems, and the fact that it meets constraints of signal discrimination efficiency by keeping relevant qualitative properties scrupulously apart, suggest that phenomenal consciousness was itself involved in the evolution process.

Selection pressure on qualitative experience

Our claim is that phenomenal consciousness is optimally designed to perform its function, which in our view is to label perceptual and mental states. It is associated with the output of each modal sensory processing where it makes relevant signals the most discernible. This is exactly what we expect from a perceptual labelling device designed by natural selection. If we accept this hypothesis and think that phenomenal consciousness has been directly produced by evolution to fulfil an adaptive function, then we may consider (1) that phenomenal consciousness is phenotypic and (2) that neural states underlying phenomenal states only exist because the latter have an adaptive function. In this sense, phenomenal consciousness is part of the phenotype, exactly as bird song in our example. Underlying neural devices are not themselves phenotypic, since they are just a link in the long chain going from genes to phenomenal consciousness. If we follow the analogy with bird song, optimality of qualitative experience can be directly understood, whereas the optimality of underlying neural states would only appear through a reference to phenomenal properties.

From this perspective, phenomenal consciousness is what led the evolution of cognitive systems towards increasing discriminatory capacities. If phenomenal qualities were epiphenomenal, our perceptions would not give rise to such a variety of phenomenal states. The richness and the vividness of our phenomenal repertoire suggests that it is the direct product of natural selection. Under this hypothesis, qualitative experience has to be seen as a driving element in the evolutionary process which produced both our rich perception of the environment and our ability to discriminate mental states. It is thus indirectly responsible for our ability to learn efficiently.

Conclusion

We presented phenomenal consciousness as modular. Qualitative properties of experience are associated with sensory modalities, they are and remain distinct even if they can be integrated into multimodal and conceptual representations of objects and events. According to the hypothesis presented here, an adaptive function of phenomenal consciousness is to be found in relation to this integration involving qualitative information. Qualitative properties play the role of labels. Through the combinatorial power of a binding mechanism based on synchronous firing of neurones, representations may be multimodal and yet preserve contextual and modally distinguished perceptual aspects. Conscious organisms are thus able to discriminate among their perceptual representations. They are neither highly specialised robots nor purely abstract general problem solvers. Phenomenal consciousness allow them to better cope with the wide range of situations found in a complex ecological environment.

Higher-order cognitive processes have to be sensitive to qualitative properties of experience in order to determine the source of mental representations. According to our hypothesis, this is made possible by the fact that qualitative properties play the role of labels that carry information about the origin of representations.

The structural features of phenomenal consciousness, its modularity and the variety of qualitative properties within each modality, are in accordance with what we expect from a labelling device. On the other hand, alternative accounts in terms of neural states that consider qualitative properties as epiphenomenal can hardly explain the richness and the vividness of the qualitative repertoire. Phenomenal consciousness should be considered as a proper phenotypic character. Phenomenal consciousness is what natural selection could act upon. Any increase in qualitative variety was likely to induce a more probable survival of individuals. This might explain why phenomenal properties of experience, which seem to be optimally designed for the labelling of representations, were selected and designed by evolution.

Notes

1 The feeling of being a single entity, the fact that some recalled events look familiar, the feeling of "ownership" about our mental states, the first-person point of view, the ability to observe aspects of our cognitive functioning are other important features of what is called consciousness. Nevertheless, all of them are different aspects of consciousness, each one might be related to different cognitive functions and may eventually call for different accounts [START_REF] Zalla | Unité et multiplicité de la conscience : une étude critique des théories contemporaines à la lumière d'une hypothèse modulariste[END_REF]]. 2 In the modular theory of consciousness put forward by R. [START_REF] Jackendoff | Consciousness and the Comuputational Mind[END_REF], only the intermediate level, where sensory information has been processed in a modality specific way but has not yet reached central representations, supports awareness. 3 The reader may object that topological information is transmitted as such, from map to map, in neural visual processing. But what is conveyed here is signal, not information. Neighbouring relations are present in the matrix for an external observer, but they do not exist as such for the brain until they are detected. And they are lost afterwards. An edge detector may use topology among pixels. At the output of this detector, topological relationships between pixels do not exist anymore, simply because at this stage pixels are no longer represented. Topology among edges is preserved in the signal, only because it has not yet been detected. 4 We speak here of biological plausible circuitry as we imagine it, since it is technically possible to perform anything with neurones, even compute square roots.

5 Suengas and Johnson's experiments [1988, p.388] also demonstrated that both emotion when recalling imaginary events and lack of clarity when recalling real events reduce qualitative differences between these two types of memories and thus tend to generate some kind of confabulation. 6 A syndrome associated with deep lesions in the right posterior, non-linguistic hemisphere is characterised by the patient's denial of "ownership" of his paralysed, left arm. Conversely, normal subjects experience the loss of a limb very much as a loss of "a part of themselves". We can suppose that the lack of proprioceptual qualitative states is the cause of one's misattribution of parts of the body. 7 For instance, characteristics of bird songs produced by different neighbouring species are very different. The male bird can thus be correctly identified by females of its species. 8 By contrast, a physiologist would not be interested in territory signalling. She would consider syrinx as phenotypic and the ability to produce a distinctive song as a way to assess syrinx fitness. 9 Strictly speaking, the adaptive value should be assessed at the gene level [Dawkins 1978]. Neutralists [START_REF] Kimura | The neutral theory of molecular evolution[END_REF]] have claimed that random shifts are an important aspect to explain evolution ; however the probability that complex functional characteristics emerge from random shift is virtually zero. 10 In French, these phonemes are present in words like plat, vie, roue. In English, apple starts with [ae]. Better examples for [a] would be words like lie and now in which the first part of the diphthong is considered. 11 According to the modular description that we adopted, this happens at a certain level of input analysis, at the output of sensory modules. 12 Colour blind subjects being of course excluded. 13 In digital communications, possible waveforms should be chosen so that the energy of their difference is maximal.

