PhD Christel Bidet-Ildei
email: christel.bidet@univ-poitiers.fr

Yoshiyuki Tamamiya

Kazuo Hiraki

Observation and action priming in anticipative tasks implying biological movements

Keywords: anticipation, biological motion, action priming, observation priming

The purpose of this study was to assess the effect of short-term priming in perceptual anticipation tasks involving point-light biological motions. After the production or the observation of a relevant or an irrelevant movement (action vs. observation priming), eleven right-handed volunteers were asked to anticipate, as quickly and accurately as possible, the end-point of a pointing movement after the stimulus vanished upon completion of 60 % of the total movement. Our results indicate that perceptual accuracy is significantly affected only with relevant observation priming. This suggests that perceptual anticipation tasks' involving point-light biological motions implies specific perceptual competencies.

INTRODUCTION

Decades of research have demonstrated the remarkable capacity of humans to detect and recognize biological motions. Reduced depiction, based only on dots of lights placed on the limbs of an actor (i.e., a point-light display paradigm), is sufficient to identify the produced action [START_REF] Johansson | Visual perception of biological motion and a model for its analysis[END_REF] and to resolve both the gender [START_REF] Kozlowski | Recognizing the sex of a walker from dynamic point-light displays[END_REF] and identity of a person [START_REF] Beardsworth | The ability to recognize oneself from a video recording of one's movements without seeing one's body[END_REF][START_REF] Loula | Recognizing people from their movement[END_REF]. One of the most remarkable abilities is that humans are able to anticipate subsequent components of a human motion sequence. For instance, in a two-forced choice handwriting task, Kandel, Orliaguet and Boë (2000) show that adults are able to predict the second letter of a digram ("l" or "n") after observing the first letter ("l"). This capacity for perceptual anticipation also manifests itself when observers are asked to point with a mouse the final position of a simple arm movement in which the last segment of the trajectory is hidden [START_REF] Martel | Anticipating the terminal position of an observed action: Effect of kinematic, structural, and identity information[END_REF][START_REF] Pozzo | Kinematic features of movement tunes perception and action coupling[END_REF]. In this case, participants are able to estimate the terminal location of the reaching movement after the stimulus vanished, even though only 60 % of the total movement duration was observed [START_REF] Martel | Anticipating the terminal position of an observed action: Effect of kinematic, structural, and identity information[END_REF].

The most common theoretical explanation for this visual anticipation capacity is based on the motor theory of perception, which suggests that an individual's motor competencies are connected to their perceptions of movements of other humans [START_REF] Hommel | The Theory of Event Coding (TEC): a framework for perception and action planning[END_REF][START_REF] Jeannerod | Neural simulation of action: a unifying mechanism for motor cognition[END_REF][START_REF] Prinz | Perception and action planning[END_REF][START_REF] Rizzolatti | The mirror-neuron system[END_REF][START_REF] Schütz-Bosbach | Perceptual resonance: action-induced modulation of perception[END_REF]. This point of view is illustrated in various neurophysiological, behavioral and neuropsychological works that demonstrate that the motor system is significantly involved in the anticipation of perceptual events. In this context, brain-imaging studies have shown that the anticipation of sequential patterns activates specific brain areas involved in the production of motor sequences. For instance, [START_REF] Chaminade | Is perceptual anticipation a motor simulation? A PET study[END_REF] show that the left premotor cortex and the right intraparietal sulcus are activated during anticipation of pointing movements while the left frontal operculum and the superior parietal lobule are activated during the anticipation of writing movements.

Moreover, increased premotor activation is revealed when observers have to anticipate the final state of an occluded human action [START_REF] Stadler | Predicting and memorizing observed action: differential premotor cortex involvement[END_REF]. Additionally, [START_REF] Graf | Predicting point-light actions in real-time[END_REF] show that the capacity to judge whether a static image corresponding to a well continuation of a motor sequence previously observed improves when the timing of the visual presentation of the static image matches the timing of the execution of the motor sequence. This suggests that predictive competencies are based on real-time, covert motor activation. Recently, [START_REF] Springer | Motor execution affects action prediction[END_REF] have shown that the efficiency of this real-time process increases when the perceptual anticipation task is associated with the simultaneous execution of an action. Finally, recent experiments show that anticipation of a human's movement goal improves when predictions are made on biological point-light motions [START_REF] Elsner | Humans Anticipate the Goal of other People's Point-Light Actions[END_REF][START_REF] Martel | Anticipating the terminal position of an observed action: Effect of kinematic, structural, and identity information[END_REF]. This suggests that anticipation competencies could be based on kinematics information which is known to be a marker of human action production (e.g., [START_REF] Viviani | The effect of movement velocity on form perception: geometric illusions in dynamic displays[END_REF].

