Dr Jean-Luc Schwartz

Anahita Basirat

Lucie Ménard

Marc Sato

The Perception for Action Control Theory (PACT): a perceptuo-motor theory of speech perception

Keywords: perceptuo-motor interaction, speech perception, vowel perception, perceptual organisation, multisensory interactions, neurocomputational model, dorsal route

It is an old-standing debate in the field of speech communication to determine whether speech perception involves auditory or multisensory representations and processing, independently on any procedural knowledge about the production of speech units or on the contrary if it is based on a recoding of the sensory input in terms of articulatory gestures, as posited in the Motor Theory of Speech Perception. The discovery of mirror neurons in the last 15 years has strongly renewed the interest for motor theories. However, while these neurophysiological data clearly reinforce the plausibility of the role of motor properties in perception, it could lead in our view to incorrectly deemphasise the role of perceptual shaping, crucial in speech communication. The so-called Perception-for-Action-Control Theory (PACT) aims at defining a theoretical framework connecting in a principled way perceptual shaping and motor procedural knowledge in speech multisensory processing in the human brain. In this paper, the theory is presented in details. It is described how this theory fits with behavioural and linguistic data, concerning firstly vowel systems in human languages, and secondly the perceptual organization of the speech scene. Finally a neurocomputational framework is presented in connection with recent data on the possible functional role of the motor system in speech perception.

Introduction

It is an old-standing debate in the field of speech communication to determine whether speech perception involves auditory or multisensory representations and processing, independently on any procedural knowledge about the production of speech units (for a review, [START_REF] Diehl | Speech perception[END_REF]; or on the contrary if it is based on a recoding of the sensory input in terms of articulatory gestures, as posited in the Motor Theory of Speech Perception [START_REF] Liberman | A motor theory of speech perception[END_REF][START_REF] Liberman | The motor theory of speech perception revised[END_REF][START_REF] Liberman | On the relation of speech to language[END_REF]. The discovery of mirror neurons (for reviews, [START_REF] Rizzolatti | Neurophysiological mechanisms underlying the understanding and imitation of action[END_REF][START_REF] Rizzolatti | The Mirror-Neuron System[END_REF] in the last 15 years has strongly renewed the interest for motor theories. However, while these neurophysiological data clearly reinforce the plausibility of the role of motor properties in perception, it could lead in our view to incorrectly deemphasise the role of perceptual shaping, crucial in speech communication. The so-called Perception-for-Action-Control Theory (PACT) aims at defining a theoretical framework connecting in a principled way perceptual shaping and motor procedural knowledge in speech multisensory processing in the human brain. In the following, the theory will be presented in details in Section I.

Sections II and III will describe how this theory fits with behavioural and linguistic data, concerning firstly vowel systems in human languages, and secondly the perceptual organization of the speech scene. Section IV will consider a neuro-computational framework in connection with recent data on the possible functional role of the motor system in speech perception. The conclusion will address some perspectives in relation with language emergence.

I. The Perception for Action Control Theory (PACT) of speech perception

I.1. Action shapes perception

The Motor Theory of speech perception originated in the 50s [START_REF] Liberman | The role of selected stimulus variables in the perception of the unvoiced stop consonants[END_REF]Liberman, 1957) to tackle the problem of invariance and variability in relation with coarticulation phenomena (see [START_REF] Perkell | Invariance and Variability in Speech Processes[END_REF]. It was developed from the following statement, based on many experiments in speech synthesis. The coarticulation-driven composition of articulatory commands during speech production is non-linearly transformed into a complex composition of acoustic features, so that the acoustic properties of speech sounds are not invariant but context dependent, and the correspondence between sounds and phonemes is far from transparent.

However, speech percepts seem to be more directly related to gestures than to sounds. To recall two classical examples: the same acoustic feature, e.g. a stop burst centred at 1400 Hz, may be perceived as a labial [p] or a velar [k] depending on the following vowel, while conversely a single percept [d] may be associated to either a rising or a falling second formant transition whether the following vowel is [i] or [u]. According to Liberman andcolleagues (1952,1954), there results are driven by implicit information about coarticulation mechanisms. This led to the more general claim that speech perception involves recovery of articulatory gestures, or invariant motor commands, capitalising on the knowledge the listener has of speech production mechanisms (Liberman et al., 1967;[START_REF] Liberman | The motor theory of speech perception revised[END_REF]. Recent reviews of the Motor Theory (e.g. [START_REF] Liberman | On the relation of speech to language[END_REF][START_REF] Galantucci | The motor theory of speech perception reviewed[END_REF] recapitulate the major experimental arguments in favour of the Motor Theory (or its variant, the Direct Realist Theory of speech perception, [START_REF] Fowler | An event approach to the study of speech perception from a direct-realist perspective[END_REF].

They can be summarised in two parts.

The first series of arguments deal with the possible existence of a functional connection between speech perception and production systems, thanks to which the motor system would be accessed on line during the speech perception process. A number of behavioural data support this hypothesis, beginning by two classical studies. In the "selective adaptation" task, [START_REF] Cooper | Feature processing in the perception and production of speech[END_REF] showed how reiterated perception of a given stimulus not only affects the perception of following stimuli (the classical selective adaptation paradigm, Eimas & Corbitt, 1973) but also their production. Conversely, repetitive articulation of a speech stimulus also affects the identification of following stimuli [START_REF] Cooper | Articulatory effects on speech perception: a preliminary report[END_REF][START_REF] Cooper | Articulatory effects on speech perception: a second report[END_REF]). In the "close shadowing" task, [START_REF] Porter | Speech-production measures of speech perception: Rapid shadowing of VCV syllables[END_REF] and [START_REF] Porter | Rapid reproduction of vowel-vowel sequences: evidence for a fast and direct acoustic-motoric Linkage in Speech[END_REF] showed that the time between perceiving a speech item and reproducing it was so short that it suggested a direct selection of the adequate gesture within the perception task itself. A number of further studies, reviewed in [START_REF] Galantucci | The motor theory of speech perception reviewed[END_REF], also provide behavioural evidence that perceiving a speech sound activates action representations compatible with the input stimulus.

Another body of evidence in favour of a link between speech perception and production comes from recent neurophysiological studies. The discovery of mirror neurons in the macaque's brain and of a putative mirror system in humans (see [START_REF] Rizzolatti | Neurophysiological mechanisms underlying the understanding and imitation of action[END_REF][START_REF] Rizzolatti | The Mirror-Neuron System[END_REF], for a review) has provided strong evidence for the existence of an actionobservation matching system. Mirror neurons are a small subset of neurons, found in the macaque ventral premotor cortex and the anterior inferior parietal lobule, that fire both during the production of goal-directed actions and during watching to a similar action made by another individuals. [START_REF] Kohler | Hearing sounds, understanding actions: action representation in mirror neurons[END_REF] later extended the mirror neuron property to listening to the sound of an action, introducing "audiovisual" mirror neurons in monkeys. The existence of the mirror-neuron system thus suggests that actions may be understood in part by the same neural circuits that are used in action performance. In addition to action understanding, the human mirror-neuron system has been proposed to play a fundamental role in speech processing (e.g., [START_REF] Rizzolatti | Language within our grasp[END_REF][START_REF] Arbib | From Monkey-like Action Recognition to Human Language: An Evolutionary Framework for Neurolinguistics[END_REF][START_REF] Gentilucci | From manual gesture to speech: a gradual transition[END_REF] by providing a neurophysiological mechanism that creates parity between the speaker and the listener. In agreement with this view, brain areas involved in the planning and execution of speech gestures (i.e., the left inferior frontal gyrus, the ventral premotor and primary motor cortices) and areas subserving proprioception related to mouth movements (i.e., the somatosensory cortex), have been found to be activated during auditory, visual and/or auditoryvisual speech perception (e.g., Möttonen et al., 2004;[START_REF] Wilson | Listening to speech activates motor areas involved in speech production[END_REF][START_REF] Ojanen | Processing of audiovisual speech in Broca's area[END_REF][START_REF] Skipper | Listening to talking faces: Motor cortical activation during speech perception[END_REF][START_REF] Pekkola | Perception of matching and conflicting audiovisual speech in dyslexic and fluent readers: an fMRI study at 3T[END_REF][START_REF] Pulvermuller | Motor cortex maps articulatory features of speech sounds[END_REF][START_REF] Wilson | Neural responses to non-native phonemes varying in producibility: evidence for the sensorimotor nature of speech perception[END_REF][START_REF] Skipper | Hearing lips and seeing voices: how cortical areas supporting speech production mediate audiovisual speech perception[END_REF]. Transcranial magnetic stimulation (TMS) studies also demonstrated that motor-evoked potentials recorded from the lips or tongue muscles are enhanced during both passive speech listening and viewing, when stimulating the corresponding area of the left primary motor cortex [START_REF] Sundara | Observation-execution matching system for speech: A magnetic stimulation study[END_REF][START_REF] Fadiga | Speech listening specifically modulates the excitability of tongue muscles: a TMS study[END_REF][START_REF] Watkins | Seeing and hearing speech excites the motor system involved in speech production[END_REF][START_REF] Watkins | Modulation of motor excitability during speech perception: the role of Broca's area[END_REF][START_REF] Roy | Phonological and lexical motor facilitation during speech listening: A transcranial magnetic stimulation study[END_REF]. Importantly, this speech motor 'resonance' mechanism appears to be articulatory specific, motor facilitation being stronger when the recorded muscle and the presented speech stimulus imply the same articulator [START_REF] Fadiga | Speech listening specifically modulates the excitability of tongue muscles: a TMS study[END_REF][START_REF] Roy | Phonological and lexical motor facilitation during speech listening: A transcranial magnetic stimulation study[END_REF]. The specificity of this speech motor resonance mechanism is also suggested by two recent functional magnetic resonance imaging (fMRI) studies showing somatotopic patterns of motor activity in the ventral premotor cortex during both producing and listening to or viewing lips-and tongue-related phonemes [START_REF] Pulvermuller | Motor cortex maps articulatory features of speech sounds[END_REF][START_REF] Skipper | Hearing lips and seeing voices: how cortical areas supporting speech production mediate audiovisual speech perception[END_REF]. Perception in this framework would be mediated by "motor ideas" (Fadiga et al., 2000) represented in the brain by sensori-motor neurons enabling to access these "ideas" both through acting and through observing or hearing somebody else acting (though see a more critic view on this point in a recent opinion paper by [START_REF] Lotto | Reflections on mirror neurons and speech perception[END_REF].