Authors' names are listed alphabetically. oct. 1996


Jean-Louis Dessalles

Why is language well-designed for communication?

Selection through iterated learning explains no more than other non-functional accounts, such as universal grammar, why language is so well-designed for communicative efficiency. It does not predict several distinctive features of language like central embedding, large lexicons or the lack of iconicity, that seem to serve communication purposes at the expense of learnability.

Christiansen and Chater rightfully observe that communicatively arbitrary principles, such as UG, are unable to explain why language is adequate for communication. The same criticism can be addressed, however, to their own account. If the main driving force that led to language emergence is learnability rather than communicative efficiency, language should be locally optimal for the former and not for the latter. Evidence suggests that, in several respects, the exact opposite is the case.

What would language be like if, as the authors claim, the cultural selection of learnable languages were "stronger" than the biological selection of brains designed for efficient communication? If language can compare with a "viral" entity that gets selected for its ability to resist vertical cultural transmission, we predict for instance iconic signifiers, especially gestures, to win the contest. Yet, although analogue resemblance makes learning almost trivial, linguistic evolution shows that non-iconic signifiers tend to prevail, even in sign languages.

The "viral" theory of language does not explain the size of lexicons either. Ideally, an expressive code is easiest to learn, and resists iterated transmission best, if words are limited in number and have separate and unambiguous meanings. Yet, real vocabularies include tens of thousands of words, massive near synonymy and many rare unpredictable word combinations [START_REF] Briscoe | Language learning, power laws, and sexual selection[END_REF]. Such evidence suggests that there may be some "viral" cause for the existence of plethoric lexicons, but its action is opposite to what is expected from selection for learning efficiency.

Language, as mainly shaped by selection through repeated learning, is supposed to mirror the general human induction bias. Efficient induction systems [START_REF] Solomonoff | Complexity-based induction systems: Comparisons and convergence theorems[END_REF], including human learning [START_REF] Chater | The search for simplicity: A fundamental cognitive principle?[END_REF] and analogy making [START_REF] Cornuéjols | Analogie, principe d'économie et complexité algorithmique[END_REF] are guided by a complexity minimization principle. If languages were the bare expression of a simplicity-based induction device looping on itself, we should expect the complexity of languages to converge to a minimal amount. A similar claim is that general-purpose learning devices, except in rote learning mode, produce only "good shapes" (Gestalten), i.e. structures that are left invariant by operations forming an algebraic group (Dessalles, 1998a). Language has not, so far, been described as involving good shapes. For instance, syntactic structures, contrary to many other aspects of cognition, cannot be induced as invariants of transformation groups [START_REF] Piattelli-Palmarini | Théories du langage -Théories de l'apprentissage[END_REF]) and seem to thwart general inductive processes [START_REF] Piattelli-Palmarini | Evolution, selection and cognition: From 'learning' to parameter setting in biology and in the study of language[END_REF].

In a bio-functional account of language emergence, learnability puts limits on what is admissible, but is subordinate to communicative functions. The two main proximal functions of language in our species, as revealed by the observation of spontaneous language behavior, are conversational narratives and argumentative discussion [START_REF] Bruner | Actual minds, possible worlds[END_REF][START_REF] Dessalles | Why we talk -The evolutionary origins of language[END_REF]. From a bio-functional perspective, iconicity is dispensable if the problem is to express predicates for argumentative purposes [START_REF] Dessalles | Why we talk -The evolutionary origins of language[END_REF]. Lexical proliferation is predicted if the problem is to signal unexpectedness in narratives and to express nuances in argumentative discussion [START_REF] Dessalles | Why we talk -The evolutionary origins of language[END_REF]. And language-specific learning bias is expected if early language performance makes a biological difference. Let us consider a fourth example to show that functional aspects of language could evolve at the expense of learnability.

Non-functional accounts of language, including cultural selection through iterated learning, do not account for the existence of central embedding (the fact that any branch may grow in a syntactic tree), a feature present in virtually all languages. Recursive syntax has been shown to emerge through iterated learning, but only when individuals already have the built-in ability to use recursive grammars to parse linguistic input (e.g. [START_REF] Kirby | Learning, bottlenecks and the evolution of recursive syntax[END_REF]. A bio-functional approach to language provides an explanation for the presence of central embedding in language. As soon as the cognitive ability to form predicates is available, possibly for argumentative purposes [START_REF] Dessalles | Why we talk -The evolutionary origins of language[END_REF], predicates can be recruited to determine the arguments of other predicates. This technique is implemented in computer languages such as Prolog. To express "Mary hit Paul" for listeners who do not know Mary, the speaker may use "Mary ate with us yesterday" to determine the first argument of "hit". Prolog achieves this through explicit variable sharing, whereas human languages connect phrases for the same purpose: "The girl who ate with us yesterday hit Paul" [START_REF] Dessalles | Why we talk -The evolutionary origins of language[END_REF].