Although several experiments demonstrate the intervention of the motor system during visual anticipation of human movements, the specific mechanism implied in this intervention stays an open issue. Two possibilities can be envisaged. First, in agreement with a recent study of [START_REF] Jantzen | Dorsal stream activity and connectivity associated with action priming of ambiguous apparent motion[END_REF], we could consider that the intervention of motor system in visual anticipation task implying human movements is based on sensorimotor integration. In this case, sensory feedback of compatible actions would be integrated to improve the anticipation processing. Alternatively, the intervention of motor system could be based on the activation of specific motor representation. This hypothesis is consistent with the idea proposed by [START_REF] Pozzo | Kinematic features of movement tunes perception and action coupling[END_REF] that perceptual anticipation is sustained by an "internal representation of potential action related to the stimulus". To distinguish these two hypotheses, the present study aimed to directly compare the facilitation of perceptual anticipation after visual and action priming. It is known that production and observation of biological movements are connected in a lot of characteristics in particular concerning the activation of a specific motor representation. For instance, several studies have documented equivalent levels of cerebral activation when humans produce or observe a human action [START_REF] Decety | Brain activity during observation of actions. Influence of action content and subject's strategy[END_REF][START_REF] Filimon | Human cortical representations for reaching: mirror neurons for execution, observation, and imagery[END_REF][START_REF] Rizzolatti | Localization of grasp representations in humans by PET: 1. Observation versus execution[END_REF]. However, some researchers argue that the equivalence between production and observation is not systematic [START_REF] Bidet-Ildei | Observing or producing a motor action improves later perception of biological motion: Evidence for a gender effect[END_REF][START_REF] Bidet-Ildei | Visual preference for isochronic movement does not necessarily emerge from movement kinematics: a challenge for the motor simulation theory[END_REF]Bidet-Ildei & Orliaguet, 2008;[START_REF] Bidet-Ildei | Rôle des représentations motrices dans la perception visuelle des mouvements humains[END_REF][START_REF] Pavlova | Perception and production of biological movement in patients with early periventricular brain lesions[END_REF]. For example, the detection of point-light walking movements embedded in a mask is independent of the walking capacity of adolescents with periventricular lesions [START_REF] Pavlova | Perception and production of biological movement in patients with early periventricular brain lesions[END_REF]. In the same manner, Bidet-Ildei and Orliaguet (2008) have shown in children that specific kinematic parameters as movement speed are independent of motor-perceptual coupling (see [START_REF] Meary | Four-day-old human neonates look longer at non-biological motions of a single point-of-light[END_REF] for similar evidence in human newborns). Overall, these findings show that action representation is not strictly related with motor competencies and suggest that action representation could be more pregnant during the observation than the execution of actions. In this theoretical context we asked participants, after a brief period of visual or action priming, to anticipate the final position of a point-light pointing movement that vanished before the final position was reached. If perceptual anticipation capacity is based on sensorimotor integration we should observe better performance after action relevant priming. On the contrary, if perceptual anticipation capacity is based more on action representation, we should observe a higher influence of visual relevant priming than equivalent action priming.

METHOD

Participants

Eleven right-handed healthy adults (6 male and 5 female) aged between 18 and 27 years (mean age= 24.3 years, standard deviation= 2.6 years) participated in this experiment. These participants were recruited as volunteers from the University of Tokyo. All participants provided written informed consent for the protocol, which was in accordance with the Declaration of Helsinki and approved by the Ethics Committee of the University of Tokyo. None of the participants reported any sensory or motor deficits, and none had previous experience with this stimulus condition. All participants had normal or corrected-to-normal vision.

Apparatus and stimuli

Each participant sat in an unlit room in front of a 17" CRT computer screen (EIZO FlexScan F520, spatial resolution: 1024 * 768 pixels, sampling rate: 60 Hz) at a viewing distance of 69 cm. The visual angle subtended by the screen to eye-level was 17.4 ° vertical (V) * 25.9 ° horizontal (H). Body movements were limited by establishing contact between the participant's chest and a table placed between the participant and the screen (see figure 1B).

The stimuli were created in the same manner as in our previous work [START_REF] Martel | Anticipating the terminal position of an observed action: Effect of kinematic, structural, and identity information[END_REF]. They consisted of animations showing a point-light display sequence depicting a pointing movement of either 20 or 30 cm of distance. This target movement was represented by 4 dots placed on the shoulder, elbow, wrist and forefinger of an actor. In a pre-experimental session, an actor (who did not participate as a subject in the present experiment) produced a number of pointing motions in a horizontal plane with the right hand. His performance was filmed from above using the 3D ultrasound Zebris system (Zebris, http://www.zebris.de/) by using markers placed on his articulation points (shoulder, elbow, wrist and forefinger). This system allows for the recording of spatial and temporal parameters of movement (spatial resolution, 0.2 mm; temporal resolution, 200 Hz). The actor was instructed to produce as fast and accurately as possible ten pointing movements toward a circular target (ø = 1 cm) placed at a distance of 20 or 30 cm in front of him. For each distance, a point-light reaching movement was generated with Matlab software by using the mean coordinates recorded by the Zebris system.