The second series of arguments deal with the claim that this connection would play a critical role in speech perception. This is the "perceiving speech is perceiving gestures" argument, illustrated by a number of data in which complex links between gestures and sounds due to coarticulation effects seem to be parsed out in perception, as in the previously cited examples about plosives perception. In these various examples reviewed by [START_REF] Galantucci | The motor theory of speech perception reviewed[END_REF], it seems that the listener, who obviously "knows something" about speech production, exploits this knowledge to disentangle the complexity of the acoustic input and access functional units more directly related with motor commands. The same reasoning is proposed for multisensory perception, e.g. audiovisual (but also audio-haptic) interactions in speech perception, which are claimed to be related to the knowledge the listener has of the multisensory coherence of a speech gesture [START_REF] Fowler | The perception of phonetic gestures[END_REF]; see also [START_REF] Schwartz | Ten years after Summerfield ... a taxonomy of models for audiovisual fusion in speech perception[END_REF]. This is connected to the general statement by Viviani that perception involves a "procedural knowledge of action", as nicely exemplified by his data showing how the knowledge of the laws of human movement modifies the perception of hand movements, in terms of both configurational and kinematic properties [START_REF] Viviani | Biological movements look uniform: evidence of motorperceptual interactions[END_REF].

In a recent discussion paper at the last LabPhon Conference, [START_REF] Schwartz | Filling the perceptuo-motor gap[END_REF] claimed that the various studies presented in the corresponding session all contained some elements of answers to the question: "what does a listener know about a speaker's gesture, and how is this knowledge useful for perception?". It was shown how some "procedural knowledge" on vowel reduction (for the paper by [START_REF] Solé | What is and what is not under the control of the speaker: intrinsic vowel duration[END_REF] and voicing assimilation [START_REF] Kuzla | Compensation for Assimilatory Devoicing and Prosodic Structure in German Fricative Perception[END_REF] could actually intervene in the course of speech perception. However, one case presented a difficulty. It consisted in the observation by [START_REF] Mielke | Covert /®/ allophony in English: variation in a socially uninhibited sound pattern[END_REF] that there may be different strategies in the production of an [®] in American English, including "bunched" and "retroflex" lingual configurations [START_REF] Delattre | A dialect study of american rs by x-ray motion picture[END_REF][START_REF] Tiede | A new taxonomy of American English /®/ using MRI and ultrasound[END_REF] -partly driven, according to the study by Mielke and coll., to the context in relation with articulatory ease -though the sound is quite similar in both cases. Here comes a problem for the Motor Theory of speech perception: what happens when the relationship between gestures and sounds is many-to-one (various gestures for a single sound), and hence the gesture cannot be, in theory, recovered from the sound without additional pieces of information?

This means that at most some abstract representation of the gesture can be recovered, rather than a complete articulatory configuration (see [START_REF] Guenther | A theoretical investigation of reference frames for the planning of speech movements[END_REF].

The problem is not only technical, actually, but also theoretical: what makes an [®] an [®] appears to be related to the acoustic content of the unit rather than to its articulatory characteristics.

In this case, it seems that the link between perception and action goes the other way round, perception shaping action and not the inverse.

I.2. Perception shapes action

The case of [®], as many others (see e.g. [START_REF] Savariaux | Compensation strategies for the perturbation of the rounded vowel [u] using a lip-tube: A study of the control space in speech production[END_REF][START_REF] Savariaux | Compensation strategies for the perturbation of French [u] using lip-tube. II. Perceptual analysis[END_REF][START_REF] Perrier | About speech motor control complexity[END_REF] suggests that a gesture is characterised by its functional value, likely evaluated in auditory terms. This is not easy to deal with in the framework of the Motor Theory, apart if one conceives a vocal tract configuration as a global coordination oriented towards a functional phonetic goal [START_REF] Mattingly | The global character of phonetic gestures[END_REF].

The solution out of this difficulty is to acknowledge that gestures are not pure motor units, but rather perceptuo-motor units, gestures being shaped by perception. Let us take a first example, that is lip rounding. Consider what it means to round the lips starting from [i]. Decreasing the lip area from the unrounded configuration first does not change the sound almost at all. Then, it suddenly dramatically changes it into an [y]-like sound, because of both acoustic and auditory reasons [START_REF] Abry | Plateaus, catastrophes and the structuring of vowel systems[END_REF]. In this example, it should become clear that "lip rounding" does not mean anything per se, and cannot be defined in pure articulatory terms. Rounding the lips means decreasing the lip area at a value lower than the critical value under which the sound changes significantly.

Gestures are not only shaped by perception, but also selected in relationship with their perceptual (acoustic-auditory) value. Let us consider vowel systems in human languages. Assuming that the Motor Theory of Speech Perception is correct, what would be the possible predictions in terms of oral vowel systems in human languages? There are basically three degrees of freedom for producing oral vowels: height, front-back position, and rounding. What would be the best threevowel system in this space? The system [i a u] is a very good choice, in terms of articulatory dispersion, combining the high front unrounded [i], the high back rounded [u] and the open [a]. It is indeed part of most languages, as shown by the available data on vowel systems in human languages [START_REF] Maddieson | Patterns of sounds[END_REF]. However, [y a µ] should be as good a choice: it just consists in associating lip rounding with tongue frontness, and lip unrounding with tongue backness. It combines articulatory features differently, but the difference cannot be assessed in articulatory terms. However, this second system never appears in human languages. The reason for this is clearly auditory. Auditory perception is a kind of lateral projection of this 3-D space, in a 2-D (F1, F2) space in which [i u] is much better (in terms of dispersion) than [y µ]. Altogether, the prevalence of [i a u] and the absence of [y a µ] clearly shows that gestures are selected in relation with their perceptual value (Fig. 1).

Notice that not only does the "audibility" of a gesture intervene in the selection of speech gestures, but also its "visibility", as shown by the largely developed use of the [m]-[n] contrast in human languages, poorly audible, but quite visible (Schwartz et al., 2007).

These two examples can be related to two major theories of speech communication.

Stevens's Quantal Theory [START_REF] Stevens | The quantal nature of speech: Evidence from articulatory-acoustic data[END_REF][START_REF] Stevens | On the quantal nature of speech[END_REF] capitalises on nonlinearities of the transformation from articulatory to acoustic/auditory features, which sometimes provide a "quantal" behaviour in the sense that some regions of the motor command do almost not change the sound, while others change it abruptly -as in the lip rounding case. Stevens suggests that this provides natural boundaries for designing phonetic contrasts exploited in human languages such as the [i]-[y] contrast in French. Lindblom's Hyper-Hypo or Adaptive Dispersion Theory [START_REF] Liljencrants | Numerical simulations of vowel quality systems: The role of perceptual contrast[END_REF][START_REF] Lindblom | Phonetic universals in vowel systems[END_REF][START_REF] Lindblom | On the notion of possible speech sound[END_REF]) exploits acoustic-auditory differentiation as a driving force shaping vowel systems (Liljencrants & Linblom, 1972) -and possibly, according to Abry (2003) and Schwartz & Boë (2007), also plosive systems. In both cases, it appears that gestures are indeed perceptuo-motor units shaped by their perceptual properties and selected for their functional/perceptual value for communication.

I.3. Integrating motor and perceptual knowledge inside PACT

What is the state of affairs at this point? If action shapes perception and perception shapes action in return, are we in a never-ending loop? Let us advance step by step.

On the one hand, the involvement of procedural knowledge about speech production in the course of speech perception is at odds with auditory theories, relying on sensory transduction followed by categorisation processes and assuming that all the information is in the acoustical signal. On the other hand, we argue that the decision process should operate on auditory features rather than articulatory configurations, gestures being shaped by perception and selected in relation with their functional perceptual value. This is at odds with pure motor theories, which consider that perceptual representations should be gestural in nature.

A possible synthesis could be envisioned in the framework of the Theory of Event Coding developed by Prinz and colleagues (see e.g. [START_REF] Hommel | The theory of event coding (TEC): A framework for perception and action planning[END_REF] predicting that perceptual contents and action goals are cognitively coded in a common representational medium by composite codes of their distal features. However, there is a specificity of speech actions, which are communicative actions driven by the need to generate communicative stimuli that support the linguistic code. This is the reason why gestures should be considered as perceptually-shaped units in the case of speech. Altogether, these two principles -that perception involves knowledge of action, and that action aims at perceptually-shaped gestures -provide the basis of PACT.