Predicates P 1i can therefore be used to determine arguments in a given predicate P 1 ; but each P 1i may require further predicates P 1ij to determine its own arguments. This possibility leads to recursive syntactic processing that produces central embedded phrase structures. Models that ignore functions like predicate argument determination cannot account for the necessity of embedded phrase processing. They merely postulate it, either as a consequence of some fortuitous genetic accident [START_REF] Chomsky | Reflections on language[END_REF] or deduce it from a general cognitive ability to perform recursive parsing [START_REF] Kirby | Learning, bottlenecks and the evolution of recursive syntax[END_REF]. But then, the adequacy to the function is left unexplained as well. No single genetic accident and no selection through repeated learning can predict that phrase embedding will efficiently fulfill predicate argument determination. Only a bio-functional approach that derives the existence of phrase embedding from its function can hope to explain why recursive processing came to exist and why it is locally optimal for that function.

From a phylogenetic perspective we may wonder why, if human languages have been selected to be easily learned, chimpanzees are so far from acquiring them, spontaneously or not. One must hypothesize some yet unknown qualitative gap between animal and human general learning abilities. Invoking such "pre-adaptation" remains, for now, non-parsimonious. Not only is the emergence of "pre-adaptations" not accounted for in iterated learning models and more broadly in non-functional models, but their subsequent assemblage into a functional whole remains mysterious as well. Biofunctional approaches to language emergence avoid the "pre-adaptation" trap. They do not attempt to explain why a given feature did not occur in other lineages by invoking the lack of required "pre-adaptations".

Language is not a marginal habit that would be incidentally used in our species. It has dramatic influence, not merely on survival but on differential reproduction, which is what determines natural selection. Individuals who fail to be relevant are excluded from social networks and become preferential victims (Dessalles 1998b;[START_REF] Dessalles | Why we talk -The evolutionary origins of language[END_REF]. Given the crucial impact of conversational performance on reproductive success, it would be highly unlikely that human brains could have evolved independently from language.


Jérôme Lodewyck

Michal Zawada

Luca Lorini

Mikhail Gurov

Pierre Lemonde

Observation and cancellation of the dc Stark shift in strontium optical lattice clocks

We report on the observation of a dc Stark frequency shift at the 10 -13 level by comparing two strontium optical lattice clocks. This frequency shift arises from the presence of electric charges trapped on dielectric surfaces placed under vacuum close to the atomic sample. We show that these charges can be eliminated by shining UV light on the dielectric surfaces, and characterize the residual dc Stark frequency shift on the clock transition at the 10 -18 level by applying an external electric field. This study shows that the dc Stark shift can play an important role in the accuracy budget of lattice clocks, and should be duly taken into account.

I. INTRODUCTION

The development of optical clocks with neutral atoms trapped in an optical lattice has rapidly progressed over the last decade, now surpassing the best microwave cesium clocks with a stability slightly above 10 -15 / √ τ and an accuracy at the 10 -16 level [START_REF] Katori | Ultrastable optical clock with neutral atoms in an engineered light shift trap[END_REF], [START_REF] Takamoto | An optical lattice clock[END_REF], [START_REF] Ludlow | Sr lattice clock at 1 × 10 -16 fractional uncertainty by remote optical evaluation with a ca clock[END_REF], [START_REF] Baillard | An optical lattice clock with spin-polarized 87 Sr atoms[END_REF], [START_REF] Lemke | Spin-1/2 optical lattice clock[END_REF], [START_REF] St | The 87 Sr optical frequency standard at PTB[END_REF]. In addition, a large room for improvement remains, as the stability of lattice clocks is still two orders of magnitude away from the quantum projection noise limit associated with the large number of probed atoms, and the limits in accuracy will likely be pushed in the near future. Optical lattice clocks are therefore good candidates for future optical frequency references. Among them, the development of strontium lattice clocks has received a large attention, with a number of systems already in operation and under construction.

Many systematic effects in optical lattice clocks arise from the electromagnetic environment of the trapped atoms. The frequency of the atomic transition is displaced by the laser beams forming the optical lattice in which the atoms are confined, through the ac Stark effect, although this displacement is mostly canceled at the 'magic' wavelength [START_REF] Katori | Ultrastable optical clock with neutral atoms in an engineered light shift trap[END_REF], [START_REF] Takamoto | An optical lattice clock[END_REF], [START_REF] Westergaard | Lattice-induced frequency shifts in sr optical lattice clocks at the 10 -17 level[END_REF]. Other radiations are also involved, such as the clock laser itself, and, more importantly, the blackbody radiation emanating from the vacuum vessel at room temperature. This later systematic effect is now under extensive study [START_REF] Th | Tackling the blackbody shift in a strontium optical lattice clock[END_REF] as it is the current limitation to the accuracy of strontium lattice clocks. The influence of the static magnetic field is also considered: the linear Zeeman effect is canceled by alternatively probing magnetic sub-states with opposite quantum numbers m F split by a bias magnetic field, while its quadratic component is accurately estimated from the measurement of the magnetic field given by the Zeeman splitting. However, the dc Stark shift resulting from the interaction of the atoms with a static electric field had never been considered in the accuracy budget of optical lattice clocks, although an electric field as low as 1 kV/m is enough to induce a shift almost two orders of magnitude larger than the claimed accuracy of optical lattice clocks. Because optical lattice clocks usually require a lot of optical access to cool and trap the atoms, a large solid angle of the environment is made of dielectric material that do not effectively shield the atoms from stray electric fields.

In this article, we report on the observation, cancellation and characterization of a dc Stark frequency shift detected by comparing the two strontium lattice clocks in operation at LNE-SYRTE. This effect was induced by the presence of electric charges on the surface of curved mirrors placed a few centimeters from the atoms.

