Amélie Rochet-Capellan

Jean-Luc Schwartz

An articulatory basis for the Labial-to-Coronal effect: /pata/ seems a more stable articulatory pattern than /tapa

Keywords: numbers: 43, 70Aj, 43, 70Fq

Rochet

-Capellan & Schwartz: An articulatory basis for the Labial-to-Coronal effect -1 -This paper investigates the coordination between the jaw, the tongue tip and the lower lip during repetition with rate increase of Labial-to-Coronal (L a C o ) Consonant-Vowel-Consonant-Vowel disyllables (e.g./pata/) and Coronal-to-Labial (C o L a ) ones (e.g. /tapa/) by French speakers. For the two types of disyllables: (1) the speeding process induces a shift from two jaw cycles per disyllable to a single cycle; (2) this shift modifies the coordination between the jaw and the constrictors and (3) comes with a progression towards either a L a C o attractor (e.g. (/pata/ or /tapa/) /patá/ /ptá/) or a C o L a one (e.g. (/pata/ or /tapa/ /tapá/ /tpá/). Yet, (4) the L a C o attractor is clearly favored regardless of the initial sequencing. These results are interpreted as evidence that a L a C o CVCV disyllable could be a more stable coordinative pattern for the lip-tongue-jaw motor system than a C o L a one. They are discussed in relation with the so-called LC effect that is the preference for L a C o associations rather than C o L a ones in CV.CV disyllables in both world languages and infants' first words.

Introduction

The analysis of CV.CV disyllables in world languages lexicons reveals that Labial-to-Coronal (L a C o ) sequences (e.g. /pata/) are about 2.5 times more used than Coronal-to-Labial (C o L a ) ones (e.g. /tapa/) [START_REF] Locke | Phonological Acquisition and Change[END_REF]MacNeilage et al., 2000aMacNeilage et al., , 2000b;;[START_REF] Rousset | From lexical to syllabic organization: favored and disfavored cooccurrences[END_REF]. This asymmetry, also displayed in infants' first words in the course of ontogeny [START_REF] Ingram | Fronting in child phonology[END_REF][START_REF] Locke | Movement patterns in spoken language[END_REF], MacNeilage et al., 2000a, 2000b), is known as the "LC effect". The existence of the same trend in infant and adult speech lead MacNeilage and Davis (2000b) to propose a "substancebased" explanation of the LC effect in the framework of the "Frame-Content Theory" of speech development [START_REF] Macneilage | The frame/content theory of evolution of speech production[END_REF]. Yet, there are some problems with their explanation, letting space for further hypotheses. The present investigation is an attempt to test an original explanation of the LC effect, based on motor principles likely to intervene both in speech development and in on-line adult speech production.

1.1. The "simple first" hypothesis MacNeilage and Davis (2000aDavis ( , 2000b) ) proposed that the LC effect might result from the conjunction of two principles in the course of speech development: (1) labial proto-consonants (or closants in the Frame-Content terminology) would be easier to produce by infants than coronal ones, and (2) in order to deal with the complexity of the mental lexicon access required in the production of their first words, infants would focus on easier forms. Thus, they would select sequences of actions beginning by the simpler gesture, supposed to be the labial one. The preference for L a C o disyllables in development would then have been preserved in adults' languages.

The assumed greater simplicity of labial gestures is one component of the Frame-Content theory of speech motor control development [START_REF] Macneilage | The frame/content theory of evolution of speech production[END_REF][START_REF] Macneilage | The frame/content theory of evolution of speech production[END_REF]Davis, 2000a, 2000b). This theory claims that speech production begins in babbling with phonation associated with repeated vertical oscillations of the jaw. Apart from vocal tract pre-settings, the other articulators would stay stable all along the jaw cycles. Thus, besides their alimentary function, jaw cycles would induce alternation of closing and opening patterns (closants and vocants) and constitute the frame of speech. Then, the speech segmental content (independent control of consonants and vowels inside the frame) would progressively emerge from the development of the central and peripheral motor control of the other articulators. According to MacNeilage and Davis, for neutral vocal tract pre-setting, that is purely passive tongue and lips, jaw upwards gestures would induce a labial closure of the vocal tract and so, the production of labial closants ("pure frames"). On the contrary, coronal closants would require an active motion of the tongue in pre-setting ("fronted frames"). Therefore, labial closures would be easier to realize than coronal ones. However, the "simple first" explanation suffers in our view from several weaknesses. Firstly, it is a bit ad hoc. Secondly, the "simpler" nature of labial closants in babbling has been questioned by [START_REF] Vilain | From idiosyncratic pure frame to variegated babbling: evidence from articulatory modelling[END_REF] in a study exploiting articulatory models of the vocal tract. They compared three models with different morphologies -coming from the statistical analysis of cineradiographic data recorded from three different adult speakers. They simulated pure frames on the three models, by applying jaw upward commands, all other articulators being kept passive, and they obtained different kinds of closures. In one model the jaw raise resulted in closing the lips first, producing a labial contact, while in another one it first resulted in a closure inside the vocal tract, producing a coronal contact. In the last model, a labial and a coronal contact were obtained almost in synchrony. Therefore, both /ba/, /da/ and /bda/ could be pure frames according to the vocal tract morphology. This could explain why analyses of babbling inventories do not display a preference for labial patterns over coronal ones [START_REF] Locke | Phonological Acquisition and Change[END_REF].

Another problem concerns the extrapolation from infants to adults. Indeed, there is no reason to believe that labial plosives would stay simpler than coronal ones in adult speech, both involving a specific articulatory control superimposed on the jaw, respectively for the lips in labials and for the tongue in coronals [START_REF] Munhall | Articulatory evidence for syllabic structure[END_REF]. Finally, the "simple first" hypothesis tends to consider the articulatory control of L a C o and C o L a utterances as a sequence of independent phonetic segments while it might be better characterized as coordinative structures [START_REF] Kelso | The dynamical perspective on speech production: data and theory[END_REF][START_REF] Nittrouer | Phase relations of jaw and tongue tip movements in the production of VCV utterances[END_REF].

An alternative hypothesis based on jaw-tongue-lips coordination

It is widely assumed that some coordinative patterns are more stable and economic than others and that the repetition of a multi-effectors gesture with rate increase would attract the system toward its most stable coordinative mode. This has been first established for inter-limb coordination [START_REF] Hoyt | Gait and the Energetic of Locomotion in Horses[END_REF][START_REF] Haken | A theoretical model of phase transitions in human hand movements[END_REF] and then, extended to articulators' coordination in speech production [START_REF] Kelso | The dynamical perspective on speech production: data and theory[END_REF]Lindblom, 1990a;[START_REF] Kelso | Dynamic patterns: The self-organization of brain and behavior[END_REF]. Such criteria of stability and economy have been introduced in substance-based criteria shaping the sound systems of human languages (e.g. [START_REF] Stevens | On the Quantal Nature of Speech[END_REF]Lindblom, 1990b). In this vein, it has been shown that rate increase induced repeated VC syllables to switch towards cycled CV ones [START_REF] Stetson | Motor Phonetics[END_REF][START_REF] Tuller | The production and perception of syllable structure[END_REF][START_REF] De Jong | Effects of syllable affiliation and consonant voicing on temporal adjustment in a repetitive speech-production task[END_REF]. This led the authors to conclude that CV sequences are more stable than VC ones, which could explain the preference for CV syllables over VC ones in language inventories [START_REF] Redford | The relative perceptual distinctiveness of initial and final consonants in cvc syllables[END_REF][START_REF] Rousset | From lexical to syllabic organization: favored and disfavored cooccurrences[END_REF].

These studies provide an interesting framework for attempting to anchor the LC effect in the properties of the articulatory motor system rather than in a pure developmental framework.

More precisely, we assume that a labial constriction followed by a coronal one could be a better coordinative pattern for the lip-tongue-jaw system than the coronal-then-labial one. This assumption is inspired from [START_REF] Sato | Multistable syllables as enacted percepts: a source of an asymmetric bias in the verbal transformation effect[END_REF]. The authors supposed that in CCV or CVC sequences involving a labial and a coronal consonant, there was a trend to anticipate the coronal constriction during the labial one, rather than the inverse. Hence, lips and tongue were more inphase in /ps / than in /sp /. The reason is that in /ps /, the tongue tip may be prepared for /s/ during the labial closure, so that the /p/ release is almost synchronous with the tongue-driven onset of the next /s/. On the contrary, in /sp /, the lips must stay open during the production of the /s/, before labial closure followed by the /p/ release, so that the tongue-lip synchrony is difficult to achieve (see also [START_REF] Byrd | Influences on articulatory timing in consonant sequences[END_REF][START_REF] Zsiga | Acoustic evidence for gestural overlap in consonant sequence[END_REF]Surprenant and Goldstein, 1998, for compatible data in English;and Chitoran et al., 2002 for Georgian). This articulatory asymmetry between the two sequences of consonantal gestures could be a part of the LC effect explanation in both language development and world languages lexicons.

An experimental scenario

The presumed greater stability of the coordination in L a C o compared with C o L a could be investigated through the speeding paradigm. The slow regular repetition of C 1 VC 2 V disyllables should be characterized by a regular alternation of C 1 and C 2 gestures over time. On the contrary, rate increase could lead to asymmetries, the C 2 constriction being either closer to the previous or to the next C 1 one. This would induce the reduction of either the vowel after C 1 or after C 2 . The French language, under study here, displays a stress on the word last syllable. Therefore, in the repetition with rate increase of a CVCV disyllable, French speakers should rather reduce the first vowel, and a CCV re-syllabification could be expected. Hence, two hypotheses may be contrasted. In a first one, there would be no asymmetry between L a C o and C o L a coordination, and the French speakers would preserve the original disyllable prosody through speeding. In a second experiment based on articulatory measurements, we studied how the articulatory coordination between jaw, tongue and lips would be reorganized in order to subtend the progression from one constriction per jaw cycle (e.g. /pata/) towards two constrictions per jaw cycle (e.g. /ptá/). A prediction was that the two constrictions would be realized on a single jaw cycle before the fading of the vowel was completed (e.g. /patá/). In other words, the variation of the articulation place between the first and the second constriction would allow the production of two syllables on a single jaw cycle, while most studies of jaw movements associate the jaw cycle with the CV syllable [START_REF] Nelson | Mandibule movements during increasingly rapid articulations of single syllables : Preliminary observations[END_REF][START_REF] Ostry | Human jaw movement in mastication and speech[END_REF]Perkell and Zandipour, 2002;Perkell et al., 2002).