The Perception-for-Action-Control Theory considers that speech perception is the set of mechanisms that enable not only to understand, but also to control speech, considered as a communicative process. This leads to two consequences. Firstly, perception and action are costructured in the course of speech development, which involves both producing and perceiving speech items. In consequence, the perceptual system is intrinsically organised in reference to speech gestures, and in relation with the structure of the action system. This is why it includes, in one way or another, an implicit procedural knowledge of speech production. Secondly, perception provides action with auditory (and possibly visual) templates, which contribute to define the gesture, providing them objectives, organisation schemes and functional value. This is how PACT incorporates the "gesture shaping" component.

In PACT, the communication unit, through which parity may be achieved, is neither a sound, nor a gesture, but a perceptually-shaped gesture, that is a perceptuo-motor unit. It is characterised by both its articulatory coherence -provided by its gestural nature -and its perceptual value -necessary for being functional. Let us take an example to make the concept hopefully clearer. In the framework of the Frame-Content Theory, MacNeilage (1998) argues that the jaw cycles pre-existing in mastication in ontogeny -and possibly in phylogeny -play a bootstrap role initiating dynamic vocalisations at the birth of speech. The key point is that jaw cyclic gestures are shaped by the acoustic/auditory transformation in a quasi quantal way in Stevens's terms (see Fig. 2). Actually, when the jaw is lowered, the sound is vocalic, while when the jaw is raised, it results in a closure alternating silence and burst, characteristic of a plosive, with possibly an intermediate stage generating a friction noise characteristic of a fricative. These two non-linearities -from vowel to fricative through the harmonic-to-turbulent switch of acoustic mode, and from fricative to plosive through vocal tract closure -naturally generate a consonant-vowel alternation leading to syllables in all human languages. Syllables in this scenario are neither articulatory nor perceptual structures, but perceptuo-motor in essence: jaw gestures non-linearly shaped by audition in a quantal way.

We shall review in Sections 2 and 3 two series of works we did in the past years in this perspective, leading in both cases to introduce perceptually shaped gestures as a basis for explanation of perceptual processes. Then we shall discuss in Section 4 how recent data about perceptuo-motor connections in the human brain fit with PACT. In light of these data, we shall propose some elements of a neurocomputational architecture and discuss about two possible implementations, involving either implicit articulatory knowledge stored in perceptual areas, or explicit involvement of the speech production system in the perception routine.

II. Vowels as auditorily-shaped gestures

The conception of vowel systems as auditorily-optimized structures obeying perceptual dispersion principles for maximizing their efficacy of communication founded a major break in the Chomskyan conception of an autonomous "language organ". This opened the route towards what [START_REF] Lindblom | Phonetic universals in vowel systems[END_REF][START_REF] Lindblom | On the notion of possible speech sound[END_REF]) called "substance-based" theories of human languages.

II.1. From DT to DFT, vowel systems are an auditory-driven communication system

The initial push towards such "substance-based" theories of human languages was provided by [START_REF] Liljencrants | Numerical simulations of vowel quality systems: The role of perceptual contrast[END_REF] who proposed that vowel systems aimed at maximising distances between their formant sets, just as electrical charges in a bounded space would maximise their distances because of repelling Coulomb forces. Liljencrants & Lindblom defined a "Dispersion Theory" (DT) in which a vowel system with a given number of vowels is characterised by a global "energy" summing the inverse squared distances between their F1 and F2 values within an (F1, F2) triangular space bounded by articulatory constraints. They showed that this lead to rather adequate predictions of 3-vowels systems [i a u], four-vowel systems [i e a u], five-vowel systems [i e a o u], compatible with vowel inventories (e.g. [START_REF] Maddieson | Patterns of sounds[END_REF]. Further evolutions of the theory were proposed by e.g. [START_REF] Lindblom | Can the models of evolutionary biology be applied to phonetic problems[END_REF][START_REF] Lindblom | Phonetic universals in vowel systems[END_REF], [START_REF] Diehl | Increasing realism of auditory representations yields further insights into vowel phonetics[END_REF], etc. [START_REF] Schwartz | The dispersion-focalization theory of vowel systems[END_REF] added another perceptual driving force according to which vowel acoustic configurations are also attracted towards "focal" configurations in which there is proximity between two consecutive formants, F1 and F2 (e.g. for [u] and [a]), F2 and F3 (e.g. for [y]), or F3 and F4 (typically for [i]). Focal vowels correspond to more stable auditory patterns, which actually seem to drive both infant and adult perception (e.g. [START_REF] Schwartz | A strong evidence for the existence of a large scale integrated spectral representation in vowel perception[END_REF][START_REF] Schwartz | Asymmetries in vowel perception, in the context of the Dispersion-Focalisation Theory[END_REF] and children production (e.g. [START_REF] Ménard | The role of vocal tract morphology in speech development: Perceptual targets and sensori-motor maps for French synthesized vowels from birth to adulthood[END_REF]. Focalisation would intervene in shaping vowel systems, just as "focal colours" shape the categorisation of colours in human languages [START_REF] Rosch-Heider | Universals in color naming and memory[END_REF]. This lead to the "Dispersion-Focalisation Theory of vowel systems" (DFT) combining dispersion and focalisation and providing a good fit of vowel systems from 3 to 9 vowel qualities, including best systems and their preferred variants [START_REF] Vallée | Phase spaces of vowel systems : A typology in the light of the Dispersion-Focalisation Theory (DFT)[END_REF].

One important problem however remains in these simulations: vowel systems seem to balance the number of categories in their front unrounded and back rounded series (with e.g. [START_REF] Schwartz | The dispersion-focalization theory of vowel systems[END_REF]. This regularity in the distribution of vowels within a given system may be related to a principle defined by [START_REF] Ohala | Moderator's introduction to symposium on phonetic universals in phonological systems and their explanation[END_REF] as "Maximal Use of Available Features" (MUAF), according to which systems would combine features in a systematic way. In the case of vowel systems, height features (e.g. high, mid-high, mid-low) would be combined with tongue/lip configuration features (e.g. front unrounded or back rounded) to provide balanced systems.

II.2. The distribution of height controls, a motor-driven idiosyncrasy

In a recent paper, Ménard et al. (submitted) observed a curious idiosyncrasy in the production of vowel height in French or Canadian French. A first aspect of this idiosyncrasy is that subjects vary a lot in the way they distribute vowels along the F1 dimension (Fig. 3a). Some speakers may produce high and mid-high vowels (e.g. Such inter-individual variation is not completely surprising, and compatible with a variant of the DT, called Adaptive Variability Theory (AVT) [START_REF] Lindblom | On the notion of possible speech sound[END_REF] This regularity, statistically assessed, is very interesting because it does not seem to obey any perceptual law -and it seems to provide a kind of analogous to the MUAF principle, though not on logical values ("features") but on continuous cues (F1). Ménard et al. interpreted this pattern as due to a stability of tongue height from one member of the mid-high or mid-low series to another, MUAF becoming what they proposed to call a "Maximal Use of Available Controls" (MUAC) principle. In their reasoning, speakers select a given articulatory command -vowel height -at a rather arbitrary value in an idiosyncratic way, but once selected, this value is transferred from one vowel to another. More precisely, they suggested that in the course of speech development, the young speaker would achieve a sufficient control within a given series, and then transfer the adequate (sufficient and subject-dependent) control to another tongue/lip configuration, thus achieving stable mid-high and mid-low series (with stable tongue height and thus, as shown by acoustic simulations, stable F1 values) thanks to a MUAC principle.

In this reasoning, the developmental articulatory pathway is supposed to be crucial in the achieved shape of the vowel system for a given speaker. This illustrates how vowels are, in fact, more than pure "sounds" selected for their perceptual values; but rather, vocalic gestures organised developmentally and shaped by their acoustic/auditory properties.

II.3. The categorisation of height features, a perceptual idiosyncrasy in mirror

In a later study, Ménard & Schwartz (submitted) tested a possible perceptual counterpart of this motor idiosyncrasy, by submitting the speakers of the previous study to a speech perception task aiming at determining their vocalic prototypes. For this aim, an articulatory model of the vocal tract, VLAM, was used [START_REF] Boë | Modeling the growth of the vocal tract vowel spaces of newly-born infants and adults. Consequences for ontogenesis and phylogenesis[END_REF], from Maeda, 1979). This model enables to synthesise vowel stimuli covering the vowel triangle, for various sizes of the vocal tract corresponding to various ages between birth and adulthood. A "younger" vocal tract is characterised in VLAM by both larger formants and larger F0 values [START_REF] Ménard | Auditory normalization: of French vowels synthesized by an articulatory model simulating growth from birth to adulthood[END_REF]. Stimuli were generated for covering the vowel space corresponding to two ages: an "adult" one and a younger one, depending on the subject's age.

Subjects were asked to identify the vowel stimuli within one of the 10 categories for French oral vowels [i e E y O ø u o ç a]. Four major results emerged from this study (Fig. 3b,c). Firstly, subjects are variable as listeners, as they are as speakers, with large inter-individual variation for category centres from one subject to another. Secondly, F1 grouping is displayed for perception as for production, mid-high and mid-low series being characterised by stable F1 values for each subject, and different from subject to another. Thirdly, the distribution of F1 values for a given subject for the two vocal tract sizes are correlated, indicating the existence of normalisation mechanisms prior to identification along the idiosyncratic prototypes. Last but not least, the distribution of F1 values for perception and production are correlated, that is, the way a given subject produces vowel height (idiosyncratically) is mirrored in the way he/she perceives vowel height.