This paper is organized as follows. First, we describe the motivations and design goals of optical cavities for the optical lattice in lattice clocks. Then, we report on the observation of a large frequency shift induced by the presence of electric charges on the mirrors of such a cavity. In section IV, we demonstrate experimental procedures to eliminate and characterize these charges. This papers ends with a prospective study of cavity designs that protect the atoms from electric charges.

II. OPTICAL LATTICES FORMED IN OPTICAL CAVITY

The light shifts induced by the optical lattice is the main specificity of optical lattice clocks. For alkaline-earth atoms considered for optical clocks, the shift associated with the scalar polarizability of the atomic levels involved in the clock transition is by far the largest one, but it is completely canceled at the magic wavelength of the optical lattice. However, other frequency shifts (due to the tensor polarizability, hyperpolarizability and multi-polar polarizabilities [START_REF] Katori | Ultrastable optical clock with neutral atoms in an engineered light shift trap[END_REF], [START_REF] Brusch | Hyperpolarizability effects in a Sr optical lattice clock[END_REF], [START_REF] Westergaard | Lattice-induced frequency shifts in sr optical lattice clocks at the 10 -17 level[END_REF]) do not exhibit such a cancellation, and, although they are orders of magnitude smaller than the scalar shift, require a wellengineered optical lattice in terms of power and polarization in order to characterize and control them. For this purpose, lattices in optical cavities provide for a clean and reproducible trapping potential for the atoms. Furthermore, the power buildup in the resonator enables us to reach very large trap depths thanks to which we could recently measure the various lattice light shifts for strontium atoms to an unprecedented level [START_REF] Westergaard | Lattice-induced frequency shifts in sr optical lattice clocks at the 10 -17 level[END_REF]. This study tackled many of the main effects that used to limit the accuracy of strontium lattice clocks. We expect that similar optical cavities will be implemented in other lattice clocks to measure the light shift effects with other atomic species.

We have built two optical lattice cavities at LNE-SYRTE, with different cavity designs. The first one consists of a L shaped resonator with two meniscus mirrors of curvature radius 200 and 125 mm separated by 320 mm. Inside the cavity lay the two windows of the vacuum chamber and a 45 plate playing the role of a polarization discriminator. The eigen-mode of the cavity has a waist of 89 µm where the Sr atoms are trapped. The finesse of the cavity is 80 and the intra-cavity optical power goes up to 12 W. The resonance frequencies of the two polarization eigen-modes of the cavity are distinct enough to ensure a stable polarization with a pure linearity (> 99.9% in terms of electric field). The length of the cavity is locked with a Hänsch Couillaud (HC) system to keep the laser light on resonance.

For the second clock a rather different design has been chosen. We opted for a linear cavity entirely placed under vacuum with a length of 60 mm. The plane-concave mirrors are used as the vacuum chamber windows and are articulated through flexible bellows. Their curvature radius is 35 mm, thus forming a lattice with a 56 µm waist and a finesse of 150. The length of the cavity is locked by a Pound-Drever-Hall (PDH) scheme. This newer design brings a number of improvements over the first design:

• The line-width of the cavity is larger, thus dramatically reducing the efficiency of the laser frequency-to-amplitude noise conversion by the cavity. Indeed, the amplitude noise in the cavity parametrically heats the atoms and eventually expels them from the trapping potential [START_REF] Savard | Laser-noise-induced heating in far-off resonance optical traps[END_REF].

With the first cavity design, a pre-stabilization of the tapping laser frequency on a reference cavity is required, and the trapping lifetime is limited to about 500 ms. With the second design, the parametric heating is no more a limit for the lifetime of the trapped atoms. • The polarization eigen-modes are degenerated, which allows the complete study of the polarization dependence of the polarizability and hyper-polarizability of the clock transition.

• Because of the absence of intra-cavity optical elements, we have a unity impedance matching of the input beam with the cavity. Combined with a higher finesse and a smaller waist, higher trap depths can be reached. • A short cavity placed under vacuum enables a solid design insensitive to mechanical drifts and dust accumulation. Moreover, the PDH lock requires less tuning than the HC scheme. With this configuration, the long term operation of the clock is more reliable and suits more efficiently the long integration times usually required for metrology experiments. Effectively, this cavity has been in operation for many months with only minor realignment.

Optical lattices in cavities enables us to access very large trapping depths. With the first clock, depths as large as U = 1000 E R (E R being the recoil energy associated with the absorption of one lattice photon) could be achieved. In the second clock, with its finer cavity, trapping potentials as deep as 2600 E R were observed. These depths are ideally suited to explore the polarizability and hyper-polarizability of the clock Figure 1. Temporal decrease of the frequency difference between the two strontium clocks. The exponential decay is due to the slow discharge of the charges through the mirror substrate. We measured a time constant of 264 days for the frequency decay corresponding to 528 days for the charge because of the quadratic nature of the Stark effect. Knowing the static polarizability α of the strontium clock transition [START_REF] Porsev | Determination of sr properties for a high-accuracy optical clock[END_REF], we can deduce the magnitude of the electric field at level of the atoms of 3.4 kV/m at t = 0. Inset: closer view on the first points. On a short time scale, no variation of the frequency shift was observed as the frequency difference between the two clocks averages down with a statistical uncertainty below 10 -16 after one hour of measurement.

transition. Moreover, with such large trap depths, the second stage Magento-Optical Trap (MOT) usually used to bridge the temperature gap between the Sr MOT using the intense 1 S 0 -→ 1 P 1 transition at 2 mK and the shallow dipoletrapping depth is not required. Instead, we use the 'drain' technique [START_REF] Brusch | Hyperpolarizability effects in a Sr optical lattice clock[END_REF] in which the deep lattice is overlapped with the optical lattice as well as two laser beams tuned on the 1 S 0 -→ 3 P 1 (689 nm) and 3 P 1 -→ 3 S 1 (688 nm) transition with a waist slightly smaller than the optical lattice. With these two beams, the atoms from the MOT that cross the lattice position are drained in the 3 P 0 and 3 P 2 metastable state and therefore escape the MOT dynamics. Among these, the coldest fraction accumulate in the lattice dipole trap. Within a few 100 ms, about 10 4 atoms are loaded in the optical lattice, scaling as U 3/2 .