2. An acoustic study of LC and CL stability

The aim of this first study is to test if L a C o sequences are more stable than C o L a ones in a speeding paradigm, using an acoustic measure of vowel reduction.

Method

Speakers and phonetic material

28 native French speakers with no speech or hearing problem and naive as to the purpose of the study participated in the experiment. The phonetic material consisted in six CVCV disyllables: three L a C o (/pata/, /pasa/ and /fata/) and their three C o L a counterparts (/tapa/, /sapa/ and /tafa/). This choice aimed at contrasting L a C o and C o L a sequences while controlling for a possible "plosive-to-fricative" effect. Indeed, in the framework of "sonority" principles [START_REF] Selkirk | On the major class features and syllable theory[END_REF][START_REF] Clements | The role of the sonority cycle in core syllabication[END_REF] it could be supposed that utterances containing a fricative and a plosive would switch towards plosive-fricative attractors (e.g. /tfa/, /psa/), rather than the L a C o attractors /pta/, /psa/ and /fta/ expected in the framework of our basic assumptions. The /a/ vowel was selected to provide an open configuration, mainly involving the jaw, with basically no strong requirements on the lips and the tongue. The six disyllables were randomly arranged inside three lists. The three lists were presented to each speaker in different orders. Hence each disyllable was presented three times to each speaker. The test began with three items provided as training stimuli (/vada/, /daba/ and /dava/).

Procedure

In order to simplify post-treatments the experiment was assisted by a computer program that allowed to record utterances and to display a kind of visual metronome. The participants were sitting in front of a computer connected to a microphone. The instruction was to repeat the disyllable displayed on the screen at an increased and then decreased rate following as much as possible the rhythm of a visual flash. The aim of this flash was not to precisely control the speakers' rate but rather to give global timing marks in order to homogenize acceleration and deceleration phases between speakers and to attempt to drive the speakers to their limit rate. It was mentioned that the flash would reach rates impossible to follow and the instruction was to maintain a rate as high as possible during this period until the decelerating phase. Moreover, speakers were encouraged to go on without interruption even if they perceived any kind of transformation of the uttered sequence.

The experiment began by the three practice trials. Then, the three lists were successively displayed. Each trial started with the presentation of the CVCV sequence to repeat, displayed in the middle of the screen. Speakers initiated the flash pressing the "space" key. A blue square took the place of the disyllable on the screen for one second, followed by the flash. The flash was an alternation of a black and a white square. The instruction was to produce one syllable on the black flash and the other on the white one. Duration of each square presentation started at 300 ms and progressively decreased to reach 125 ms at the middle of the acceleration phase and 50 ms at the end of the acceleration phase (8 s). The progression was shaped in order to obtain a large amount of rapid productions. The timing of the deceleration phase was the symmetric of the acceleration one. Thus, each trial record lasted 16 s. Speakers sometimes took a breath inside the whole acceleration-deceleration process, but this occurred seldom and at random positions inside the 16 s utterance. Then the speaker spontaneously resumed the process at about the same tempo he/she had reached before the breath.

Hypotheses and measurements

The Maxima and minima of energy curves were automatically detected and then hand-selected as vowels and consonants (Figure 1). When a speaker's production evolved towards a single consonantal cluster (e.g. /ptá/), the labeling rule was to mark the "vanished" vowel and the two consonants around as a same event (e.g. Figure 1, right). Speakers' errors such as repetition of a single syllable or pauses were removed. From these data, two parameters were computed for each CVCV or CCV sequence correctly produced (now referred as "utterances"): (1) the utterance duration and (2) the variation of intensity between the two vowels, that is the difference between the intensity of the vowel after the coronal consonant (V Co ) and the intensity of the vowel after the labial consonant (V La ):

= I(V Co ) -I(V La )

The operational hypothesis was that I would be positive for both the L a C o and the C o L a utterances for fast productions, with very high positive values indicating a shift towards a single L a C o CCV syllable (e.g. from /pata/ or /tapa/ to /ptá/, as in Figure 1).

----Figure 1 ----

Results

The analyses excluded two speakers, one because he did not manage to alternate the labial and the coronal constrictions at a fast rate and the other for recording problems. ----Figure 2 ----

Detailed analysis according to the disyllables and to the speakers

In order to investigate more precisely possible reorganizations of productions in the speeding process, the analysis of I was restricted to utterances with durations shorter than 300 ms. In spite of the flash, the utterance durations varied a lot according to the speaker and to the trial. Therefore, for the analysis, we kept only speakers achieving a sufficient amount of quick data with the following criterion: for each of the six CVCV logatoms, the speaker should have produced at least five quick utterances (durations shorter than 300 ms) for at least two of his/her three trials. Then, only the two trials with the largest number of quick utterances were analyzed.

This criterion excluded five speakers. In the following, only the utterances with durations less than 300 ms are analyzed, for the two selected trials of the 21 remaining subjects.

Global means of the 42 trials are positive for all disyllables, and all significantly different from 0 except for /sapa/ (Table I, first row). Moreover, the distribution of the 42 means according to their sign (last four rows in Table I) shows that the ratio of positive means related to negative ones (rows 3 and 5) is 2.5 for /pata/ and /tapa/, 7.4 for /fata/, 3.6 for /tafa/ and 1.8 for /pasa/ whereas it is 1 for /sapa/. A Chi-square test shows significant difference between the frequencies of positive vs. negative means ( 2(1) = 6.6, p<.001). Restricted to means that significantly differ from 0 (rows 4 and 6) the prevalence of positive means over negative ones is even greater with a ratio of 4.75 for /pata/, 2.5 for /tapa/, 3 for /pasa/, 1.4 for /sapa/, 13 for /fata/ and 5.3 for /tafa/ (frequency of significant positive means significantly different from significant negative ones, 2 (1) = 6.8, p<.001). Hence, the speeded repetition of L a C o and C o L a CVCV disyllables more often evolves towards the corresponding L a C o attractor (eg. /pasá/ or /psá/) than the C o L a one (eg.

/sapá/ or /spá/). However, C o L a forms appear in some cases for both L a C o and C o L a trials.

Furthermore, two additional factors tend to emerge. Indeed, there is some trend that I values are larger, and more systematically positive, for L a C o than for C o L a trials on one hand, and for /pata/-/tapa/ and /fata/-/tafa/ than for /pasa/-/sapa/ trials on the other hand.

----Table I ----

Considering that two attractors seem to coexist, though with a preference for the L a C o one, we wondered whether they might coexist for a given speaker. For this aim, Figure 3 (left) displays the relation between mean I values for the first and the second selected trial, for each speaker and for each CVCV sequence, provided that the two means significantly differed from 0.

It appears that mean I values are both significantly positive for 41 cases, both significantly negative for 7 cases and have different signs in 5 cases. Therefore, the dominant behavior is to evolve towards the same attractor for the two trials with a large preference for the L a C o compared to the C o L a one. Yet, in some cases, the attractor differs from one trial to the other. Then, grouping all logatoms, it appears that some speakers tend to be "L a C o stable" (e.g. S12, right part of the Figure 3) whereas others are, in a lower extend, rather "C o L a stable" (e.g. S23). Finally, some speakers are "bi-stable": the category of attractor they evolved towards changes according to the trial (S20) or to the logatom (S6).

----Figure 3 ----In summary, the speeded repetition of L a C o and C o L a logatoms could evolve both towards the L a C o or the C o L a attractor. Yet, and despite variability between disyllables and between speakers, the L a C o attractors are largely favored compared to the C o L a ones. Thus, /patá/-/ptá/ is the favorite attractor for /pata/ and /tapa/; /pasá/-/psá/ for /pasa/ and to a lower extend for /sapa/ and /fatá/-/ftá/ for both /fata/ and /tafa/.

3. An articulatory study of L a C o and C o L a stability

The main purpose of the second study is to understand from EMMA recordings how the articulatory system could evolve with rate increase from a L a C o or a C o L a CVCV disyllable towards either a L a C o or a C o L a attractor, with a preference for the first one.

Method

Speakers and phonetic material

The participants were five college-aged French speakers (three females -S1, S3 and S4and two males -S2 and S5), without any speech or hearing deficits. They did not participate in Experiment 1. The L a C o and C o L a disyllables were the same as in the acoustic study. Two duplicated CVCV logatoms (/papa/ and /tata/) were added as control items.

Procedure, data recording and processing

The instruction was to continuously repeat the disyllable enounced by the experimenter starting at a slow rate and then at an increasingly rapid rate up to the highest possible one. The speakers then had to progressively decrease the rate to return to the initial slow one. Contrary to the first experiment, no external tempo driver was used because of the already difficult conditions of EMMA recording. Speakers were encouraged not to stop their productions even if they seemed different from the initial sequence. The eight disyllables were repeated three times with orders that varied from one subject to another. The articulator motions were tracked over time at 500 Hz using a Cartens Electromagnetic Articulograph. Two reference transducers, one at the bridge of the nose and the other on the gums above the upper teeth allowed to correct the data for head movements. The three measurement points were: the jaw (transducer placed beneath the lower teeth); the Tongue Tip (TT, transducer placed at about 1 cm from the tip of the tongue) and the Lower Lip (LL, transducer just beneath the vermilion border of the lower lip). These raw measurements were then processed according to classical procedures (see Hoole P., 1996 for more details). These involved translation and rotation correcting for head movement, resulting in setting the origin at the reference coil located on the upper incisors, the vertical axis as the line joining the reference coils on upper incisors and nose bridge, and the x-axis aligned with the occlusal plane. The acoustic signal was simultaneously recorded by the way of a microphone fixed on the articulograph helmet and then digitized at 20 kHz.

Hypotheses

The first hypothesis is that the mandible would evolve from a one-cycle-per-syllable Instead, the hope was that inter-speaker variability would enable to observe both kinds of attractors, in order to be able to characterize them articulatorily.

relationship

Measurements

Processing of acoustical data was the same as in Experiment 1. It led to computation of the duration and intensity variation between the two vowels ( I) for each utterance (CVCV or CCV items correctly produced). Articulatory analysis used articulator trajectories preprocessed by a low-pass Chebychev filter (cutting frequency at 15 Hz). Jaw maxima and minima (Figure 4, row 2) were automatically detected on the jaw trajectory, allowing its segmentation into closing and opening phases (up and down strokes). Post-processing discarded movements realized during pauses or errors (detected from the acoustical labeling). For the correct utterances, two criteria were used in order to discard productions subtended by too small jaw movements. Firstly, we checked for the regular alternation of opening and closing strokes; secondly, we removed strokes with an amplitude lower than 0.1 mm, which is the claimed precision of the EMMA system.