There is not so many evidence of a link between idiosyncrasies in perception and production (though see [START_REF] Bell-Berti | Some relationships between speech production and perception[END_REF], cited in Galantucci et al., 2006;[START_REF] Villacorta | Sensorimotor adaptation to feedback perturbations on vowel acoustics and its relation to perception[END_REF]. The present data clearly suggest that there is a co-construction of motor and perceptual representations in the course of speech development for vowel height, as suggested in PACT.

II.4. Summary

The PACT scenario for vowel height production and perception assumes that vowel gestures are built in development through a process involving articulatory exploration and composition, and perceptual tuning (including sufficient dispersion, and focalisation), leading to a series of prototypes used by the subject for production and perception. A model presented in Fig. 4 displays how perception would function in this scenario, starting with perceptual cue extraction and normalisation, followed by categorisation based on prototypes defined in common by perception and production. Vowels in this scenario appear as auditorily-shaped articulatory gestures, displaying both an articulatory coherence expressed by the MUAC principle and acquired in the course of development, and a perceptual coherence associated to the communicative value of phonetic units, expressed by the normalised auditory cues characterising the vowel content for categorisation, and obeying dispersion and focalisation principles (DFT).

III. Speech Scene Analysis as a perceptuo-motor process

Auditory Scene Analysis (ASA) is known since the classical book by [START_REF] Bregman | Auditory Scene Analysis: The Perceptual Organization of Sound[END_REF] as the set of mechanisms enabling the listener to organise an auditory scene, grouping coherent ingredients within streams or sources and disentangling one stream from another thanks to various cues such as temporal, spectral, fundamental frequency or localisation coherence. 15 years later, [START_REF] Remez | On the perceptual organization of speech[END_REF] noticed that a speech stream is characterised by pieces of information displaying some degree of incoherence, associated to the quick succession of silences, bursts, noises, or harmonic sections. However, the "speech scene" seems perceived in a coherent way, suggesting that specific mechanisms could be in charge of gluing the various pieces together, because of the "special" nature of speech (likely, its motor nature, in the framework of the Motor Theory which served as a background for the paper by Remez and coll.). We will show in the following that speech scene analysis is a natural framework for displaying the role of perceptuomotor interactions in speech perception.

III.1. The Verbal Transformation Effect, a paradigm for studying Speech Scene Analysis

In the last years, Sato, Basirat, Schwartz and coll. developed a series of works exploiting the "Verbal Transformation Effect" (VTE; [START_REF] Warren | An auditory analogue of the visual reversible figure[END_REF][START_REF] Warren | Illusory changes of distinct speech upon repetition -The verbal transformation effect[END_REF]) as a way of assessing Speech Scene Analysis mechanisms. In the VTE, a given stimulus presented in loop may lead to switches from one percept to another (for example, the rapid repetition of the word "life" produces a perceptual transform into "fly" and "fly" back into "life" and so on). The VTE thus pertains to a family of paradigms producing multistable perception, as in the visual modality with the Necker's cube or binocular rivalry, in which the subject alternatively perceives the stimulus in one or another way, and displays more or less regular switches from one to the other. Multistability reveals various ways in which a given scene may be perceived, and has proved to be an efficient paradigm for characterising the emergence of percepts in the human brain (e.g., [START_REF] Leopold | Multistable phenomena: Changing views in perception[END_REF][START_REF] Blake | Visual competition[END_REF].

III.2. Auditory, phonetic and lexical mechanisms in the VTE

In a series of works with the VTE, Pitt and colleagues contributed to better specify the VTE methodology, exploiting such ingredients as the number of displayed forms, the time spent on each form, the frequency of switches, etc [START_REF] Pitt | The source of a lexical bias in the Verbal Transformation Effect[END_REF], 2002;Shoaf & Pitt, 2002). They displayed three series of mechanisms at work in the VTE. The basic mechanism is a re-segmentation process changing the segmentation boundary from one position to another (e.g. from "life life life" to "fly fly fly"). A second series of phonetic/lexical/semantic mechanisms enrich the number of possible forms by introducing modifications of the input stimuli at each of these three levels, e. [START_REF] Remez | On the perceptual organization of speech[END_REF] considered as unlikely for normal speech perception. For example, Pitt & Shoaf (2002) show that in repetitions of one-syllable words including a consonantal cluster with an initial fricative in the onset or a final fricative in the coda, there was a strong tendency that listeners separated the fricative from the rest of the information, verbal transformations thus involving the sequence without the fricative into a foreground stream, and the fricative constituting a kind of background stream segregated from the remaining phonemes (e.g.: for [skin] repetition, [kin] or

[gin] reported as transformations and [s] as the background stream).

III.3. The role of articulatory coherence in the VTE

In a first series of experiments, [START_REF] Sato | Multistable syllables as enacted percepts: a source of an asymmetric bias in the verbal transformation effect[END_REF] wondered whether articulatory coherence could be shown to display a role in the VTE. For this aim, they focused on a French nonsense analogous of the "life"-"fly" transform, in which a "pse" non-word transforms into "sep" and the other way round. They suggested that in a perceptuo-motor paradigm in which the subject produces the sequence in loop and searches for a transformation, [ps´] should display a larger articulatory coherence than [s´p] because of articulatory synchrony between the consonantal gestures. Indeed, in [ps´], the labial gesture for [p] and the tongue gesture for [s] can be prepared in anticipation and launched almost in synchrony, the lips being opened for [p] on a vocal tract prepared for [s] with the tongue tip already in contact with the palate, so that the [s] friction can be immediately produced after the labial burst at the syllable onset. On the contrary, in [s´p], the two gestures respectively happen at the syllable onset for [s] and coda for [p]. The switch from "sep sep sep" to "pse pse pse" would hence consist in a resynchronisation, and conversely, the switch from "pse pse pse" to "sep sep sep" would involve a desynchronisation of the two consonantal gestures. The prediction was that the synchronous [ps´] would be more stable than the asynchronous [s´p] and therefore, a larger number of transformations from [s´p] to [ps´] would be observed than the other way round (Fig. 5a). This is what was actually showed, both in an overt and in a covert mode [START_REF] Sato | Multistable syllables as enacted percepts: a source of an asymmetric bias in the verbal transformation effect[END_REF].

In a further study focused on CVCV alternations such as [pata] vs. [tapa], Sato et al. (2007a) were able to produce similar effects of articulatory coherence, though in a completely auditory VTE paradigm. They capitalised on the so-called labial-coronal effect, according to which labial-coronal (LC) sequences such as [pata] (a labial plosive [p] in the first syllable, followed by a coronal plosive [t] in the second one) are more frequent in human languages [START_REF] Macneilage | On the origin of internal structure of word forms[END_REF]. An articulatory explanation of the LC effect was provided by [START_REF] Rochet-Capellan | An articulatory basis for the labial-to-coronal effect: /pata/ seems a more stable articulatory pattern than /tapa[END_REF] according to which the LC sequence can be realised on a single jaw cycle with an anticipation of the coronal gesture inside the labial one, the reverse being impossible (anticipating the lip closure would hide the audibility of the coronal). Therefore, the CL sequence would be less well coordinated than the LC one. A prediction was that the LC coherence would lead in the VTE to a larger stability for [pata] than for [tapa], the listener chunking "patapatapata" sequences into LC

[pata] items compatible with jaw cycles, rather than into CL [tapa] items (Fig. 5b). This was actually displayed in experimental data (Sato et al., 2007a).

The fact that articulatory constraints may act on the emergence and stabilization of verbal transformations strongly suggests that they partly rely on motor neural processes. This is in keeping with several recent fMRI and intracranial electro-encephalographic (iEEG) studies demonstrating that articulatory-based representations play a key part in the endogenously driven emergence and stabilization of auditory speech percepts during a verbal transformation task [START_REF] Sato | Multistable perception of speech forms in working memory: An fMRI study of the verbal transformation effect[END_REF][START_REF] Kondo | Neural mechanisms of auditory awareness underlying verbal transformations[END_REF][START_REF] Basirat | Parieto-frontal oscillatory synchronization during the perceptual emergence and stabilization of speech forms[END_REF].

III.4. Multimodal speech scene analysis

In a further series of experiments, Sato et al. (2007b) tested whether visual speech could intervene in the VTE. They showed that lipread speech actually produced switches as auditory speech, and that the visual content of an audiovisual sequence contributed to the audiovisual switches by increasing or decreasing the stability of the sequence whether the auditory and visual stimuli were coherent or incoherent. Furthermore, they showed that visual changes in the seen material could drive the audiovisual switches over time, as displayed when a stable audio "sepsep" sequence is dubbed on a video alternation of "sep" and "pse", resulting in switches from "sep" to "pse" perfectly synchronous with the video switches (Fig. 5c). They suggested that lip opening gestures could provide onset cues for audiovisual speech segmentation, driving the percept towards the item beginning with the most visible onset. This is in line with a previous experiment by [START_REF] Schwartz | Seeing to hear better: Evidence for early audio-visual interactions in speech identification[END_REF] in which lip gestures happen to enhance the detection of audio cues necessary for categorisation. In this study, subjects had to discriminate voiced and unvoiced syllable onsets such as [tu] vs. [du] embedded in noise and presented at random temporal positions. Seeing the lip gesture provided a temporal cue useful for the auditory detection of prevoicing, resulting in increasing the audiovisual performance (Fig. 5d). This provides another example of "multimodal speech scene analysis".