III. OBSERVATION OF ELECTRIC CHARGES ON THE CAVITY

MIRRORS

Upon completion of the in-vacuum cavity for the second clock, a frequency difference on the order of 10 -13 was observed between the two strontium lattice clocks, orders of magnitude higher than the realized and expected accuracies of lattice clocks. This frequency difference has been attributed to the dc Stark shift induced by the presence of electric charges on the reflecting surface of the cavity mirrors. Quantitatively, if δE is the norm of the electric field induced by the charges, the stark shift δν on the clock frequency reads:

δν = - 1 2 α(δE) 2 , ( 1
)
where α is the difference between the static polarizabilities of the excited and fundamental states of the clock transition.

Over time, these charges slowly migrate through the mirror substrate to eventually reach the opposite plane surface itself connected to the electrical ground. Figure 1 shows the effect of the exponential discharge of the mirrors with a time constant of 528 days. On the short term, however, the stark shift shows itself very stable as it does not degrade the stability of the frequency difference between the two clocks over time scales of a few 10 4 seconds. In addition, it shows that building two similar frequency references is a crucial test for establishing the accuracy budget of the clocks.

By finite element modeling (FEM) of the mirrors geometry and ground planes of the vacuum chamber, we estimate the electric capacity of the optical cavity on the order of 1 pF, the difference of electric potential between the mirrors on the order of 1 kV, which yields an electric charge of ±1 nC. Subsequently, from the time constant of the discharge, we can estimate the resistivity of the mirror substrate at the level of 10 18 Ωm. The resistivity of glass is mainly determined by the amount of impurities it embeds, and therefore can span several orders of magnitude, from 10 15 Ωm to 10 18 Ωm. However, optical grade glasses usually feature the least impurity hence the highest resistivity, in agreement with our model and measurements.

The origin of the charges trapped on the mirrors is unclear, as we could not purposely charge them in a reproducible manner. These charges must originate from static electricity accumulated while the mirrors were assembled, because we observed that the pumping process of the vacuum chamber from atmospheric pressure does not recharge the mirrors.

Systematic shifts induced by patch charges is a known effect for ion clocks, in which the isolating dielectric of the electrodes or the collecting optics can easily be charged by the ionizing process. Fortunately, the motion of the ion usually reveals the presence of these charges [START_REF] Poitzsch | Cryogenic linear ion trap for accurate spectroscopy[END_REF]. To the best of our knowledge, this papers reports the first observation of the effect of electric charges in neutral optical clocks. Furthermore, the large magnitude of the effect, four orders of magnitude higher than the targeted accuracy of strontium clocks raises some concerns whether this effect might reveal itself to be a show-stopper for the further development of lattice clocks. Indeed, although an optical cavity close to the atoms may be an extreme scenario, the operation of optical lattice clocks requires a large optical access where dielectric component are prone to charge accumulation. In the following sections, we describe experiments we have conducted to eliminate and characterize the residual amount of charges as well as prospects for further reduction, if required.

IV. DISCHARGE AND MEASUREMENT OF PATCH CHARGES

The removal of trapped charges from the insulated surface of the mirrors inside the vacuum chamber was achieved by shining UV light on the mirrors through the vacuum chamber optical accesses. Such a technique had been successfully developed and characterized for gravitational wave detectors [START_REF] Pollack | Charge management for gravitational-wave observatories using uv leds[END_REF]. The most probable mechanism is the extraction of charges from the surface by the photoelectric effect. The charges would then follow the electric field lines to the ground. Each 'UV' tag represents a 30 s UV illumination with a broadband UV lamp centered on 360 nm distributed over a large solid angle. After each illumination, the Stark shift is halved, but a fast relaxation of the charges is observed shortly after (note that at 2000 s, the lattice wavelength was stabilized on the magic wavelength introducing a gap of several Hz in the frequency difference). After the illumination with a UV LED centered on 365 nm and a power of about 100 mW during 30 hours, the two strontium clock frequency matched at the 10 -16 level. Another mechanism based on the creation of unstable paramagnetic centers in silica has also been reported [START_REF] Ugolini | Discharging fused silica test masses with ultraviolet light[END_REF], but given the thickness of the mirrors and the discharge rate we obtained, this effect is unlikely to occur in our case. Figure 2 shows the reduction of the Stark shift on the clock transition after 2 minutes of illumination with UV light down to the 10 -15 level. After keeping the UV light for two consecutive nights, the frequency difference between the clocks became compatible with zero within the accuracy budget of the strontium lattice clocks on the order of 10 -16 .