Table II summarizes the total number of correct utterances and the number of correct utterances kept for further articulatory analysis. Then, TT and LL events were labeling on articulatory signals, with the help of acoustic events. TT constrictions were defined as the highest TT positions between the vowel after the labial consonant (V La ) and the vowel after the coronal consonant (V Co ) (Figure 4, row 3). Similarly, LL constrictions were defined as the highest LL positions between V Co and V La (Figure 4, row 4). This method (similar to Hertrich and Ackermann's one, 2000) discards secondary maxima due to passive motion induced by the jaw gesture that comes with the other constriction.

----Table II ----Then, jaw cycles were separated into an up stroke and its adjacent down stroke. The jaw cycle duration was the sum of its up and down stroke durations and the jaw cycle amplitude was the mean of its up and down stroke amplitudes (Figure 5.A). Each constriction event was assigned to the jaw cycle that temporally included it and positioned relatively to this jaw cycle (Figure 5B). This measure, expressed in percent of jaw cycle, constitutes an indicator of the phasing relationship between the jaw and constrictors. Finally, the coordination between TT and LL was characterized by the duration of the labial-to-coronal phase (time from LL constriction to TT constriction D LaCo ), normalized by the whole disyllable duration (time from a LL constriction to the next LL one D LaLa ):

(LaCo/LaLa) = D LaCo / D LaLa ----Figure 5 ----

Results

Acoustic analysis

The ----Figure 6 ----Moreover, the mean values for utterances with durations shorter than 300 ms (Table III) represent the panel of behaviors observed in the first experiment. Indeed, as a subset of speakers of the acoustic study, S3 fails to produce enough rapid productions. This may indicate that this speaker rarely shifts towards a CCV structure. Then, S1 and S4 tend to favor the C o L a attractor (more negative than positive means) especially for /pasa/ and /sapa/ for S1 and for the three C o L a disyllables for S4. On the contrary S2 and S5 tend to favor the L a C o attractor for all disyllables except for S5's /sapa/ productions. Interestingly, these behaviors correspond to both the speakers' profiles and the variability between sequences observed in the first experiment.

----Table III ----

Global analysis of jaw cycles

The /tapa/ utterance samples displayed on Figure 4 illustrate the speeding process generally observed for a L a C o or a C o L a trial. First, during the acceleration phase (left), each constriction comes with its own jaw gesture. Yet, the amplitude and the duration of the jaw cycle for the labial constrictions tend to be smaller than for the coronal constrictions. Then, rate increase induces the progressive fading of the jaw cycle that supports the labial constriction, resulting in a single jaw cycle for the two constrictions. For this example, the plot of the acoustic signal (top) shows that the speaker does not evolve towards a CCV syllable but produces two syllables on a single jaw cycle (splitting phase, middle). Finally, the deceleration progressively brings the system back to its initial state with one jaw cycle per syllable (right).

Plots of jaw cycle durations (y) against CVCV durations (x) for all speakers' productions (Figure 7, first row) and separately for the duplicated (left) and variegated (right) CVCV sequences show that duplicated productions are grouped around the (y=x/2) line (i.e. two jaw cycles for one disyllable) whereas both L a C o and C o L a ones are distributed around the (y=x/2) and (y=x) lines (i.e. two vs. one jaw cycle per disyllable). This distribution splitting occurs for utterance durations less than about 400-450 ms. Analyses of I in the acoustic study showed that the shift towards a CCV cluster, characterized by high I values, occurs for durations shorter than 300 ms. Therefore, the shift towards a single cycle for two syllables occurs before the total suppression of one of the two vowels (that is, for longer utterance durations).

----Figure 7 Altogether, this portrait is coherent with the fact that at a fast rate L a C o / C o L a sequences can be produced on one single jaw cycle. The progression towards a single cycle limits jaw motion and keeps it at a more comfortable duration for the fastest production rates. Thus, it probably limits the energy consumption.

In order to analyze the shift from two jaw cycles to a single one, the speaker's productions were classified into three periods (e.g. Figure 4). The "splitting" phase includes the utterances realized on a single jaw cycle. The "acceleration" and the "deceleration" phases include the utterances realized on two jaw cycles respectively before and after the splitting phase.

Patterns of jaw motion in the acceleration phase

The amplitude and duration of jaw opening and closing strokes in the acceleration phase are plotted on Figure 8, for each speaker, respectively for duplicated (left) and L a C o / C o L a (right) sequences. Strokes are seldom shorter than 50 ms, which agrees with the 100 ms threshold observed for the jaw cycle durations (Figure 7). Separate regression lines are superimposed for labial and coronal cycles. In their study of jaw motion for duplicated sequences (/sa/), [START_REF] Nelson | Mandibule movements during increasingly rapid articulations of single syllables : Preliminary observations[END_REF] used the same kind of display. A theoretical analysis allowed the authors to define a limit parabolic curve, such that for a given value of the maximum acceleration during a jaw stroke, the corresponding amplitude vs. duration points were always on the right of the limit parabolic curve. Without entering their explanation in too many details, the principle is basically that to achieve a given trajectory extent in a given duration, the acceleration along the trajectory must achieve a sufficient maximum value. Reciprocally, for a given maximum acceleration value and a given trajectory extent, duration cannot be lower than a limit, specified by the parabolic curve. On the figure, we have superimposed three such limit curves, respectively corresponding to maximum acceleration values of 0.25 g, 0.50 g and 1.5 g (see also [START_REF] Nelson | Physical principles for economies of skilled movements[END_REF]).

It appears that for duplicated utterances (Figure 8, left), speakers S3 and S4 follow a different pattern from speakers S1, S2 and S5. For S3 and S4, stroke amplitude decreases with duration for both the labials (in /papa/) and the coronals (in /tata/) cycles. Moreover, the distribution of amplitude vs. duration tends to follow the theoretical curve corresponding to an acceleration of 0.25 g. On the contrary, amplitude does not decrease with duration for S1, S2 and S5. Moreover, while S1 keeps a small amplitude for all durations, rate increase leads S2 and S5

beyond the 0.25 g limit curve, towards an acceleration up to 0.5 g. Therefore, to support rate increase, speakers either decrease the jaw stroke amplitude without increasing acceleration (S3 and S4) or increase acceleration without changing amplitude (S1, S2 and S5). These results agree with the study by [START_REF] Sonoda | Effetct of Speaking Rate on Jaw Movements in Vowel Sequence[END_REF]. These authors observed that jaw motion speeding induced either an increase in velocity and little change in amplitude or little velocity change and a decrease in amplitude (see also Lindblom, 1990a for a link with phonetic variations; and Perkell et al. 2002 for a more detailed discussion of intersubject differences).

----Figure 8 ----

The jaw behavior is more homogeneous for the L a C o / C o L a group. For all speakers, there is a global decrease of amplitude with duration (Figure 8, right) and the distributions more or less follow the 0.25 g curve. Furthermore, the means of amplitude and duration tend to be smaller for the strokes that subtend the labial constrictions as compared to the strokes that subtend the coronal constrictions, with significantly negative differences in a number of cases, and no significantly positive difference (Table IV). This pattern is modulated by two factors. Firstly, there are individual differences, with, for example, larger differences between the labial and coronal jaw strokes for speaker S3, and smaller for speaker S2. Secondly, the labial to coronal difference is increased for /pasa/ and /sapa/ logatoms (for which the labial jaw cycle is of a significantly lower amplitude and duration for all speakers) and decreased for /fata/ and /tafa/ logatoms (for which the difference is significant only for Speaker 3). This portrait suggests that in the acceleration phase, the passage from two jaw cycles to a single one rather corresponds to a progressive removal of the "labial cycle", as displayed in Figure 4. We shall now explore how the reorganization between jaw, tongue and lips occurs in the "splitting" phase.

----Table IV ----

Coordination between jaw and constrictors in the acceleration, splitting and deceleration phases

For most of the productions realized on two jaw cycles (during the acceleration and the deceleration phases), the TT and LL constrictions are around 0% of the jaw trajectory (Figure 9, top and bottom). This indicates that each constriction occurs when the mandible is at its highest position. So, TT and LL successively move in phase with the jaw. Yet, distributions look less peaked at 0% for TT (Figure 9, right top and bottom) than for LL (Figure 9, left top and bottom)

for both duplicated and L a C o / C o L a groups. Moreover, for L a C o and C o L a logatoms, there was some trend for more variations during the deceleration phase than during the acceleration phase.

This could be interpreted as a sign of hysteresis, considering the much more complex portrait in the splitting phase.

In the splitting phase, the labial and coronal constrictions must reorganize within one jaw cycle. The basic trend for reorganization is clearly displayed in Figure 9: the tongue constriction is around the highest jaw position (phase 0%), while the labial constriction is around the lowest position (phase 50% or -50%). On this basic pattern, a global shift towards the right of the jaw cycle can appear, with a coronal constriction in the beginning of the opening phase (around 25%)

and a labial constriction in the beginning of the closing phase (around -25%). Two factors seem to intervene in this possible shift. The first factor is speaker variability, with S2 more involved in the right shift than all other speakers. The second factor is the nature of the labial and coronal consonants. Thus, for /pasa/ and /sapa/, the coronal constriction is strictly located around the jaw highest position (0% phase) while for /fata/ and /tafa/ it is often shifted towards positive values.

The correspondence with the predominance of the coronal jaw cycle upon the labial one in terms of amplitude and duration, displayed in the previous section, is obvious. It is rather straightforward. Indeed, the coronal fricative /s/ induces a specifically high jaw position, generally higher than all other phonemes, including the coronal plosive /t/ or /d/ [START_REF] Keating | Comments on the jaw and syllable structure[END_REF]Lindblom, 1983;[START_REF] Keating | Variability in jaw height for segments in english and swedish VCVs[END_REF]Lindblad and Lundquist, 1999, see also data for French in Rhardisse and Abry, 1994). Hence the coronal jaw cycle is increased, and the coronal constriction is more strictly stuck on the highest jaw value. On the contrary, the labiodental /f/, also inducing a high jaw position, increases the labial jaw cycle, which induces the shift towards the right, with higher values of the labial constriction in the jaw trajectory. This global pattern is also modulated by the difference between L a C o and C o L a attractors.