III.5. Summary

In these experiments, the speech scene analysis process appears to be driven by both perceptual and motor coherence. The listener combines general auditory scene analysis (ASA) mechanisms (leading to e.g. streaming processes) with articulatory principles grouping the acoustical -and visual, if present -pieces of information in a coherent way, limiting streaming as

IV. A neurocomputational framework for PACT

PACT is built upon a central concept, that speech units are perceptuo-motor in nature, with two related main assumptions: firstly, perceptual representations are shaped by articulatory knowledge; secondly, identification proceeds from perceptual representations, which contain the communicative value of the speech gesture. We shall discuss how these assumptions fit with neurophysiological data, how they can be incorporated into a computational model, and what kinds of experimental questions remain in this framework.

IV.1. Does the dorsal/ventral cortical network fit with PACT?

A strong lesson of the last twenty years is the very convincing accumulated evidence for sensorimotor connections between temporal auditory and audiovisual regions, parietal somesthesic/proprioceptive areas and frontal motor and premotor zones inside a dorsal cortical network as shown in Section I. The dorsal route is a candidate for establishing parity between sensory and motor representations in the framework of the mirror neurons theory (see [START_REF] Rizzolatti | Neurophysiological mechanisms underlying the understanding and imitation of action[END_REF][START_REF] Rizzolatti | The Mirror-Neuron System[END_REF] for a review). For [START_REF] Hickok | Towards a functional neuroanatomy of speech perception[END_REF], 2004, 2007) it is the network involved in learning the control and processing of phonemic units for speech communication and is thought to be used strategically to assist in working memory and sub-lexical tasks. This provides a basis for one of the two major assumptions in PACT: that perceptual and motor representations of speech are learnt and structured together, and connected in a principled and functional way. However, [START_REF] Hickok | Towards a functional neuroanatomy of speech perception[END_REF], 2004, 2007) consider that speech comprehension under ecologically valid conditions, i.e. when sounds are ultimately transformed into lexical/conceptual meaning, exploits another route, different from the dorsal one, and that they call the ventral route. The dorsal and ventral pathways both imply initial auditory processing in the primary auditory cortices along the supratemporal plane and subsequent phonological processing in the middle and posterior portions of the superior temporal sulcus (STS). At this point, the ventral stream however diverges from the dorsal one towards posterior middle and inferior portions of the temporal lobes involved in lexical and semantic processing Note that [START_REF] Scott | The functional neuroanatomy of prelexical processing in speech perception[END_REF] differ on the precise temporal anatomy of the ventral route, going, in their view, from the primary auditory cortex towards the anterior part of the STS and possibly later connected with prefrontal areas for lexical, syntactic or semantic processing. Whatever the precise anatomy of the ventral route, the crucial role of temporal areas in speech comprehension is in good agreement with the second PACT assumption, that perceptual representations provide the basis for identification of phonetic units.

In summary, the dorsal route would ensure a co-structuration of perceptual and motor representations in a coherent way -and the possibility to call for motor routines in the course of perceptual processes: this remains to be further discussed in Section IV.3. And the ventral route would provide the decision/comprehension process based on perceptual (auditory, visual) representations before lexical and semantic access.

IV.2. Ingredients for a computational PACT architecture

To explore computational architectures, let us begin by asking a functional question. What is precisely the role of perceptuo-motor interactions in speech perception? Within PACT, we suggest two kinds of answers.

Firstly, the perceptuo-motor link contributes to structure perceptual categories in reference to their motor content. This is the core ingredient of motor theories. Consider the [di] vs. [du] case quoted in Section I.1. Various pieces of acoustic evidence, different from one vocalic context to the other, point to the same consonantal gesture. This is taken by [START_REF] Liberman | The motor theory of speech perception revised[END_REF] as an indication that acoustic cues are translated into articulatory features. However, another reasoning is that the articulatory link enables to define acoustic/auditory classes of equivalence, or to natural phonetic categories. Interestingly, these two different reasoning lead so different theoreticians as [START_REF] Liberman | The motor theory of speech perception revised[END_REF] and [START_REF] Sussman | Linear correlates in the speech signal: The orderly output constraint[END_REF] to call for the same neuronal analogy in auditory processing: that is, the neuronal columns for auditory localisation in the inferior colliculus of the barn owl. [START_REF] Wagner | Representation of interaural t time difference in the central nucleus of the barn owl's inferior colliculus[END_REF] or Konishi et al. (1988) showed how different phase relations between the two ears, related to the same orientation in the 3D space (same azimuth) for tones of different frequencies, were organised within orientation dominance columns enabling to recover the invariant character of the distal event from the variable proximal stimulus. This was considered by Liberman & Mattingly as a typical case of modular "heteromorphic" system connecting the proximal stimulus with its distal cause, just as the speech proximal acoustic stimulus is heteromorphically transformed into its distal cause, the articulatory gesture. But it was considered by [START_REF] Sussman | Linear correlates in the speech signal: The orderly output constraint[END_REF] as a case of efficient exploitation of the available auditory information, just as their "locus equation" would provide an invariant acoustic correlate of plosive place in spite of the variable vocalic context, with no need for articulatory inversion.

Our reasoning is a synthesis of both versions: the information for decision is actually acoustic/auditory (and visual); but the equivalence classes, organising the proximal stimulus into natural categories, are defined in relationship with gestural commands. This would explain how listeners recover sources of variability related with coarticulation, reduction, etc: not through articulatory-to-acoustic inversion, but through categories based on representations and boundaries defined within the perceptual space, though organised in reference with the perceptuo-motor link, as in the experimental data on vowel production and perception presented in Section II. This is described in Fig. 6, which proposes a general computational architecture for PACT. The link between "Motor schemas" (combining prototypes and production laws) and "Auditory categorization", acquired through co-structuring of perceptual and motor representations, enables to incorporate knowledge of the speech production mechanisms inside auditory categorisation mechanisms (through sensori-motor speech maps and trajectories). For example, the fact that the speaker has "experienced" the various auditory consequences of closing the lips for "b" results in a structuring of the auditory space into a natural "closed lips" category which implements what Viviani called the "procedural knowledge of action". This is the way the "variability" due to coarticulation is dealt with in PACT.

The second use of the perceptuo-motor link relates to what [START_REF] Skipper | Hearing lips and seeing voices: how cortical areas supporting speech production mediate audiovisual speech perception[END_REF] call "prediction" in their model of audiovisual speech perception involving the temporo-parieto-frontal connection, though "prediction" is a principle that might appear a bit vague. Indeed, considering the speed and accuracy of the auditory system, it is not really obvious if motor-based predictions could be really and systematically helpful. However, there could be in one case: when there is a lack of information, say because of noise or in a foreign language. this case, the perceptuo-motor link could enable to indicate where the lacking information could be searched. This is the case in the "speech scene analysis" problem described in Section III, in which audiovisual and perceptuomotor links appear important for better organising the scene, segmenting it appropriately, and recovering adequate auditory pieces of information in the case of a noisy communication (as in [START_REF] Schwartz | Seeing to hear better: Evidence for early audio-visual interactions in speech identification[END_REF]. This is incorporated in Fig. 6 in terms of "integration" that is incorporating speech production knowledge about possible sensory trajectories through "receptive field" tuned on these trajectories.

IV.3. Connecting computational principles with neurocognitive data

The two potential roles of a perceptuo-motor connection could actually rely on quite different types of implementations, and could hence be related to different sets of experimental data.

The co-structuring component suggests the possibility to connect perceptual and motor representations for speech communication in a principled way. This is what we proposed to call "sensori-motor speech maps" capitalising on previous works on speech robotics in our lab (Abry and Badin, 1986). In an experimental behavioural task on the discrimination of jaw height, tongue position or lip rounding in vowel perception, we showed [START_REF] Schwartz | Hearing the tongue and lips of vowel gestures: a new differential paradigm[END_REF] that subjects are to a certain extent able to have direct access to vowel articulatory features, which suggests that sensori-motor maps do exist, for vowels at least, in the human brain. Recent fMRI data on perceptual or articulatory maps for vowels or consonants [START_REF] Obleser | Cortical representation of vowels reflects acoustic dissimilarity determined by formant frequencies[END_REF][START_REF] Obleser | Magnetic brain response mirrors extraction of phonological features from spoken vowels[END_REF][START_REF] Obleser | Vowel Sound Extraction in Anterior Superior Temporal Cortex[END_REF][START_REF] Pulvermuller | Motor cortex maps articulatory features of speech sounds[END_REF]) confirm and provide neurophysiological correlates of such maps.

Interestingly, these maps appear to be dynamic and provide a way by which changes in production can result in changes in perception. Indeed, in a recent study, [START_REF] Shiller | Perceptual recalibration of speech sounds following motor learning[END_REF] showed that after a speech production experiment with auditory perturbations leading to motor after-effects, perception was recalibrated in relation with this motor after-effect. They suggested that the perceptuo-motor link coordinating perceptual and motor representations is in fact plastic and susceptible to modifications relating articulatory changes to perceptual changes.