In spite of the apparently fast discharge of the cavity mirrors and the good agreement of the two strontium clocks, a measurement of the electric field at the level of the atoms is still required to independently estimate the residual magnitude of the dc Stark effect that should appear in the clock accuracy budget. For this, we creates an electric field E at the level of the atoms with an electrode placed a few centimeters away from the atoms, outside the vacuum chamber, which . A voltage of V = 1 kV on the electrode is enough to induce a shift ∆ν = 26 Hz (or ∆ν/ν = 6 × 10 -14 in fractional units, ν = 429.228 THz being the Sr clock frequency) on the frequency of the clock transition. Because of the quadratic behavior of the Stark effect, this frequency shift should stay constant when the voltage of the electrode is reversed. If not, it reveals a residual electric field δE in the direction of the applied field E [START_REF] Matveev | Systematic frequency shifts in spectroscopy of 1s-2s transition in atomic hydrogen[END_REF] (see figure 3). Quantitatively, the residual systematic frequency shift δν due to the residual field δE reads:

δν = (∆ν + -∆ν -) 2 16∆ν , (2)
where ∆ν ± is the Stark shift on the clock frequency when the voltage ±V is applied on the electrode. This expression is remarkable in the sense that only the difference between the Stark shifts needs to be measured with a good precision. Furthermore we can see that the average Stark shift ∆ν acts as a leverage to measure δν with a good accuracy without requiring large integration times. Finally, when switching the polarity of the electrode, the difference δV between the absolute values of the voltage has to fulfill δV /V ≪ δE/E so that the voltage imbalance δV is not interpreted as a residual electric field. In our experiment, we have δV /V ≃ 10 -5 , which is sufficient for all practical purposes. For an electric field E non-orthogonal to the axis of the cavity, we measured ∆ν + -∆ν -= 0.1 ± 0.4 Hz (or (∆ν + -∆ν -)/ν = (2 ± 9) × 10 -16 in fractional units) after an integration time of 400 s. Taking into account the angle between the electrode and the cavity, this yields a residual systematic frequency shift due to the charges compatible with zero with an uncertainty well within the target accuracy for optical lattice clocks:

δν ν = 0 ± 1.5 × 10 -18 . (3)
To be complete, this experiment should be reproducted with three non-orthogonal directions of the applied electric field E. However, because the electrode has been carefully placed so that the electric field is not orthogonal to any specific plane on the vacuum vessel, and because the UV light is applied in an isotropic way, we can safetly conclude that this experiment shows that the UV light very efficiently removed the trapped charges.

Interestingly enough, after 400 s of integration, we could observe a revival of charges with a random walk behavior. After one hour, this new charge accumulation induced an electric field of about 130 V/m at the level of the atoms giving a Stark shift of δν/ν = 1.5×10 -16 . This accumulation can be dramatically enhanced by the same UV light used for discharging when shone in conjunction with the electric field A longer length means a small electric field experienced by the atoms. As expected, the effective length rises when the shield thickness increases or when the shield aperture reduces. The graph indicates the accuracy levels reached for given effective lengths assuming that the cavity mirrors are charge as we initially observed in our experiment. We can observe that the charges are efficiently screened, even for casual geometries of the shielding cylinder.

E. This observation backs the photo-electric interpretation of the effect of the UV light: UV photon can extract charges from the vacuum chamber material, and these charges follow the electric field lines, in this later case generated by the electrode. However, these charges rapidly disappeared within a few minutes after the electric field E and UV light were switched off, indicating that they were accumulating on a dielectric surface with a larger conductivity than the cavity mirrors, most probably the vacuum chamber window close to the electrode.

V. SHIELDING THE ATOMS FROM STRAY ELECTRIC FIELDS

Although we have seen in the previous section that charges could be easily removed using UV light, a better design of the vacuum chamber in which the atoms are shielded from the dielectric surfaces of the cavity mirrors could help to avoid the burden of repeatedly checking that no dc Stark shifts impairs the accuracy of the clock. For this purpose, we modified our finite element modeling of the vacuum chamber with the addition of grounded cylindrical pieces, made of conductive metal, that cover the surface of the cavity mirrors. These pieces are characterized by their thickness and by the inner diameter of the cylinder through which the lattice light reaches the mirrors. The FEM simulation gives the effective length of the cavity defined as the ratio between the difference of electric potential on the surface of the mirrors (imposed by the amount of charges trapped on the mirrors) and the electric field experienced by the atoms. For the unshielded cavity, the effective length is 30 cm. Figure 4 shows that a conductive cylinder with a 4 mm internal diameter and a thickness of 4 mm would be sufficient to reduce the Stark shift from 10 -13 to 10 -19 .

A more drastic method to get rid of charge accumulation on dielectric materials would be to cover the entire surface of the vacuum chamber with a thin layer of titanium using a sublimation pump. This thin layer would make the surface conductive, efficiently driving the charges to the ground. However, it is unclear whether such a process would preserve the finesse of the optical cavity.

VI. CONCLUSION

In this paper, we have proposed the first study of the dc Stark effect in optical lattice clocks. We have demonstrated that charges can accumulate to the point where they induce a shift orders of magnitude larger that the claimed clock accuracy. Then, we have proposed efficient methods to remove and characterize these charges. This study shows that the Stark effect, when carefully controlled, can play a negligible role in the clock accuracy budget, at the level of 10 -18 . This work has received funding from the European Community's FP7, ERA-NET Plus, under Grant Agreement No. 217257, as well as from CNES and ESA (SOC project)

Figure 2 .

 2 Figure 2. Frequency difference between the two strontium lattice clocks. Each 'UV' tag represents a 30 s UV illumination with a broadband UV lamp centered on 360 nm distributed over a large solid angle. After each illumination, the Stark shift is halved, but a fast relaxation of the charges is observed shortly after (note that at 2000 s, the lattice wavelength was stabilized on the magic wavelength introducing a gap of several Hz in the frequency difference). After the illumination with a UV LED centered on 365 nm and a power of about 100 mW during 30 hours, the two strontium clock frequency matched at the 10 -16 level.