Articulatory characterization of L a C o and C o L a attractors

L a C o utterances should be characterized by both a higher intensity of the second vowel over the first one (positive value of , Table III) and a proximity of the labial and next coronal constrictions ( (LaCo/LaLa) smaller than 50%, Table V). Notice however that data selection for these two analyses satisfy different criteria, that is utterance duration less than 300 ms for the acoustic analysis and utterances produced on a single jaw cycle for the articulatory analysis.

Despite these differences, the two analyses are relatively coherent.

Indeed, means of (LaCo/LaLa) tend to be higher than 50% for S1, which agrees with the C o L a profile that characterizes this speaker in the acoustic space. On the contrary, all (LaCo/LaLa) means for S2 are smaller than 50%, which confirms the speaker's tendency to be L a C o -stable. For S4, means of (LaCo/LaLa) are smaller than 50% for all L a C o sequences whereas they are greater than 50% for /sapa/ and /tafa/, which agrees with acoustic data showing that this speaker tends to be L a C o -stable for the L a C o logatoms and C o L a -stable for the C o L a ones. For S5, (LaCo/LaLa) means are smaller than 50% for all the sequences except /pasa/ and /sapa/, which confirms his acoustic profile: L a C o for /pata/, /tapa/, /fata/ and /tafa/ but rather C o L a for /sapa/. Finally, results

for S3 show that though this speaker does not often reach utterance durations shorter than 300 ms (Table III), she evolves towards a single jaw cycle for all sequences. Moreover, she tends to be L a C o -stable with (LaCo/LaLa) values always significantly smaller than 50% except for /fata/.

Therefore, quite logically the L a C o attractor corresponds to a decrease of the temporal distance between the labial and the next coronal constriction, while the C o L a attractor corresponds to a decrease of the temporal distance between the coronal and the next labial constriction.

However, there is a clear predominance of switches towards the L a C o attractor (e.g. /patá/ or /ptá/) over the 21 speakers of the acoustic study (Experiment 1). This agrees with the hypothesis according to which the anticipation of the tongue gesture during the labial constriction, typical of L a C o CVCV' (e.g. /patá) or CCV' (e.g. /ptá/) sequences, is much more likely than the anticipation of the labial gesture during the tongue constriction.

Discussion

The present work aimed at testing the hypothesis that the L a C o sequencing is a more stable articulatory pattern than the C o L a one. The results of Experiment 1 provide arguments for this hypothesis for French. Furthermore, the results of Experiment 2 allow to better describe the articulatory phenomena that subtend this asymmetry.

4.1. The L a C o / C o L a coordinative system "on line"

The results of the articulatory study confirm that rate increase could induce the repetition of L a C o and C o L a CVCV sequences to evolve from one jaw cycle per syllable towards a single jaw cycle for two syllables until a single CCV syllable is eventually achieved. This progression seems more economic regarding jaw motion. In this process, the determination of the attractor (L a C o vs. C o L a ) may result from the interaction of several factors.

The motor program shift

At the beginning of the repetition process, the speakers produce one jaw cycle per syllable. During this period, each constriction occurs at the highest jaw position. Then, with rate increase, the amplitude and the duration of the jaw strokes decrease together until reaching a threshold value (near 50 ms for an opening or a closing gesture). Coupled with the anticipation of one constriction during the previous one, this induces the shift on a single cycle. This shift rests on the reorganization of the tongue-lip-jaw coordination. Globally, two coordinative patterns are displayed to achieve the two constrictions on a single jaw cycle (Figure 10, right), depending on the initial profile of jaw motions (Figure 10, left). Firstly, and in most of the cases, the jaw cycle that carries the labial constriction becomes shorter and smaller than the jaw cycle that carries the coronal constriction (Figure 10, top left). Hence, when rate increases, the labial cycle reaches the threshold duration earlier than the coronal one. This induces the preservation of the phasing between the jaw and the tongue tip highest positions, whereas the lower lip constriction occurs during the jaw closing gesture or at the end of the opening gesture (Figure 10, top right). This is typically the case for Speaker 3, and for /pasa/ and /sapa/ utterances. Secondly, when no asymmetry appears between the two cycles during the acceleration phase (Figure 10, bottom left), the tendency is to observe a dephasing of both TT and LL in reference to the jaw cycle. Thus, the LL and TT constrictions respectively occur during the closing gesture and during the opening gesture (Figure 10, bottom right). This is the trend for Speaker S2 and for /fata/ and /tafa/ utterances.

These results are interesting in the context of previous articulatory studies [START_REF] Nelson | Mandibule movements during increasingly rapid articulations of single syllables : Preliminary observations[END_REF][START_REF] Kelso | A qualitative dynamic analysis of reiterant speech production: phase portraits, kinematics, and dynamic modeling[END_REF][START_REF] Hertrich | Lip-jaw and tongue-jaw coordination during rate-controlled syllable repetitions[END_REF], modeling works [START_REF] Redford | The relative perceptual distinctiveness of initial and final consonants in cvc syllables[END_REF][START_REF] Redford | Constrained emergence of universals and variation in syllable systems[END_REF][START_REF] Oudeyer | The origins of syllable systems: An operational model[END_REF] or theories [START_REF] Macneilage | The frame/content theory of evolution of speech production[END_REF] that always associated the jaw cycle to the CV syllable. Moreover, patterns of mandible motions are close to data published in the literature and tend to confirm the general principle of economy of effort.

Economy of effort

Various possible indicators of economy of effort have been proposed in the literature, e.g. maximum speed or jerk that is acceleration derivative [START_REF] Nelson | Physical principles for economies of skilled movements[END_REF]. In their already mentioned study, [START_REF] Nelson | Mandibule movements during increasingly rapid articulations of single syllables : Preliminary observations[END_REF] characterized jaw strokes by the maximum acceleration compatible with the observed movement, which is the indicator we used in Section 3. Furthermore, they showed that during the repetition of /sa/ with rate increase, the jaw motion progressed in a way that tended to minimize energy consumption. In their data, the durations of jaw opening and Furthermore, estimation of the maximal acceleration value in Figure 8 indicates that it is generally smaller for the L a C o and C o L a sequences compared to the duplicated ones. Taken together, these results suggest that when rate increases, the coordination of the two constrictions on a single jaw cycle might be more economic for the jaw than the alternation of two jaw cycles.

Multi-determinism of a bistable system

The results show that with rate increase, the lips-tongue-jaw system could evolve either towards a L a C o attractor (e.g. /pata/ /patá/ /ptá/) or a C o L a one (e.g. /pata/ /tapá/ /tpá/).

Thus, the coordinative system is bistable. Yet, according to the first study, the L a C o attractor prevails. On this basic pattern, a number of other additional factors are observed or could be assumed to play a role. Firstly, the initial coordination provides a bootstrap towards the corresponding attractor: the preference for the L a C o attractor is less marked for the repetition of C o L a CVCV sequences as compared to L a C o ones. This is particularly the case for /pasa/ and /sapa/, with /pasa/ more frequently evolving towards the L a C o attractor whereas /sapa/ often evolved towards the C o L a one. This is likely to be due to the fact that, in spite of the instruction to continue even if transformations occurred, some speakers tried to respect the initial structure and even tended to reduce the speed when they perceived a shift to the reverse sequencing. Secondly, the C o L a attractor was more often observed for /sapa/ as compared to /tapa/ and /tafa/. The difference between /sapa/ and the other C o L a disyllables could rest on articulatory factors. Indeed, analysis of the jaw cycles in the second experiment showed that in the acceleration phase, the asymmetry between the jaw cycles that come with the labial constriction and the jaw cycles that come with the coronal one are more systematic for /pasa/-/sapa/ than for the other sequences, which was interpreted in terms of the need for a specific high jaw position for achieving /s/. This could favor the progression towards the C o L a attractor for /pasa/ and /sapa/. Thirdly, inter-speaker differences, which could come from specific individual patterns of coordination, lead to differences in the selection of one or the other attractor, as displayed in Section 2.2.2.

But a number of other factors, not studied in the present work, could also intervene in the selection of one or the other attractor. An obvious one is the selected language. Indeed, it is likely that the CCV re-syllabification process observed here results from the fact that these forms are possible in French and that the language stresses the last syllable of a word. Therefore, future experiments should be done to assess the L a C o greater stability in languages with other prosodic behaviors or phonological constraints. Other additional experiments should also be done on the role of the vowel context, the consonantal mode (e.g. voiced or nasal consonants), etc.

Nevertheless, in spite of all these established or potential additional factors, the L a C o attractor is globally the dominant one over all speakers and disyllables in the present study, which suggests that it could indeed be the most stable and economic, at least in French.

Substance-based selection of phonological sequences

The underlying assumption of this work is that world languages inventories might be shaped by substance-based constraints such as properties of the motor control system. Thus, some sequencing of syllables might be preferred because they are easier to produce in the sense that they are more economic (less energy consuming) or more stable (easier to control). In this framework [START_REF] Liljencrants | Numerical simulations of vowel quality systems: the role of perceptual contrast[END_REF][START_REF] Lindblom | Self-organizing Processes and the Explanation of Languages Universals, in Explanations of Linguistic Universals[END_REF][START_REF] Kelso | Dynamic patterns: The self-organization of brain and behavior[END_REF][START_REF] Davis | Acquisition of serial complexity in speech production: A comparison of phonetic and phonological approaches to first word production[END_REF], the present study brings at least two interesting contributions: (1) the production of duplicated CVCV sequences may be more costly than the production of CVCV sequences that vary the constrictor from the first to the second consonant, and (2) the L a C o sequencing seems more stable than the reverse C o L a one.

Variegated rather than duplicated

Statistical analyzes of world languages lexical inventories show that in C 1 VC 2 V sequences, lexicons favor associations that vary the articulation places in C 1 and C 2 , rather than those that repeat the same constriction (MacNeilage and Davis, 2000aDavis, , 2000b;;[START_REF] Rousset | From lexical to syllabic organization: favored and disfavored cooccurrences[END_REF].