The other component, that is prediction/integration, could possibly involve on-line call to the motor system for completing auditory speech scene analysis. There are however two caveats for this assumption. Firstly, after all, it is not necessary to call for motor simulation if the structure of the auditory representations already includes clues about perceptuo-motor trajectories. Secondly, if an explicit call for motor processes is indeed necessary, the prediction is that the call would be particularly necessary in adverse conditions, e.g. noise, audiovisual coordination, foreign language.

This fits well with experimental data showing an increase in frontal activity in speech perception exactly in these conditions, that is noise (e.g., [START_REF] Binder | Neural correlates of sensory and decision processes in auditory object identification[END_REF][START_REF] Zekveld | Top-down and bottomup processes in speech comprehension[END_REF], audiovisual integration (e.g., [START_REF] Jones | Brain activity during audiovisual speech perception: An fMRI study of the McGurk effect[END_REF][START_REF] Ojanen | Processing of audiovisual speech in Broca's area[END_REF][START_REF] Pekkola | Perception of matching and conflicting audiovisual speech in dyslexic and fluent readers: an fMRI study at 3T[END_REF][START_REF] Skipper | Hearing lips and seeing voices: how cortical areas supporting speech production mediate audiovisual speech perception[END_REF], or foreign language (e.g., [START_REF] Callan | Phonetic perceptual identification by native-and second-language speakers differentially activates brain regions involved with acoustic phonetic processing and those involved with articulatoryauditory/orosensory internal models[END_REF][START_REF] Wilson | Neural responses to non-native phonemes varying in producibility: evidence for the sensorimotor nature of speech perception[END_REF]. However, no clearcut experiment proves in an unambiguous way a decisive role of motor and premotor areas in speech perception. Hickok & Poeppel (2001, 2004, 2007) repeatedly notice that frontal lesions impairing speech production do not impede speech perception. Some weak perturbations in auditory speech tasks have been obtained by temporarily disrupting the activity of the motor system through TMS [START_REF] Meister | The essential role of premotor cortex in speech perception[END_REF]d'Ausilio et al., 2009;[START_REF] Sato | A mediating role of the premotor cortex in speech segmentation[END_REF]. These perturbations remain however small and not at the level to prove that perception needs action. The question hence stays rather open for future works.

IV.4. Summary

In summary, two functions could be associated with the perceptuo-motor connection.

Firstly, perceptual representations in the temporal pole would be structured in relationship with motor representations in the parieto-frontal pole, through the dorsal route in the course of speech development, and later on, all along life, through dynamic adaptations in various learning conditions. Secondly, the motor system could be involved, perhaps more in adverse conditions, in order to provide a better specification of possible auditory and visual trajectories and enhance speech scene analysis. This would exploit the dorsal route on line, but comprehension would operate basically in the temporal pole within the ventral route.

V. Conclusion

PACT provides a tentative way to connect perceptual and motor representations in a principled way in line with behavioural and neurophysiological data. PACT starts from the acknowledgement that speech units are shaped by a double set of articulatory and sensory (audiovisual) constraints, and hence are characterised by both a motor and a perceptual coherence.

This enables to derive a number of predictions from PACT towards the way human languages emerge in development [START_REF] Serkhane | Building a talking baby robot: A contribution to the study of speech acquisition and evolution[END_REF][START_REF] Serkhane | Infants' vocalizations analyzed with an articulatory model: A preliminary report[END_REF] and phylogeny (Schwartz et al., 2007;Moulin-Frier et al., 2008). Vowels, consonants and syllables are conceived, in this process, as the result of a set of perceptual and motor optimisation processes, in line with a late view by Chomsky himself: "The language faculty interfaces with other components of the mind/brain. (...) Recent work suggests that language is surprisingly 'perfect' in this sense, satisfying in a near-optimal way some rather general conditions imposed at the interface" (Chomsky, 1996, pp. 29-30). Language as an "optimal" or at least "optimised" communication system based on perceptually-shaped articulatory gestures provides in our view the framework in which a "neural theory of language" should be further explored and developed in the near future. around two boundary values, separating plosives from fricatives and fricatives from vowels. This is a typically quantal relationship in the sense of [START_REF] Stevens | The quantal nature of speech: Evidence from articulatory-acoustic data[END_REF][START_REF] Stevens | On the quantal nature of speech[END_REF] (see text).

Figure captions

(b) The consequence is that inside jaw cycles, there is a natural division between a consonantal pole for high jaw positions and a vocalic pole for low jaw positions. which is hence more stable and attracts a larger number of transformations [START_REF] Sato | Multistable syllables as enacted percepts: a source of an asymmetric bias in the verbal transformation effect[END_REF].

(b) Labial-Coronal effect in the VTE: in "patapata…" auditory sequences, [pata] sequences are chunked since they may be produced on a single jaw cycle (Sato et al., 2007a).

(c) Audiovisual VTE. If a "psepsepse" auditory sequence is dubbed with a video sequence switching between [ps´] and [s´p] the audio percept switches accordingly: the video stream binds the audio percept in the VTE (Sato et al., 2007b). target, and which enhances the detection of the audio prevoicing [START_REF] Schwartz | Seeing to hear better: Evidence for early audio-visual interactions in speech identification[END_REF].

Fig. 6 -A PACT architecture for speech perception

The perceptuo-motor link leads to co-structuration of sensory and motor maps, and may also involve online calls to motor representations for improving the integration of speech material in the speech scene.

  [i e] and [u o] or [i e E] and [u o ç]) and also, when they exist, in their non peripheral series such as central or front rounded (e.g. [i e E] and [y O ø] in French) while no perceptual principle ensures such an equilibrium in the DT or DFT

  [i] and [e]) quite close together and mid-high and mid-low (e.g. [e] and [E]) quite far apart, others widely separate [i] and [e] but have [e] and [E] close together, others having [E] and [a] close together. These differences exist both for French and Canadian French, and whatever the speaker's age between 4 and adulthood (the distribution being evaluated in a normalised F1 space, the F1 values of [i] and [a] playing the role of normalised references between which [e] and [E] may be placed at various positions from one speaker to another).

  according to which vowel systems do not search to optimise perceptual distances but to achieve sufficient contrast for communication. However, a striking observation by Ménard et al. is that for each subject, vowels tend to be grouped along stable F1 values, that is, if [e] is close to [i], then [O] is close to [y] and [o] is close to [u], [e], [O] and [o] having quite neighbour F1 values; and the same for [E], [ø] and [ç].

  g.: [pold] for [pod] repetition. A third mechanism displayed by Pitt et al. consists in auditory streaming. This is precisely what Bregman (1980) capitalised on for general Auditory Scene Analysis mechanisms, and what

  Remez et al. suggested, and favouring articulatory coherent streams such as [ps´] over [s´p], [pata] over [tapa], coherent audiovisual scenes over incoherent ones, relying on visible labial onsets. Here again, speech should be conceived as what it is: neither auditory, nor motor, but perceptuo (multisensory) -motor in nature.

Fig. 1 -

 1 Fig. 1 -The articulatory three-dimension space of oral vowels (a), together with its auditory (b) and visual (c) projections (see text).

Fig. 2 -

 2 Fig. 2 -Jaw cyclic gestures shaped by the acoustic/auditory transformation in a quasi quantal way (a) When jaw position varies from low to high, the acoustic/auditory features change suddenly

Fig. 3 -

 3 Fig. 3 -Perceptuo-motor distribution of vowel heights in French Typical distributions for subjects having either close high and mid-high vowels (Type 1), or close mid-high and mid-low (Type 2) or close mid-low and low (Type 3). (a) Left column: typical production data; (b) Middle column: typical perceptual data at the same age; (c) Typical perceptual data with a smaller vocal tract (larger formant values)

( d )

 d Audio-visual speech scene analysis and perception in noise. The categorization of audio syllables beginning by a voiced consonant, such as [du], is improved by presentation of the video input, thanks to the temporal cue provided by the labial gesture onset towards the rounded[u]


Eric Lichtfouse
email: eric.lichtfouse@dijon.inra.fr

Sustainable agriculture as a central science to solve global society issues

Keywords: sustainable agriculture, climate change, biodiversity, agronomy for sustainable development, organic farming, pest control, food, water, soil, pesticide, farming system

Serious global issues such as poverty, illness, food prices, climate changes, global market, pollution, pest adaptation and resistance, soil degradation, decreasing biodiversity and desertification can be explained by the increasing artificialization of human society. Since most issues are now intertwined they cannot be solved anymore by the classical fireman approach. In that respect, the structure of actual science and governmental institutions are probably outdated and should evolve to meet global challenges. Unexpectedly, agronomy appears as a central science to solve current societal issues because agronomists are trained to manage the input of many disciplines such as plant biology, soil science, climate sciences, ecology and chemistry.