Figure 3 .

 3 Figure 3. DC Stark shift of the frequency of the clock transition as a function of the electric field generated by an electrode placed in the vicinity of the atoms. The dashed parabola shows the Stark shift on the clock frequency without any residual field. A residual field δE due to charges slightly displaces the center of the parabola (thick solid parabola), thus introducing a difference ∆ν + -∆ν -in the frequency of the clock transition when probed with opposite voltages applied on the electrode. This sketch illustrates the large leverage of this technique: even a minute frequency shift δν on the clock transition in the absence of electrode voltage would translate in a large frequency unbalance in the presence of the electric field E.

Figure 4 .

 4 Figure 4. Effective electrical length of the optical lattice cavity for different shielding geometries. A longer length means a small electric field experienced by the atoms. As expected, the effective length rises when the shield thickness increases or when the shield aperture reduces. The graph indicates the accuracy levels reached for given effective lengths assuming that the cavity mirrors are charge as we initially observed in our experiment. We can observe that the charges are efficiently screened, even for casual geometries of the shielding cylinder.


Hélène Duez

Bart Staels

Circadian control of epigenetic modifications modulates metabolism. Subtitle: Circadian HDAC3/NCoR recruitment to DNA regulates hepatic lipid homeostasis

A new study in Science reveals how circadian epigenetic modification of DNA drives diurnal gene expression in mouse liver and is required for the maintenance of lipid homeostasis.

Numerous physiological processes (including sleep/wake cycles, body temperature, blood pressure, hormone secretion, metabolic pathways, etc display diurnal variations. These circadian rhythms are programmed centrally by light, allowing alignment to seasonal variations of night/day cycles, and peripherally by food availability. Disruption of these circadian rhythms (due to frequent jetlag, sleep restriction or rotating shift work) predisposes to metabolic disorders and cardiovascular diseases [START_REF] Duez | Nuclear receptors linking circadian rhythms and cardiometabolic control[END_REF] . Genome-wide studies indicate that the expression of 10-15% of genes cycles in many organs involved in metabolic control, such as the liver, and cardiovascular function, such as the vascular wall and the heart. Although many of the transcription factors of the clock machinery also exert metabolic regulatory activities, the molecular mechanisms driving the rhythmic expression pattern of metabolic genes have been unclear until now.

Feng et al. now identify circadian variations in chromatin modifications as one contributing mechanism [START_REF] Feng | A circadian rhythm orchestrated by histone deacetylase 3 controls hepatic lipid metabolism[END_REF] . In a series of elegant experiments they show that histone deacetylase 3 (HDAC3) binding to DNA occurs in a circadian manner, with more than 14,000 specific binding sites present during the inactive light phase, whereas only 120 sites are occupied during the dark phase active feeding period in mouse liver. This rhythmic DNA binding pattern was retained in constant darkness, but reversed upon feeding restriction to daytime, indicating that this binding rhythm is under the control of the clock machinery and can be entrained by food availability in the liver. HDAC3 is a histone lysine deacetylase, which modifies chromatin compaction, hence rendering DNA less accessible for transcription. In line, Feng et al. observed an opposite circadian pattern of H3K9 acetylation and recruitment of RNA polymerase II to gene transcription start sites, revealing a parallel rhythmicity between HDAC3 recruitment and repression of gene transcription. Since histone modifying enzymes do not bind directly to DNA, the authors went on to show that HDAC3 recruitment to DNA oscillates in phase with the circadian expression of the nuclear receptor and transcriptional repressor Rev-erbα, and that HDAC3-binding regions overlap with binding sites for Rev-erbα and the nuclear co-repressor NCoR. Consistent with this model, the same group previously reported abnormal histone acetylation and circadian profiles of gene repression in mice expressing a mutant NCoR protein unable to interact with HDAC3 [START_REF] Alenghat | Nuclear receptor corepressor and histone deacetylase 3 govern circadian metabolic physiology[END_REF] .

Together these data indicate that rhythmic recruitment of a HDAC3/Rev-erbα/NCoR repressive complex to specific sites in the DNA elicits a strong diurnal pattern of gene expression.

What is the biological implication of these observations? In addition to being part of the molecular clock machinery 4 , Rev-erbα is a metabolic regulator controlling lipid [START_REF] Raspe | Identification of Rev-erbalpha as a physiological repressor of apoC-III gene transcription[END_REF] , glucose [START_REF] Yin | Rev-erbalpha, a heme sensor that coordinates metabolic and circadian pathways[END_REF] and bile acid metabolism [START_REF] Duez | Regulation of bile acid synthesis by the nuclear receptor Rev-erbalpha[END_REF] , as well as adipogenesis 8;9 . In line, the authors found that HDAC3/Rev-erbα/NCoR recruitment triggers an anti-phasic hepatic expression pattern of genes involved in lipogenesis, suggesting that this transcription complex exerts circadian control of lipogenesis. Surprisingly, liver triglycerides accumulated not only in hepatic HDAC3-null but also in Reverbα-null mice. This contrasts with a publication by Le Martelot et al. [START_REF] Martelot | REV-ERBalpha participates in circadian SREBP signaling and bile acid homeostasis[END_REF] , who observed markedly reduced hepatic triglycerides in livers of Rev-erbα-deficient mice. These authors reported that Rev-erbα regulates temporal SREBP1c nuclear translocation through circadian regulation of INSIG2, a lipid level sensing protein, resulting in a phase-shift of SREBP1c target genes involved in de novo lipogenesis in Rev-erbα-deficient mice. Apart from differences in the background and construction of the mouse models, it is not clear why these two studies led to these conflicting results.