This means that languages prefer e.g. "pata" to "papa". Yet, duplication could, a priori, be considered as simpler in terms of control, which could explain that it is the core behavior in canonical babbling (MacNeilage and Davis, 2000aDavis, , 2000b)). However, the articulatory data obtained here lead to propose that the preference for variegated sequencing in world languages could result in part from the fact that the repetition of the same constriction is more costly for the jaw than the alternation of two different constrictions. Indeed, the comparison of the production of L a C o / C o L a and duplicated CVCV sequences shows that quicker production rates could be achieved for L a C o / C o L a sequences with less energy consumption for the jaw, since anticipation allows producing L a C o and C o L a CVCV utterances on a single jaw cycle whereas it is impossible for duplicated ones. Thus, without involving the extreme speeds obtained here at the end of the acceleration phase, it appears that the alternation of the movement of two constrictors is probably easier and more economic than the repetition of the movement of a single constrictor for the mature speech production system. Similar principles govern other synergies such as fingers in piano playing. This results from the fact that when two different organs are involved, the motion of one organ could be anticipated during the realization of the other one, which is not possible when a single organ realizes the two successive gestures. The possibility to produce two syllables on a single jaw cycle might be generalized to other kinds of sequencing than L a C o and C o L a ones in further investigations. Moreover, the preference for the L a C o attractor in the two experiments shows that inside the preference for variegated forms, some associations might be more appropriate than others.

L a C o rather than C o L a

The results of Experiment 1 are in favor of the idea that L a C o sequences appear as a natural and coherent production unit, more stable and better "in-phase" than their C o L a inverse counterpart. This could explain the preference for L a C o utterances in infant production, rather than the "simple first" explanation previously proposed. There is actually no reason to believe that the greater stability of L a C o sequences in the present experiments on adults could be due to any "simple first" mechanism. Rather, articulatory coherence could provide a common basis to both infant data and our results. Of course, the fact that the C o L a structure may also be locally stable, though less often selected in the speeding process, is not incompatible with this reasoning.

It just suggests that C o L a sequences are also viable, though suboptimal in comparison with L a C o ones, which agrees well with typological and developmental data.

Conclusion

The experimental data analyzed here point out a major difference between L a C o and C o L a sequences at the production level: L a C o sequences can be considered as a more stable coordinative pattern for the motor system than C o L a ones. Then, a causal relationship between this asymmetry and the universal "LC effect" has been advanced. Of course, such a claim has to be improved testing L a C o and C o L a sequences stability in both other languages and more directly in infant' first words. To this aim, the paradigm and the acoustic analysis method proposed here could allow comparing L a C o and C o L a stability for speaker samples of different languages, which is necessary for generalization. Specific methodologies should be developed for studying stability in infant speech. Moreover, further articulatory investigations of jaw-tongue-lip coordination in the production of C 1 VC 2 V disyllables would be necessary to better understand the anticipation processes, and their link with the asymmetry in jaw cycles for different constriction places in C 1 and C 2 . It remains that the present study adds some new material to the already significant set of perceptuo-motor constraints likely to play a role in the shaping of phonological systems, hopefully taking substance-based approaches to language universals one step further.

Acknowledgements

This work is part of the "Patipapa" project funded by the French Ministry of Research (Action Concertée Incitative "Systèmes Complexes en Sciences Humaines et Sociales"). We also thank C. Savariaux and C. Vilain for their technical support and all the speakers for their essential participation.

Notes

1 The Praat software is developed by P. Boersma and D. Weenink, at the Institute of Phonetic Sciences. It is available to free download on their web page (http://www.fon.hum.uva.nl/praat/). constriction events were displayed on the jaw cycle normalized between -50% and 50%

with -50% at the beginning of the up stroke, 0% at the jaw maxima and 50% at the end of the down stroke.

  to a one-cycle-per-disyllable relationship when speeding L a C o or C o L a CVCV logatoms. However, duplicated L a L a (/papa/) or C o C o (/tata/) CVCV logatoms were expected not to display this behavior. The second hypothesis concerns the lips-tongue coordination inside a single jaw cycle at high rates. Two attractors were expected, either L a C o or C o L a . In the first case, the labial constriction and the next coronal constriction (respectively maximum values of LL and TT trajectories) should be close together. In the second case, the coronal constriction should be close to the next labial one. Considering the small number of speakers, and the variability observed in the first experiment, no strong claim was made about a preference for L a C o attractors.

  global pattern of I against utterance duration for L a C o or C o L a sequences is very close to the pattern observed in the first experiment (compare Figure 6, right, with Figure 2). That is, I values are close to 0 for durations longer than 300 ms whereas I values far from 0 appear for shorter durations, with more positive than negative values. On the contrary, I values are always close to 0 for duplicated L a L a or C o C o logatoms (Figure 6, left).

  ----The utterance duration histograms (Figure 7, second row) display similar values for the duplicated and the L a C o / C o L a group. Yet, the durations of duplicated utterances are never shorter than 225 ms and are mostly around 300 ms whereas around 15 % of L a C o / C o L a productions reach durations from 175 ms to 225 ms with a peak of distribution around 250-275 ms. On the contrary, the jaw cycle durations (Figure 7, third row) are clearly shorter for the duplicated productions (peak of distribution around 150 ms) than for L a C o / C o L a ones (peak of distribution around 225 ms). Finally, the jaw cycle duration is never shorter than 100 ms for all logatoms.

  closing strokes were never shorter than 50 ms. Similar values appear in the present articulatory study. Then, for one of their three analyzed speakers, Nelson et al. observed "a maximum in the movement distance as the movement times decreased into the region of 125 ms". According to the authors, this "resonance" effect shows that 125 ms could be the most economic duration for up and down jaw strokes. It would correspond to the "natural" rate of 3-4 cycles and so, 3-4 syllables per second. In our study (Experiment 2), durations of jaw cycles for L a C o and C o L a utterances are mainly around 225 ms (Figure7), which corresponds to jaw stroke durations around 112.5 ms, close to the 125 ms value observed byNelson et al. On the other hand, the production of duplicated CVCV sequences at very fast rates requires to decrease the jaw cycle down to 150 ms and so, to stroke durations around 75 ms, close to the saturation value.

Figures 900 Figure 1 :

 9001 Figures

Figure 2 :

 2 Figure 2: Intensity variation between the vowel after the labial constriction and the vowel after the coronal constriction ( I) plotted against duration for /pasa/ (left) and /sapa/ (right) CVCV and CCV correctly produced utterances for all subjects (top). Results for /pasa/ and /sapa/ are representative of the two other L a C o (/pata/, /fata/) and C o L a (/tapa/, /tafa/) pairs. Bottom: Standard deviations of I against utterances durations (100 ms meantime) for the 6 CVCV sequences. Dispersion increases when duration becomes shorter than 300 ms.

Figure 3 :

 3 Figure 3: Attractor stability and speakers' profiles. On the left: I mean (variation of

Figure 4 :

 4 Figure 4: Acoustic signal, jaw, Tongue Tip (TT) and Lower Lip (LL) displacements against

Figure 5 :

 5 Figure 5: Articulatory measurements. A: duration and amplitude of the jaw cycle are

Figure 6 :

 6 Figure 6: Intensity variation between the vowel after the labial constriction and the vowel after the coronal constriction ( I) against utterance duration for all subjects' duplicated (left) and L a C o -C o L a (right) productions. I does not vary much with duration for duplicated logatoms whereas duration around 300 ms (vertical doted line) appears as a boundary for the range of possible I values for L a C o and C o L a logatoms (see text).

Figure 7 :

 7 Figure 7: First row: Jaw cycle durations against utterance durations for all speakers'

Figure 8 :

 8 Figure 8: Jaw strokes amplitude (up and down) against duration during the acceleration

Figure 9 :

 9 Figure 9: Distribution of speakers' productions according to the position of the Lower-Lip

Figure 10 :

 10 Figure 10: Coordination between the jaw and the constrictors from two jaw cycles for a

  In this first hypothesis, L a C o CVCV sequences would evolve toward a L a C o attractor (e.g. /pata/ /patá/

/ptá/) while C o L a CVCV sequences would evolve toward a C o L a CCV attractor (e.g. /tapa/) /tapá/ /tpá/). Yet, if, as we expect, the L a C o coordination is more stable than the C o L a one, this would attract the system towards a L a C o attractor regardless of the initial CVCV sequence (e.g. (/pata/ or /tapa/) /patá/ /ptá/). Hence, in this second hypothesis, the repetition with rate increase of both L a C o and C o L a CVCV disyllables would evolve towards a L a C o attractor. The first experiment, based on acoustic measurements, aimed at selecting which of these two assumptions is true.

Tables 807 Table I :

 807I Analysis of intensity variations between the vowel after the labial and the La and V Co( I)...................................................................................................................................... 49 CVCV sequences. Dispersion increases when duration becomes shorter than 300 ms................................... 50 Figure 3: Attractor stability and speakers' profiles. On the left: I mean (variation of intensity between the vowel after the labial consonant and the vowel after the coronal consonant) of the second trial against I mean of the first trial for each CVCV and each speaker when the two means significantly differ from zero (two-tails Student test with From left to right: sample of the acceleration phase; sample of the period with one jaw cycle for two syllables (splitting) and sample of the deceleration phase. The arrows on trajectory curves represent the labeled minima and maxima for the jaw and the maxima for the constrictors. Notice the secondary maxima for TT and LL due to passive motion induced by the jaw gesture that comes with the other constriction (these secondary maxima are not incorporated in further analyses in Experiment 2). ............................................................................................. 52 Figure 5: Articulatory measurements. A: duration and amplitude of the jaw cycle are respectively the sum of its up and down stroke durations and the mean of its up and down stroke amplitudes. B: position of the Tongue Tip (P(TT)) boundary for the range of possible I values for L a C o and C o L a logatoms (see text). ............................. 54 Figure 7: First row: Jaw cycle durations against utterance durations for all speakers' productions and separately for duplicated (left) and L a C o / C o L a sequences (right). Distribution splitting observed for the L a C o and C o L a groupsshows that these disyllables could be produced on a single jaw cycle whereas duplicated disyllables are always realized on two jaw cycles. Second and third rows: distribution of utterance and jaw cycle durations for all speakers' observed when the two plosives are realized on a single jaw cycle(splitting, right). They tend to correspond to specific jaw motion profiles during the acceleration phase(left, see text for detail). .................................................... 58