Nature does not hurry, yet everything is accomplished. Lao Tzu

Starving people in poor nations, ill and fat people in rich nations, increasing food prices, climate changes, increasing fuel and transportation costs, flaws of the global market, worldwide pesticide pollution, pest adaptation and resistance, loss of soil fertility and organic carbon, soil erosion, decreasing biodiversity, and desertification are current acute problems that threaten our planet. Most current human issues can be explained by the "artificialisation" of society (Figure 1). Artificialisation began in ancient times with the start of one of the oldest human practice, agriculture. The beginnings of wheat domestication can be traced back at about 8,000 years BC [START_REF] Araus | The historical perspective of dryland agriculture: lessons learned from 10 000 years of wheat cultivation[END_REF]. At that time, humans switched progressively from a nomad life involving hunting animals and eating wild plants and fruits, typical of animal behaviour, to a more settled life by growing crops, harvesting, and storing food. Tribes rapidly invented money, for instance in the form of shells or salt, as a means to exchange goods with other tribes. That was the beginning of the financial market. Tribes also invented territories and borders. That was the beginning of nations. Early social behaviours had already benefits such as a secure food stock for winter, as well as drawbacks such as wars to conquer wealthier territories. For quite a long time, until the start of the industrial revolution around 1850, negative impacts of society were limited and local because social groups and nations were independent and small. Negative impacts of society were also restricted by time because transport of food, goods, and humans, was slow and mainly local.

Artificialisation increased dramatically during the industrial revolution. The advent of motor boats, cars, and planes allowed rapidly fast transportation of goods over long-distances. Social groups and nations began to be less and less independent, and thus relied on other countries for food and goods. This worldwide behaviour led to tremendous benefits such as medicine that has highly increased the human lifetime. But this led also to worldwide negative consequences such as world wars I and II, Chernobyl, and, more recently, climate change, poorer countries and ill-fat wealthy people.

Society issues have so far been solved using mainly the "fireman" approach, or the "pain-killer" approach as a more medical version, by which an individual problem is solved by an individual solution. A such approach does not work anymore on two grounds, at least. First, all systems, mechanisms, and activities are closely intertwined. For instance food production is closely linked to health, climate change, transportation, market, finance, and politics. Thus applying a remedy to only one element of this system will not work because the remedy will induce negative impacts on other elements in the end. Only solutions that consider the whole system and its connections will have a chance to succeed now. Second, the fireman approach does not treat the source of the problem. It only treats the negative consequence. Here, an obvious solution is to identify the problem sources, and to anticipate potential future problems.

From those observations, two advices can be given. First, actual borders, names, and field covered that separate government institutions are probably outdated. For example an agriculture department should not be separated from health, economics and transportation departments because most agriculture issues are linked to health, economics and transportation. The name, structure, mechanisms and field covered by departments should thus evolve to take into account the sources and connections of modern issues. Second, in a similar way, the division of sciences into disciplines such as physics, biology, and chemistry is not in line with actual scientific issues that are solved by an input of several disciplines. The names, structures, mechanisms, and field covered by scientific disciplines should thus evolve to adapt to actual, interconnected scientific issues.

For a long-time Agronomy has been considered as a soft, side science because food production was not really an issue in rich nations since the start of industrial farming around 1960. Now, unexpectedly, agronomists appear as the best scientists to solve current societal issues of food, climate change, health, and poverty. Indeed, agronomists are typically used to solve issues that need the input of many sciences such as plant biology, soil science, climate sciences, environmental chemistry (Lichtfouse et al., 2005), geology, sociology and economics [START_REF] Lichtfouse | Agronomy for sustainable development[END_REF](Lichtfouse et al., , 2009a)). They are also used to work on very complex research objects, the behaviour of which being seldom reproducible. Sustainable agriculture thus appears as a central science. Sustainable agriculture is thus the best fitted science to solve current issues, to anticipate future negative impacts, and to define novel practices that will make the world safer for our children. Detailed agroecological practices are published in the book Sustainable Agriculture (Lichtfouse et al., 2009b).

Figure 1 .

 1 Figure 1. Most current issues of society can be explained by the increasing artificialisation of human behaviour. The artificialisation has started to increase fast during the industrial revolution, leading to global issues.


Wouter J T Bos

Salah Neffaa

Kai Schneider

Rapid generation of angular momentum in bounded magnetized plasma

Keywords: numbers: 52.30.Cv, 52.65.Kj, 47.65.-d

Direct numerical simulations of two-dimensional decaying MHD turbulence in bounded domains show the rapid generation of angular momentum in non-axisymmetric geometries. It is found that magnetic fluctuations enhance this mechanism. On a larger timescale, the generation of a magnetic angular momentum, or angular field, is observed. For axi-symmetric geometries, the generation of angular momentum is absent, nevertheless a weak angular field can be observed. The derived evolution equations for both angular momentum and angular field yield possible explanations for the observed behaviour.

The generation of large coherent structures of the size of the flow domain is a generic feature of two-dimensional (2D) turbulence. Indeed, due to the inverse energy cascade, 2D flows show a tendency to create space filling structures. The nature of these structures and the way they are produced vary from flow to flow. In the context of Navier-Stokes turbulence the generation of a largescale domain-filling structure was predicted by Kraichnan [1] and observed in the case of forced turbulence in a periodic domain in which energy condenses at the smallest possible wave number modes [2,3]. In forced wallbounded flows this was reproduced numerically [4] and experimentally [5], and it was shown that a large scale rotating structure emerges, which dramatically reduces the level of the turbulent fluctuations [6].

A similar observation can be made in fusion plasmas, in which the dynamics share many features with 2D flows due to the imposed magnetic field. It is often assumed that in these plasmas large scale poloidal structures, called zonal flows, are beneficial for the confinement as they suppress turbulence and shear apart radially extended structures, which are largely responsible for anomalous transport [7][8][9]. The hereby created transport barriers might play a key role in the transition to an improved confinement state (H-mode) [10]. In the case of MHD turbulence the role of rotation was shown to have a similar effect on the flow, reducing the velocity fluctuations and hereby stabilizing the magnetic field [11].

In the present work we will continue the investigation of wall bounded non-ideal MHD. The generation of zonal flows through the absence of charge neutrality will not be addressed (charge neutrality being implied by the onefield MHD approximation). However, MHD allows for an affordable global description of non-uniform magnetoplasmas [12]. The present work could be related to the L-H transition through the beneficial effects of large scale poloidal rotation (which is observed in the present work) on the confinement of the plasma. The present study is also motivated by the observation that MHD-equilibria in toroidal geometry imply finite flow-fields due to the finite viscosity and resistivity [12][13][14]. In these works non-ideal MHD steady states were investigated in both the limit of small and large viscosity. In each case it was shown that the steady state contains non-vanishing velocity fields, at odds with classical static equilibria, on which decades of confinement research are based. In the present work we will not consider steady states but we will investigate the full nonlinear relaxation of non-ideal MHD with non-trivial boundary conditions in two space dimensions. The resistivity and viscosity are non-zero but small, allowing for a turbulent flow. This approach can not take into account toroidal velocities and nonuniform toroidal magnetic fields and the extension of the present approach to three dimensions constitutes therefore an important direction for further research.

In the case of decaying Navier-Stokes turbulence it is shown that the self-organization in a periodic domain will lead to a final state, consisting of two, non-interacting, counter-rotating vortices [15]. This picture changes however in the presence of no-slip walls. In this case the flow relaxes to a state with or without angular momentum, depending on the shape of the domain [16][17][18]. Indeed in circular domains without initial angular momentum the flow generally relaxes to a state free from angular momentum [19], whereas as soon as the axi-symmetry is broken the flow relaxes to a state containing a domain filling structure, containing significant angular momentum [20]. Theoretical progress has been made to explain the phenomenon in the inviscid case, based on a model of interacting vortices [21][22][START_REF] Taylor | Interacting vortices and spin-up in 2-d turbulence[END_REF].

In the case of bounded two-dimensional MHD it is not known, up to now, to which kind of state the flow relaxes and this will be addressed in the present letter. We investigate the case in which both the magnetic field and the velocity field can not penetrate into the walls. The velocity field obeys the no-slip condition at the wall, whereas the tangential component of the magnetic field can freely evolve, allowing a net current through the domain. We will focus however in the present study on the case in which no net current is initially present.

We start by writing the governing equations. In the present case we define two angular momenta: a kinetic and a magnetic one,

L u = Ω e z • (r × u)dA, L B = Ω e z • (r × B)dA (1)
in which Ω is the flow domain, r the position vector with respect to the center of the domain and u and B the velocity and magnetic field vector, respectively. Through integration by parts these quantities can also be expressed as a function of the streamfunction ψ = ∇ -2 ω and vector potential a = ∇ -2 j, respectively, with j = je z = ∇ × B the current density and ω = ωe z = ∇ × u, the vorticity:

L u = 2 Ω ψ dA, L B = 2 Ω a dA, (2)
in which a and ψ are chosen to be zero at the wall.