Chow-fed whole-body Rev-erbα-deficient mice displayed a fairly modest liver lipid accumulation compared to mice depleted in hepatic HDAC3 [START_REF] Feng | A circadian rhythm orchestrated by histone deacetylase 3 controls hepatic lipid metabolism[END_REF] . Contrastingly, de novo lipogenesis was induced to higher levels in Rev-erbα-deficient mice. Plausible, but yet to be proven explanations are compensation of Rev-erbα action by Rev-erbβ, compensation by Rev-erbα-deficiency in other tissues, and possibly, a counterbalancing increase in fatty acid oxidation. Rev-erbα also regulates hepatic glucose metabolism by repressing gluconeogenic gene expression [START_REF] Yin | Rev-erbalpha, a heme sensor that coordinates metabolic and circadian pathways[END_REF] , and chow-fed NCoR mutant mice display increased hepatic glucose output [START_REF] Alenghat | Nuclear receptor corepressor and histone deacetylase 3 govern circadian metabolic physiology[END_REF] . However, high fat-fed NCoR mutant mice are more insulin-sensitive, due to an increased energy expenditure, decreased fat mass and increased lipid burning associated with altered circadian expression patterns of genes involved in β-oxidation which were increased during the inactive light phase. In line, the >13,000 sites bound by both Rev-erbα and HDAC3 are likely not restricted to lipogenesis only and most of the processes in which these genes are engaged remain to be elucidated. Overall, it is likely that rhythmic recruitment of repressive transcriptional complexes contributes to the control of lipid metabolism. However, the versatility of the observed phenotypes suggests that the circadian clock acts as a modulator, rather than primary regulator of hepatic metabolism.

Alternatively, it cannot be excluded that other factors control the activity of clock transcription factors and/or that part of their actions are independent of their role as clock regulator.

This raises another challenging question, not addressed in the study by Feng et al.: what is the respective contribution of nutritional vs circadian regulatory signals in the control of lipogenesis? Major regulators of de novo lipogenesis are the nutritional state, with insulin and glucose playing a crucial role in the regulation of lipogenesis through the SREBP1c and ChREBP regulatory pathways. Intriguingly, Le Martelot et al. reported that dysregulation of SREBP1c target genes resulted in significantly reduced liver triglycerides only during the light phase, but not at night during the feeding phase. In addition, nutritional regulation (by fasting and high-carbohydrate refeeding) of lipogenic genes was retained in Rev-erbαdeficient mice, indicating that both circadian and nutritional signals regulate hepatic lipid metabolism through both interconnected and independent mechanisms [START_REF] Martelot | REV-ERBalpha participates in circadian SREBP signaling and bile acid homeostasis[END_REF] . This elegant study by Feng et al demonstrates that epigenomic circadian gene regulation orchestrates metabolic function, providing a mechanism linking the misalignment of circadian gene regulation to metabolic disorders. In humans, circadian disruption associated with chronic sleep disorders or rotating shift work increases the risk for the metabolic syndrome.

In addition, cardiovascular function also displays circadian variations. For instance, blood pressure and heart rate have a rhythmic profile, and acute cardiovascular events such as myocardial infarction occur predominantly in the early morning hours. A recent study indicated that shift-workers display accelerated atherosclerosis [START_REF] Puttonen | Shift work in young adults and carotid artery intima-media thickness: The Cardiovascular Risk in Young Finns study[END_REF] . Interestingly, mice with disrupted clock components display impaired vascular remodeling and aberrant ischemia/reperfusion responses, suggesting a direct role of the clock genes in vascular function 12;13 . Rev-erbα modulates the fibrinolysis cascade through regulation of PAI-1 expression [START_REF] Wang | The orphan nuclear receptor Rev-erb alpha regulates circadian expression of plasminogen activator inhibitor type 1[END_REF] , and the inflammatory response [START_REF] Fontaine | The nuclear receptor Rev-erbalpha is a liver X receptor (LXR) target gene driving a negative feedback loop on select LXR-induced pathways in human macrophages[END_REF] . Deletion of PPARγ, which regulates expression of the clock component Bmal1, in endothelial or smooth muscle cells, as well as deletion of Bmal1, results in blunted daily blood pressure variations [START_REF] Wang | Vascular PPARgamma controls circadian variation in blood pressure and heart rate through Bmal1[END_REF] . All these finding reinforce the idea that circadian anticipation is critical for normal energy homeostasis and cardiovascular function.

Major gaps in our understanding of circadian control of metabolism still remain. First, how does the nutritional state and circadian clock connect to control metabolism? Second, since it is unlikely that Rev-erb is the sole mediator of all HDAC3 actions, which are the other HDAC3 partners in the epigenomic control of metabolism? Third, what is the impact of the clock (and clock disorders) on human (patho)physiology, and, in particular, on cardiovascular diseases? Future studies should also address the question whether pharmacological modulation of nuclear receptor activity may provide a means to improve circadian disorderrelated metabolic and cardiovascular dysfunctions.

  Address for correspondence: UR1011 INSERM, Institut Pasteur de Lille, BP 245 1, rue Calmette, 59019 Lille, France. (e-mails : Bart.Staels@pasteur-lille.fr; Helene.Duez@pasteur-lille.fr).