	Tableau III: Means (m) of intensity variation between the vowel after the coronal Tableau IV: Means of amplitudes and durations for the jaw cycles that come with Tableau V: Labial-to-coronal duration means (m) normalized by the whole List of figures
	vowel after the coronal consonant for utterances shorter than 300 ms for the 42 analyzed trials (see text for the details about data selection). For each disyllable: means (dB) and are compared with 0 according to a two-tails t-test (df = 41) with Bonferroni correction (* p .01/6); standard errors; repartition of the 42 means according to their sign: for all the means and for the means that significantly differ from 0 according to a two-tails t-test with Bonferroni correction (p .05/42). pata tapa pasa sapa fata tafa Means 6.6 * 7.0 * 7.5 * 3.8 8.9 * 8.6 * Standard errors 1.9 1.9 1.7 1.7 1.7 2.1 All 30 30 27 21 37 33 Number of positive means Signif. 19 15 18 13 26 21 All 12 12 15 21 5 9 Number of negative means Signif. 4 6 6 9 2 4 and the vowel after the labial consonant for utterances shorter than 300 ms for the three trials taken together, for each speaker and each disyllable (n is the number of observations for m computation). Thirty t-tests (df = n-1) with Bonferroni correction compare each mean to 0 (° p .005, * p .05/30, ** p .01/30). pata tapa pasa sapa fata tafa n m n m n m n m n m n m 31 0.8 29 -0.1 29 -1.4* 27 -2.9** 23 -0.2 24 -0.7 25 23.2** 11 9.2 25 11.7* 20 5.5 30 20.3** 20 13.9** the labial and the coronal constrictions in the acceleration phase, for each speaker and each L a C o / C o L a pair (n is the number of observations for means computations). The significance of the difference between means was tested using t-tests (df = n-1) with Bonferroni corrections (** p .01/15, * p .05/15, NS : Not Significant). Amplitudes (mm) Durations (ms) Speaker Sequence n labial coronal p labial coronal p s1 pata/tapa 47 0,58 0,91 ** 163,6 198,6 ** pasa/sapa 38 0,59 2,06 ** 162,7 247,2 ** fata/tafa 23 0,98 0,77 NS 272,6 222,1 NS s2 pata/tapa 21 1,83 1,57 NS 231,9 244,0 NS pasa/sapa 28 1,07 1,48 ** 209,1 285,2 ** fata/tafa 23 1,00 0,83 NS 263,7 235,7 NS s3 pata/tapa 59 1,17 2,75 ** 178,3 229,7 ** pasa/sapa 55 1,01 3,46 ** 169,9 269,5 ** fata/tafa 54 2,02 2,36 ** 211,2 219,5 NS s4 pata/tapa 23 1,16 2,67 ** 174,7 247,0 ** pasa/sapa 18 1,20 3,89 ** 206,1 279,2 NS fata/tafa 33 1,57 1,34 NS 223,3 214,6 NS s5 pata/tapa 35 0,85 2,53 ** 204,7 268,0 ** pasa/sapa 14 1,08 3,78 ** 196,0 370,7 ** fata/tafa 52 1,43 1,66 NS 267,8 247,0 NS utterance duration (L a C o /L a L a ) , expressed in percent, for each speaker and each disyllable (n is the number of utterances involved in computing m). Values smaller than 50% indicate that the duration from the labial constriction to the following coronal one is smaller than the duration from the coronal constriction to the following labial one, which signals an evolution towards the L a C o attractor. On the contrary, values higher than 50% signal an evolution towards the C o L a attractor. Thirty two-tails t-tests (df = n-1) with Bonferroni correction compare each mean to 50 (* p <= .05/30, ** p <= .01/30). pata tapa pasa sapa fata tafa n m n m n m n m n m n m S1 S1 S2 20 49,3 5 53,0 27 50,8 26 51,0 31 53,7** 32 52,0 S2 25 36,3** 21 40,3* 20 41,1 22 43,9 40 30,5** 36 36,1** S3 19 44,3* 11 38,6** 22 41,5** 17 42,5** 10 44,8 23 43,7** S4 56 45,4* 48 49,5 60 46,3** 44 52,0 38 42,8** 30 51,4 S5 67 42,1** 58 40,8** 67 51,8 71 53,5* 35 37,9** 41 44,9* Figure 1: Labeling of intensity curves and measurements: intensity curves (bottom) of the acoustic signal (top) against time for three samples of a /tapa/ trial. The repetition progressively evolves from /tapa/ (left) towards /tápa/ (middle) until /ptá/ (right). L a and C o are the labial and coronal consonants; V La and V Co are the vowels respectively following L a and C o . This labeling allows computing the duration of an utterance and the intensity variation of intensity between V Figure 2: Intensity variation between the vowel after the labial constriction and the vowel after the coronal constriction ( I) plotted against duration for /pasa/ (left) and /sapa/ (right) CVCV and CCV correctly produced utterances for all subjects (top). Results for /pasa/ and /sapa/ are representative of the two other L a C o (/pata/, /fata/) and C o L a (/tapa/, /tafa/) pairs. Bottom: Standard deviations of I against utterances durations (100 ms meantime) for the 6 Bonferroni correction p .05/42). Points inside the top left and the bottom right squares correspond to cases with attractor shift from one trial to the other. Points inside the top right and bottom left squares respectively represent L a C o and C o L a stability for the two trials. On the right: I means for the two selected trials for each CVCV of four typical speakers with S20 and S6 as bi-stable profiles; S12 as L a C o stable and S23 as C o L a stable. The convex hulls of S23, S20 and S12 are displayed on the figure on the left (* : mean significantly different from zero, p .05/42). .... 51 Figure 4: Acoustic signal, jaw, Tongue Tip (TT) and Lower Lip (LL) displacements against time for samples of a 50% with -50% at the beginning of the up stroke, 0% at the jaw maxima and 50% at the end of the down stroke. .... 53 Figure 6: Intensity variation between the vowel after the labial constriction and the vowel after the coronal constriction ( I) against utterance duration for all subjects' duplicated (left) and L a C o -C o L a (right) productions. I does not vary much with duration for duplicated logatoms whereas duration around 300 ms (vertical doted line) appears as a Figure 9: Distribution of speakers' productions according to the position of the Lower-Lip (left) and the Tongue-Tip /tapa/ trial (S3). and of the Lower Lip (P(LL)); constriction events were displayed on the jaw cycle normalized between -50% and (right) constrictions relative to the jaw cycle. The three phases (acceleration, splitting and deceleration) are plotted
	separately. For the acceleration and the deceleration phases, histograms include all speakers taken together and
	both duplicated and L a C o / C o L a sequences. For the splitting phase (when utterances are realized on a single jaw
	cycle), distributions are given for each L a C o and C o L a sequence and for each speaker. ................................................ 57
	Figure 10: Coordination between the jaw and the constrictors from two jaw cycles for a disyllable to a single one.

productions. The utterance durations tend to be shorter for the L a C o / C o L a group as compared to the duplicated one whereas the contrary tends to appear for the jaw cycle

durations. ................................................................................. 55

Figure

8

: Jaw strokes amplitude (up and down) against duration during the acceleration phase (when the disyllables are realized with two jaw cycles) for each speaker and separately for duplicated (left column) and L a C o / C o L a (right column) disyllables. The parabolic curves are the theoretical minimum-time boundaries for mandible movements for three control acceleration limits (from left to right: 1.5 g, 0.5 g and 0.25 g, (see

[START_REF] Nelson | Physical principles for economies of skilled movements[END_REF]

). Regression lines and distribution means (squares and circles) are displayed separately for jaw strokes that come with labials and with coronals (see text for details)..

............................................................................................................................................ 56

Two patterns (A and B) of lower lip (LL) and tongue tip (TT) constriction positions relative to the jaw cycle are

Rochet-Capellan & Schwartz: An articulatory basis for the Labial-to-Coronal effect

Tableau II: Number of CVCV or CCV utterances correctly produced (Tot.)


Amélie Rochet-Capellan
email: amelie.rochet-capellan@icp.inpg.fr

Two CV syllables for one pointing gesture as an optimal ratio for jaw-arm coordination in a deictic task: a preliminary study

, the association of oral and brachiomanual gestures in the deictic function could be the root of the first words emergence in both ontogeny and phylogeny. This association may require a close coordination between the two gestural systems, possibly anchored in a 2:1 harmonic ratio between the natural oscillatory frequencies of the Speech frame (open-close jaw cycle) and the Sign frame (armhand-finger pointing cycle). This could explain why both languages and first words favor bi-syllabic forms (Ducey-Kaufman et al., 2005). The present study used a new paradigm to test this 2:1 ratio in adult on-line productions. The results provide first evidence that supports the 2:1 ratio and suggest new arguments for a substancebased approach of language evolution.

Introduction

Language and Action

In the last 25 years, language and action have been more and more considered as connected systems. A major evidence for this assumption is the coordination of speech and gestures in on-line face-to-face interactions [START_REF] Mcneill | Action, thought and language[END_REF]. Moreover, this coordination appears to be motor rather than purely perceptual. For example, gestures are as well involved in linguistic communication between blind people [START_REF] Iverson | Why people gesture when they speak[END_REF]. In addition, the link begins early in development, inside the corporal "babbling" of speech and hands [START_REF] Iverson | Hand, mouth, and brain: The dynamic emergence of speech and gesture[END_REF] and then with the association of words and pointing gestures in the passage from one-to two-words production [START_REF] Pizzuto | Gesturalvocal deixis and representational skills in early language development[END_REF][START_REF] Volterra | Gesture and the emergence and development of language[END_REF]. The neuroimagery data also lead to a cortical neuroanatomy in which language, perception and action share a same temporo-parieto-frontal circuit (Pulvermüller, 2005, for a review). The Broca's area itself, considered as a "pure language area" for a long time, has been shown to be involved in perception, action understanding and imitation [START_REF] Nishitani | Broca's region: from action to language[END_REF], for a review). Moreover, overlapping brain areas subtend oral and sign languages (Emmorey et al, 2002[START_REF] Macsweeney | Neural systems underlying British Sign Language sentence comprehension[END_REF] leading to suspect that some characteristics of language are modality-free (San José-Robertson et al., 2004). All this background favors the assumption of a close link between orofacial and brachiomanual actions in the emergence of language.

Deriving language from gestures [START_REF] Corballis | From mouth to hand : Gesture, speech and the evolution of right-handedness[END_REF] proposed that language could have first emerged as a manual communication system and then, progressively evolved towards mouth and speech in the course of phylogeny. On the contrary, the "Frame then Content" (FC) theory [START_REF] Macneilage | The frame/content theory of evolution of speech production[END_REF] links the emergence of speech and language to the orofacial motor control system, considering ingestive mechanisms as a primary step towards oral communication. This theory further relates universals in adult languages and infant babbling to an evolutionary-developmental scenario [START_REF] Macneilage | On the origins of internal structure of word forms[END_REF] in which jaw motor cyclicities (the frame) would have been primary. Then the independent and coordinated control of the tongue and the lips would have been mastered (the content).