A large value of the angular momentum can generally be associated with the presence of a large-scale vortical structure. By analogy we can anticipate that a large value of L B corresponds to a large-scale current density structure, and we baptize the quantity L B angular field. The evolution equations for L u and L B can be derived following the procedure described in Maassen [START_REF] Maassen | Self-organization of confined twodimensional flows[END_REF], by time deriving equations (1) and using the MHD equations:

∂u ∂t + (u • ∇)u = -∇p + j × B + ν∇ 2 u (3) ∂B ∂t = ∇ × (u × B) + η∇ 2 B ( 4
)
together with ∇ • u = 0 and ∇ • B = 0. The pressure is denoted by p, and ν and η are the kinematic viscosity and magnetic diffusivity, respectively. If we write the Lorentz force in the form:

j × B = - 1 2 ∇B 2 + (B • ∇)B, (5)
dL u dt = ν ∂Ω ω(r • n)ds + ∂Ω p * r • ds. (6)
The only difference with respect to the hydrodynamic case [18] is the pressure which is now replaced by the modified pressure p * . In most fusion plasmas, the quantity β = p/B 2 ≪ 1 to insure confinement, which means that the magnetic part of the pressure dominates. It is important to note that the pressure term in equation ( 6) vanishes in axi-symmetric domains. In this work we therefore consider both a circular and a square domain to analyze the influence of this term. The derivation of the equation for L B is analogous to the derivation for L u . The resulting equation is:

dL B dt = η ∂Ω j(r • n)ds -2ηI. (7)
We observe that there is a term involving the net current I through the domain defined by I = Ω je z dA. This term is the equivalent of the circulation in the hydrodynamic case, which is zero due to the no-slip walls. The net current is however not imperatively zero as the tangential magnetic field does not vanish at the wall. Nevertheless, a net current will not be generated if it is initially zero, which is the case in the present work. We performed computations in two different geometries: a square of size D = 2 and a circular geometry with a diameter D = 2.24. A description of the generation of the initial conditions and the numerical scheme, a spectral method with volume penalization, are given in [START_REF] Neffaa | [END_REF]. The initial velocity and magnetic field consist of correlated Gaussian noise with vanishing cross-helicity Ω u • B dA. The magnetic Prandtl number, ν/η is equal to one. The initial Reynolds number, based on the domain size is √ 2E u D/ν and yields 1960. The ratio of the magnetic and kinetic energy E B /E u = 2.3, with

E u = 1 2 Ω |u| 2 dA and E B = 1 2 Ω |B| 2 dA.
The resolution of the simulations is 512 2 Fourier modes. In each geometry 10 runs were performed starting from different statistical realizations with the same initial parameters. The numerical value of a and ψ is not automatically zero at the domain boundary. This is accomplished a posteriori by substracting a constant value at each point in the domain.

In figure 1 snapshots of the streamfunction and vector potential are shown at t * = 0.75, 3, 12 with t * = t 2E u (t = 0)/D. It can be inferred from (2) that these quantities should give a good visual interpretation of the presence of angular momentum and field. At time-instant t * = 0.75, in which inertial effects are dominant over viscous effects, it is well visible that the velocity field self-organizes into a large domain-filling structure in the square geometry, whereas in the circular geometry several structures are observed. At t * = 3 a large structure appears also in the magnetic field in the square geometry. At t * = 12 the large-scale velocity and magnetic structures in the square domain are (anti-)aligned. In the circular domain the tendency to create domain filling structures is weaker, even though the magnetic field in the circular domain shows some evidence of the formation of a large current-structure at t * = 12. To characterize the relaxation of the flows in both geometries, we also show in figure 1 the decay of the kinetic and magnetic energy in both domains, as well as the absolute value of the cosine of the alignment angle. A continuous decrease of kinetic and magnetic energy is observed and a continuous increase of global alignment.

At this moderate Reynolds number, spin-up, i.e., spontaneous generation of angular momentum, does not occur in every flow realization. Also the criterion what is strong or weak spin-up is rather arbitrary. We therefore focus first on mean quantities to illustrate the general tendency to spin-up. In figure 2 we show the absolute value of the angular momentum, averaged over 10 runs. We take the value because there is no preferential direction of the spin-up so that an average of the angular momentum would yield values close to zero for all cases. The time-evolution of |L u | and |L B | is shown for both the square and the circular geometry.

• denotes the average over 10 realizations. The quantities are normalized by L u (0) = r 2 2 E u (t = 0) and L B (0) = r 2 2E B (t = 0) , with r 2 the Euclidean norm of r. The quantity L u (t) corresponds to the value of the angular momentum of a flow in solid-body rotation with kinetic energy E u (t) , which is the flow which optimizes the value of the angular momentum for a given kinetic energy. By analogy, L B (t) is used to normalize the angular field. The following is observed: at short times L u rapidly increases in the square, but does not increase in the circular geometry. The value of L B also increases in the square, but delayed with respect to L u . In the circular geometry an increase of L B is also observed. In

L u (t) t * Sq. E B /E u =2.3 Sq. E B /E u =10 Sq. E B /E u =17 C. E B /E u =2.3 C. E B /E u =10 C. E B /E u =17
FIG. 2: (Color online) Top: Time evolution of the absolute value of the angular momentum and angular field, averaged over all realizations, normalized by Lu(0) and LB(0), respectively. In the insert the same quantities are given, normalized by Lu(t) and LB(t) (defined in the text). Bottom: timedependence of the angular momentum Lu in the square and circular geometry, normalized by Lu(0). The influence of the magnetic pressure on the spin-up in the square container is illustrated by changing the ratio EB/Eu, while keeping Eu fixed.

malized by L u (t) and L B (t). This normalization has the advantage to correct for the decay of the kinetic and magnetic energy but has the disadvantage that it is sensitive to selective decay [START_REF] Keetels | Fourier spectral computation of geometrically confined two-dimensional flows[END_REF] so that at long times we observe generation of angular momentum in each case even if its absolute value might be small. In the following we will give, where possible, an explanation for the 4 curves in figure 2. First, in the square geometry a strong spin-up of the velocity field is observed. In the hydrodynamic case, it was argued in [18,20] that the pressure term triggers the spin-up in the square geometry. The magnetic field enhances the pressure term through the magnetic pressure (p * = p + B 2 /2). If in the present case it is also the pressure term in (6) which triggers the spin-up, the effect could be enhanced by increasing the magnetic fluctuation strength B 2 . This is illustrated in figure 2 (bottom). For one run in which spin-up was observed, the initial magnetic fluctuations are increased from E B /E u = 2.3 up to E B /E u = 10 and 16.7, while keeping the initial E u fixed. The resulting spin-up is significantly stronger.

Second, for L u in the circular geometry, like in the hydrodynamic case [19], no spontaneous spin-up is observed. Increasing the magnetic-field strength does only weakly influence this result (figure 2, bottom).

Third, the interpretation of the generation of the angular field in the square geometry is less straightforward, as equation (7) does not contain a pressure term. The tendency to create large-scale magnetic structures can be attributed to the selective decay mecanism [START_REF] Matthaeus | [END_REF], which was recently shown to persist in bounded geometries [START_REF] Neffaa | [END_REF]. This does however not explain the symmetry breaking or angular momentum generation, which is the main issue of the present work. A possible trigger for the spin-up could be alignment. It is well known that the magnetic field and the velocity field tend to align so that the nonlinear term in the equation for j (or B) vanishes. Hence, the magnetic field tends to an alignment with the velocity field which acquired angular momentum through the modified pressure term. It is therefore expected that the magnetic spin-up follows the hydrodynamic spin-up after a time-scale corresponding to the alignment. Indeed, L B spins-up shortly after L u . The cosine of the angle between u and B, measuring the global alignment, is plotted in the inset of figure 1 (right). A tendency towards global alignment is observed for long times.

Fourth, in the circular geometry, the weak spin-up of the magnetic field is surprising. Higher resolution simulations are needed to clarify whether this is a viscous effect and/or a statistically more probable (maximum entropy) state? In this context we can refer to [START_REF] Taylor | Interacting vortices and spin-up in 2-d turbulence[END_REF], where, based on point-vortices, it was shown that two types of most probable states exist in a circular domain: a double vortex, free from angular momentum and an axi-symmetric flow, with finite angular momentum. This work neglected the influence of viscosity so that it is not clear how the angular momentum is acquired in the circular geometry.

We now resume our findings. Rapid generation of angular momentum takes place in bounded MHD turbulence, as long as the geometry is non-axisymmetric. The effect is enhanced by the magnetic pressure. On a slower time-scale also magnetic spin-up is observed in both geometries. It is not clear how this angular field is created. Both alignment and selective decay could be possible explanations.

FIG. 1 :

 1 FIG. 1: (Color Online) Visualizations of the stream-function ψ and the vector-potential a for both geometries. Figures for ψ are normalized by the maximum of |ψ| (and max(|a|) for a). The values of max(|ψ|) are from left to right 0.1, 0.05, 0.03; 0.06, 0.03, 0.01; and for max(|a|) 0.08, 0.06, 0.04; 0.04, 0.03, 0.01. Right: time-evolution of the kinetic and magnetic energy in both geometries. In the inset, the evolution of the absolute value of the relative cross-helicity | cos(θ)| , illustrates the global alignment of the velocity and magnetic field in both geometries. Red solid line: square geometry; green dashed line: circular geometry.

  we can absorb the first term into the pressure term of the Navier-Stokes equations by introducing the modified pressure p * = p + B 2 /2. The (B • ∇)B term does not induce new terms in the equation for L u . It vanishes in a similar way as the nonlinear term (u • ∇)u does, using ∇•B = 0 and B •n| ∂Ω = 0. The equation for L u becomes

  the inset the values of |L u | and |L B | are plotted nor-

We want to stress the implications of the present study for confinement research. Fusion plasmas are wall bounded and not axi-symmetric, so that even in the case of charge neutrality the plasma might have a tendency to create zonal flows and zonal fields, depending on the geometry of the cross-section of the plasma and the strength of the magnetic fluctuations. The present work opens several perspectives for future research, such as the influence of P r m , Re, and in particular the extension to three dimensions in which the effects of imposed magnetic fields, currents and toroidal velocities can be taken into account.

We acknowledge valuable discussions with Herman Clercx, David Montgomery and Geert Keetels. This work was supported by the ANR under the contract M2TFP.