In this "frame-then-content" sequence experimentally displayed in the course of ontogeny [START_REF] Munhall | Articulatory evidence for syllabic structure[END_REF][START_REF] Green | The sequential development of jaw and lip control for speech[END_REF], the jaw is considered as the carrier of speech gestures. This "frame dominance" would explain the preference for Consonant-Vowel syllable forms in both human languages and infant babbling. However, these "mono-modal" scenarios about language origins gave rise to criticisms [START_REF] Abry | Introduction: Vocalize to Localize? A call for better crosstalk between auditory and visual communication systems researchers: From meerkats to humans[END_REF][START_REF] Arbib | Interweaving protosign and protospeech: Further developments beyond mirror[END_REF] that let appear a consensus: the two motor systems might have in fact evolved conjointly towards an elaborated communication system involving both brachiomanual and orofacial gestures. Coming back to McNeill, the evolutionary process would have selected the capacity to associate speech and gestures in a coherent communicative process.

Deriving language from deixis

In this evolutionary process, the deictic function might have played a pivotal role. Indeed, the deictic gesture has been considered as the primary indexical sign in both phylogeny and ontogeny [START_REF] Haviland | Pointing, gesture spaces, and mental maps[END_REF]. Furthermore, according to [START_REF] Abry | Introduction: Vocalize to Localize? A call for better crosstalk between auditory and visual communication systems researchers: From meerkats to humans[END_REF] the association of voice and hand in a deictic function is a key step of both the phylogenetic and ontogenetic development of language. They assumed that language would provide a new deictic tool enabling humans to "vocalize to localize". In this "Vocalize to Localize" framework, the baby's -and by extend, the humanity's -first words would be the product of a developmental "rendez-vous" between the motor control mastery of arm-hand-finger pointing (the Sign Frame) and the jaw opening-closing gestures that subtend babbling (the Speech Frame, referring to the FC theory). This "rendez-vous" requires the coordination of the two systems, constrained and shaped by the physical and motor properties of the speech and the arm-hand systems. Indeed, [START_REF] Ducey-Kaufmann | When the Speech Frame meets the Sign Frame in a developmental framework[END_REF] displayed a 2:1 harmonic ratio between the Speech Frame and the Sign Frame for 6 French children between 6-and 18months old. In other words, the babies tend to utter two CV syllables (associated to two jaw cycles) inside one pointing gesture. According to Ducey-Kaufmann et al., since the first words would emerge from the rendez-vous between the Speech Frame and the Sign Frame, the 2:1 ratio could explain why both infants' first words and human languages favor twosyllables words [START_REF] Rousset | Structures syllabiques et lexicales des langues du monde. Données typologiques, tendances universelles et contraintes substantielles[END_REF]. In this "Vocalize to Localize" framework, the goal of this paper is to provide a new experimental evidence for the 2:1 ratio in adult deictic tasks.

Evaluating the 2:1 hypothesis in adults

Previous studies showed a coordination between speech and pointing gestures, mainly resulting from a speech adaptation [START_REF] Levelt | Pointing and voicing in deictic expressions[END_REF]Feyreisen, 1997). Other studies displayed preferential synergies between speech and finger taping motion [START_REF] Kelso | A "Dynamic Pattern" perspective on the control and coordination of movement[END_REF][START_REF] Treffner | Intentional and attentional dynamics of speech-hand coordination[END_REF]. Jaw preferential oscillatory frequency has also been estimated through dynamics studies [START_REF] Nelson | Mandibule movements during increasingly rapid articulations of single syllables : Preliminary observations[END_REF]. Yet, no study has tried to establish the favored ratio between the Speech Frame and the Sign Frame in adult. Here, we propose an original experimental paradigm testing the 2:1 hypothesis for arm-jaw coupling in a deictic task involving utterances with 1, 2, 3 or 4 CV syllables. The basic assumption is that at most two jaw cycles can be contained inside one pointing cycle. Considering that one CV syllable requires one jaw cycle, the pointing cycle should stay constant for 1-and 2-CV syllables utterances. It should then increase for 3-syllables and remain the same for 3and 4-syllables utterances. This portrait (1=2<3=4) is the focus of the present experiment. In order to measure the period of the arm-finger pointing cycle while avoiding rhythmic tasks and keeping a relatively natural deictic gesture, the task was to show a target while naming it (with 1-, 2-, 3-or 4-CV syllable(s) logatoms) twice in rapid succession.

Method Procedure

The subjects were 9 native Brazilian female speakers, all right-handed and without any speech or hearing problems. As in the princeps study by [START_REF] Levelt | Pointing and voicing in deictic expressions[END_REF], the experiment involved a gesture + speech pointing task. The subject was seated at a table. A target (red smiley icon) together with the logatom to pronounce were projected in front of her on a board during 2.5 sec +/-a random delay (with a 1-sec mean and a 0.15-sec standard deviation). The target appeared in the right visual field, either at a near or a far position. The logatom was projected at the same time in the middle of the visual field, as displayed on Figure 1. The logatom could be either /pa/, /papa/, /papapa/ or /papapapa/. It was introduced as a person's name and the target-smiley as a symbolic representation of that person. The instruction was to name and show "the person" two successive times as soon as the icon color changed (go-signal). The subject was invited to put her finger on a black square mark on the table before and after each trial. For, example, for a /pa/ item, the speaker showed the target a first time saying /pa/, put her finger back on the black square and immediately showed the target again saying /pa/. As some subjects tended to confuse /papapa/ with /papapapa/, the experimenter read the logatom aloud in order to remove the ambiguity. This methodology problem will be discussed here after. Before starting, the subject was asked to show objects in the room while naming them in order to link the task with real-world pointing situations. The experimental phase was divided into four blocks separated by a 30-sec pause. Each block started with 4 practice trials followed by 40 experimental ones, five for each of the eight experimental conditions (number of syllables) * (target position). The order of the trials was differently randomized for each block and each speaker.

Data recording

Jaw and hand motions were recorded using an optotrack system at a 100 Hz sample frequency. Two sensors were pasted on the subject's right forefinger, one on the middle of the nail and the other on the left part next to the nail. This allowed keeping the finger in the visible optotrack field when the finger turned towards the right during the pointing gesture. A third sensor was pasted between the top of the chin Figure 1: Experimental apparatus (inspired from [START_REF] Levelt | Pointing and voicing in deictic expressions[END_REF]) and the lower lip on the speaker's face. It tracked a flesh point rather than the jaw itself, but in light of the phonetic material used here, with only opening-closing movements between a stop labial consonant and an open /a/, this sensor was considered as a good indicator of jaw motion. For simplification, it is now referred as the jaw sensor. Three fixed sensors on the table and three others maintained on the subject's head provided two referentials, respectively for the finger and jaw moving sensors. The sound was recorded at a 16 kHz sampling frequency with a computer connected to a microphone. Synchronization between acoustic and optotrack signals was achieved by a beep generated on the second sound channel when the optotrack record started.

Figure 2: Acoustic signal (top) and trajectories of jaw (middle) and finger (bottom) for a /papapa/ trial. The points marked on the trajectories curves are the initiations and the arrivals of motions labeled for the jaw (J I1 , J A1 , J I2 and J A2 ) and for the finger (P I1 , P A1 , P I2 and P A2 ), see text for detail.

Data processing

After the experiment, each trial was checked in order to detect speech errors (e.g. non respect of the required number of syllables) and gesture errors (e.g. departure before the go signal). Then, finger, head and jaw sensors coordinates were projected into the table referential and then, jaw coordinates into the head referential. The trajectory of the three sensors against time was estimated running a Principal Component Analysis on x-y-z sensors coordinates. The first component explained most of the variance for all subjects (from 92% for the jaw to 98% for both finger sensors) and so, was used as the estimator of sensors trajectories against time. These signals were lowpass filtered by a Butterworth filter with a cutting frequency at 15 Hz. On these valid trials signals, onset and offset events were positioned on finger and jaw trajectories (respectively for speed increasing above or decreasing under a threshold, set at 10% of the maximum speed on the corresponding stroke). Thus, P I1 , P I2 and P A1 , P A2 are, respectively, the initiation (onset) and the apex (offset) times from the go signal for the first and the second finger pointing gesture (Figure 2, bottom). Similarly, J I1 and J I2 are the initiation (onset) events of the jaw opening motion for the first syllable of the two utterances. Finally, J A1 and J A2 are the apex (offset) events of the jaw opening gesture for the last syllable of the two utterances (Figure 2, middle). Automatic labels were checked in order to correct errors and to detect trials for which one sensor was partially hidden in the optotrack. For the finger, the rule was to take the trajectory of the left sensor if the middle one was masked (the mean correlations between the two finger sensors were above .99 for each of the x-y-z coordinates).

Experimental design and hypothesis

The experiment manipulated two within-subject factors: the number of syllables (1 vs. 2 vs. 3 vs. 4) and the target position (near vs. far). As explained in the introduction, a (1=2<3=4) portrait should be observed for the period of pointing cycle. This period was computed for each trial as the duration between the apex of the two finger gestures. It will be referred as P T :

P T = P A2 -P A1 Hence, the main hypothesis was that P T should stay stable from the 1-to the 2-syllable(s) condition. It should then increase from the 2-to the 3-syllables condition and stay stable from the 3-to the 4-syllables condition. Two main questions would remain regarding the (1=2<3=4) portrait:

(1) If actually displayed, does it resists to the increase of the pointing motion amplitude from the near-to the far-target position? (2) How is it achieved considering the different phases of pointing motion, that are the onset-to-apex duration and the post-apex phase? In order to give first element of answer to this question, the onset-to-apex pointing durations P D1 and P D2 were computed respectively for the first and second pointing gestures: P D1 = P A1 -P I1 P D2 = P A2 -P I2 These durations were compared with the total jaw motion duration necessary to produce the utterance, referred as J D1 and J D2 respectively for the first and second utterances:

J D1 = J A1 -J I1 J D2 = J A2 -J I2

Results

Factor effects on dependent variables were tested using within-subject ANOVAs. As the number of syllables has more than two levels, sphericity was systematically tested (indicated only when it could not be assumed). Comparisons between 1-and 2-(C1), 2-and 3-(C2) and 3-and 4-(C3) syllables conditions were achieved using paired t-tests with Dunn-Sidák alpha-level adjustment in the case of a priori comparisons and with Bonferroni corrections in the case of post-hoc comparisons. Non-significant tests correspond to pvalue greater than .05.

Apex-to-apex pointing duration (P T )

Figure 3 displays P T means and standard deviations for each experimental condition. Mauchly' sphericity test being significant for the number of syllables (Greenhouse-Geisser: Epsilon = .47. p < .05), ddl and p-values are given after Greenhouse-Geisser correction (which explains the noninteger values given for ddl). P T mean is 961 ms for both 1and 2-syllable(s) conditions while it increases to 1025 ms for the 3-and to 1056 ms for the 4-syllables conditions (F(1.4, 11.2) = 9.3, p < .01). Results of the three planned comparisons are: t(8) = 0.05, p = .96, for C1; t(8) = 3.1, p = 0.015, for C2 and t(8) = 2.5, p = .04, for C3. The Dunn-Sidák method shows that C3 is significant (p < (1 -(1 -0.05) 1/3 )) but not C2 (p > (1 -(1 -0.05) 1/2 )). This agrees with the (1=2<3=4) expected portrait. In addition, P T is significantly longer in the far-(1022 ms) than in the near-(979 ms) target condition (F(1, 8) = 13.9, p < .01). Yet, the interaction with the number of syllables effect is not significant (F(2, 15.8) = 1.7). Hence, the +43 ms increase from the near to the far-target condition might be too small to affect the (1=2<3=4) clustering. The analysis will now investigate if this portrait is also observed for the onset-toapex pointing duration.

Comparison of jaw (J D ) and pointing (P D ) durations

Figure 5 displays the means of jaw motions durations (J D ) and standard deviations for each experimental condition and respectively for the first (J D1 ) and the second (J D2 ) utterance.

As for P D , the utterance (first vs. second) was introduced as a third factor for the ANOVA. The results show that jaw motion duration increases with the number of syllables. From the 1-to the 4-conditions, J D is, respectively, 187, 390, 522 and 637 ms (F(3, 24) = 566, p < .0001). C1 (+203 ms), C2 (+132 ms), and C3 (+115 ms) are all significant ((t8) > 16, p < .0001). On the contrary, the target effect is not significant (near: 432 ms, far: 436 ms, F(1, 8) = 3.6).

Figure 5: Means and standard deviations of jaw motion duration (J D , see text for detail) according to the number of syllables and to the target position for the first (J D1 , left) and the second (J D2 , right) utterances.

The global comparison of J D with P D shows that the 365 ms mean observed for P D is closer to the 390 ms mean observed for J D in the 2-syllables condition than in the three other syllables conditions. The analysis of (J D -P D ) shows that the mean of the (J D -P D ) difference is -169, 34, 153 and 261 ms respectively from the 1-to the 4-syllable(s) conditions. These values significantly differ from zero at post-hoc ttests (Bonferroni Correction) for the 1-, the 3-and the 4-syllables condition (t(8) > 9, p BF < .0001) but not for the 2syllables one (t(8) = 2.2). Hence, the trend is that the duration of the pointing gesture corresponds rather closely with the duration of a sequence of two jaw cycles necessary for uttering a two-syllables component.

Discussion

Overall, the present findings agree with the 2:1 hypothesis. However, their interpretation has to be discussed in regard to a possible methodological problem: the eventuality of a higher processing load for /papapa/ and /papapapa/ than for /pa/ and /papa/.

Processing load

It could be suspected that the 1-2 vs. 3-4 clustering observed for P T is due, at least partly, to a processing load higher for the motor programming of 3-vs. 4-syllables utterances. Indeed, the visual presentation of the items on the screen induced a trend to produce much confusion between these two kinds of sequences, while it was not the case for 1-and 2syllable(s). In order to solve this problem, the experimenter gave a help to the subject by reading aloud the logatom, which avoided utterance errors. The impact of this intervention was reduced by the fact that the task is an "off- line" one: the subject waits for the go-signal to answer (e.g. [START_REF] Levelt | Pointing and voicing in deictic expressions[END_REF]). However, this could have resulted in artificially clustering pointing durations at a high value, similar for the two kinds of sequences. If this was the case, the onset of pointing (P I1 ) and jaw (J I1 ) motions for the 3-4syllables conditions might occur later than for the 1-2syllable(s) conditions.

Figure 6: Means of elapsed time from the go signal to the pointing onset (P I1 ) and to the onset of jaw opening gesture for the first vowel (J I1 ) according to the number of syllables and to the target position.

Figure 6 displays the means of P I1 and J I1 according to the target position and to the number of syllables. The P I1 mean is 348, 408, 418 and 408 ms, respectively for the 1-, 2-, 3-and 4-syllables conditions. A 2-within-factors ANOVA shows that the effect of the number of syllables is significant (F(3, 24) = 7.6, p < 0.001) while the target effect is not significant (F(1, 8) = 0.3). Moreover C1 is significant (t(8) = 3.1, p BF < 0.05) while neither C2 (t(8) = 0.7) nor C3 (t(8) = 0.8) are significant. Similarly, the number of syllables significantly affects J I1 (F(3, 24) = 10.6, p < 0.001) while it is not the case for the target position (F(1, 8) = 1.4). J I1 tends to decrease with the increase of the number of syllables: 588, 542, 516 and 471 ms, respectively for the 1-, 2-, 3-and 4syllables conditions. However, C1 (t(8) = 1.5), C2 (t(8) = 1.3) and C3 (t(8) = 2.8) are not significant (p BF > .05). Altogether, these patterns are not in favor of the "processing load" effect.

Arguments for the 2:1 hypothesis

The present study provides interesting results for the 2:1 hypothesis assumed in the Vocalize-to-Localize framework. Firstly, the oscillatory period of pointing motion is the same for 1-and 2-syllable(s) while it increases for 3-and 4syllables. On the contrary, the onset-to-apex pointing duration is rather stable for a given target position. Yet, in agreement with the 2:1 ratio, the duration of this gesture corresponds rather well with the total duration of jaw motions for the realization of two CV syllables utterances. Hence, the (1=2<3=4) clustering observed for the whole pointing cycle may mainly result from the arm-hand-finger waiting for the jaw during the post-apex period in the 3-and 4-syllables conditions. In addition, because of the lack of gesture-alone condition in this preliminary experiment, we do not know if for the 1-and 2-syllables conditions, the arm-hand-finger system keeps its natural frequency. Nevertheless, previous data by [START_REF] Levelt | Pointing and voicing in deictic expressions[END_REF] suggest that the duration of finger pointing motion with a 1-syllable utterance is close to the duration in a gesture-alone condition. Hence, two syllables might be the maximum number of syllables that could be realized on one finger pointing motion without affecting the duration of the pointing period. The lack of difference between the 3-and 4-syllables conditions and the proximity of values of jaw and pointing motion durations in the 2syllables condition strengthen this assumption.

The 2:1 ratio as a new piece in language embodiment theories Altogether, this preliminary experiment supports the assumption that the 2:1 ratio between speech and pointing observed in developmental studies [START_REF] Ducey-Kaufmann | When the Speech Frame meets the Sign Frame in a developmental framework[END_REF] also tends to appear for adult productions. This adds new evidence for a close motor link between speech and hand gestures. Moreover, considering the importance of deixis in language acquisition and especially in first-words emergence and vocabulary expansion, it seems not so "astonishing" [START_REF] Ducey-Kaufmann | When the Speech Frame meets the Sign Frame in a developmental framework[END_REF] to propose that this relationship between the durations of pointing and jaw gestures could have played a role in the course of phylogeny and may still play a role in the preference for bi-syllabic forms both in infants' first words and adults' lexicons [START_REF] Rousset | Structures syllabiques et lexicales des langues du monde. Données typologiques, tendances universelles et contraintes substantielles[END_REF]. Hence, we suggest that the 2:1 ratio assumption should be introduced as a new piece in computational models that attempt to derive phonology from substantial constraints [START_REF] Lindblom | Explaining Phonetic Variation, A Sketch of the H H Theory[END_REF][START_REF] Steels | Evolving grounded communication for robots[END_REF][START_REF] Schwartz | Linking the Dispersion-Focalization Theory (DFT) and the Maximum Utilization of the Available Distinctive Features (MUAF) principle in a Perception-for-Action-Control Theory (PACT)[END_REF]. This integration might provide a basis for explaining the preference for bi-syllabic forms in human languages.

Conclusion

The present study leads to assume that two jaw cycles is the maximum number of cycles that could be realized inside a pointing cycle without affecting the pointing duration.

Moreover, it provides a new paradigm for the investigation of the coordination between the Speech frame and the Sign frame. Of course, more investigations are needed, particularly concerning the lack of pure vocal and gestural conditions, and further assessment of the processing load problem. This is why this study is presented as a preliminary investigation. In any case, further investigations about coordination between articulatory motion and hand gestures should provide new elements for a better understanding of the relationships between oral and sign communication, and of the real nature of human language.

Figure 3 :

 3 Figure 3: Means and standard deviations of the elapsed time between the apex of the two pointing gestures (P T ) according to the number of syllables and to the target position. Onset-to-apex pointing duration (P D ) Figure 4 displays P D means and standard deviations for each experimental condition and respectively for the first (P D1 ) and the second (P D2 ) gesture. In the analysis, the gesture (first vs. second) was introduced as a third factor in a 3-within factors ANOVA (number of syllables * target position * gesture). The results show that durations for the first (364 ms) and the second (365 ms) gestures are very close to each other and do not significantly differ (F(1, 8) = 0.02). Then, gesture durations tend to be longer in the far-(P D1 = 375 ms, P D2 = 379 ms) than in the near-target condition (P D1 = 352 ms, P D2 = 350 ms). This target effect is significant (F(1, 8) = 11.4, p < .01) and does no significantly interact with gesture position (F(1, 8) = 2.4). Furthermore, P D mean is about 356 ms for both the 1-and the 2-syllable(s) conditions, and increases to 369 ms in the 3-and to 376 ms in the 4syllables conditions. The number-of-syllables effect is significant (F(3, 24) = 4.4, p < 0.05). However, C2 (+13 ms)

Figure 4 :

 4 Figure 4: Means and standard deviations of the onset-toapex pointing duration (P D ) according to the number of syllables and to the target position for the first (P D1 , left) and the second (P D2 , right) gestures

Acknowledgments

This work is part of the "Patipapa" project funded by the French Ministry of Research (Action Concertée Incitative "Systèmes Complexes en Sciences Humaines et Sociales"). It benefited from inspiring discussions with Christian Abry.