The point-light stimuli consisted of white dots (97 cd/m 2 , Ø: 0.65 ° visual angle) on a dark background (0.14 cd/m 2 ). According to our previous work on anticipation [START_REF] Martel | Anticipating the terminal position of an observed action: Effect of kinematic, structural, and identity information[END_REF], the animation included 60 % of the total duration of the pointing movement because that is the minimum duration required to observe significant anticipative competencies [START_REF] Kandel | Detecting anticipatory events in handwriting movements[END_REF]. Therefore, for the 20 and 30 cm distances, we included 23 and 36 frames (25 ms/frame), resulting in total durations of 575 ms and 900 ms, respectively. Each sequence was transformed into an avi movie with a resolution of 512 * 640 pixels and a frame rate of 40 frames/s. The target stimuli were presented at the center of the screen in a window constituting 60 % of the screen (10.5 ° (V) * 15.6 ° (H) visual angle), with sustained visual angles of 3.3 ° (V) * 3.3 ° (H) and 4.9 ° (V) * 3.3 ° (H) for the 20 and 30 cm distances, respectively. Therefore, the distance on the screen is divided by three with respect to the true distance (4 cm and 6 cm for 20 cm and 30 cm respectively). The visual presentation was in the center of the screen in vertical plane and the direction of motions was from the bottom to the top of the screen (see videos1 ).

Each experimental trial was preceded by a prime. Three types of priming were proposed.

The observation priming consisted of the visual presentation of 1) an actual pointing movement or 2) an actual applauding movement, which were presented as color movies.

The movements were previously recorded with a video camera (spatial resolution: 760 * 576 pixels, sampling rate: 30 frames/s) place above the device (see videos 1 ) and showed an actor either making a pointing (relevant observation priming condition), or making an applauding movement (2 times) (irrelevant observation priming condition).

For the relevant observation priming condition, the actor had to produce as fast and accurately as possible several pointing movements directed to a target placed in front of him in a horizontal plane. To avoid perceptual similarities between the relevant observation priming and the stimuli, the actor who produced the observation priming movements was a woman (whereas stimuli have been created from the production of a man), the distance of pointing was 40 cm (instead 20 or 30 cm used in the experimental task) and the pointing was directed toward a light toy (instead of a circular target). The light toy was a figurine representing "Pikachu", a manga character well-known in Japan.

However, visual relevant priming movements as point-light stimuli were presented vertically on the screen with a displacement from the bottom to the top of the screen.

Movement duration was 1.5 s for both pointing and applauding movements.

In contrast, the action priming consisted of each participant making either a pointing movement (relevant action priming condition) or an applauding movement (irrelevant action priming condition) under the same conditions as those used in the observation priming. For pointing movement, the participants had to reach with his/her right hand the figurine placed in front of him/her as fast and accurately as possible. During the entire movement, the forefinger was in contact with the table. The starting position was marked on the table. For applauding movement, participants had to applaud 2 times. For both applauding and pointing movements, participants had 2.5 s to execute the motions and come back in the starting position. This time duration was previously tested on 20 participants and it was sufficient to make the task. During the training's phase, the experimenter checked that this time was convenient for each participant. The experimental box was unlit during the experiment; therefore, visual feedback was not available. An additional control condition consisted of the participant observing a fixation cross for 2.5 s (neutral priming condition).

Experimental procedure

The presentation of stimuli and the registration of manual responses were controlled by E-prime software (version 2.0, http://www.pstnet.com/). Before beginning the experiment, participants were allowed to become familiarized with the point-light display. For this familiarization, participants showed 4-5 point-light movements representing human actions and had to recognize the action perceived. Next, the experimenter explained the task procedure, and participants were given time to practice producing the applauding and pointing priming movements that were required during the experimental phase. After practicing, participants were required to perform nine training trials with the same instructions and the same priming as those used in the experimental phase but with different point-light pointing stimuli. To make the experimental task clear to the participants, comments about these examples and about the experimental tasks were provided by the experimenter during the training phase. If anything was unclear, the participant was allowed repeat the training phase. The participant's task was to identify, as quickly and accurately as possible, the end-point of the point-light pointing movement after it vanished upon completing 60 % of the total movement duration. Participants indicated the location corresponding to the judged end-point of the movement by moving a mouse cursor and clicking the left button of the mouse. The mouse was positioned on the table near the participants so that the participants could easily give their responses by moving the mouse and clicking one of the buttons. The starting position of the mouse was given by contact with a block placed near the participants. To make the experimental task less monotonous and increase the variability of the required response, two different point-light stimuli (20 and 30 cm) were used.

Each trial started with a visual, action or neutral priming for 2.5 s. To clarify the task before priming, instructions appeared for 1 s before each test (i.e., "please look at the pointing movement", "please look at the applauding movement", "please look at the fixation cross", "please point at Pikachu" and "please applaud"). Each instruction appeared in Japanese and was clarified during the training phase. The instructions appeared in different colors (white, blue and yellow) to distinguish between neutral, visual and action priming (see figure 1A). The color selected for each priming condition varied between participants. As soon as the priming phase was finished, participants had to place their hand on the mouse and then a fixation cross appeared for 500 ms followed by the presentation of the point-light stimulus. Participants performed 200 trials (20 trials * 5 primings * 2 distances) in a random order, resulting in a total duration of approximately 30 min for the entire experimental session. The starting position of the mouse was given by contact with the block. "Departure" indicates the starting position of the participants in all priming conditions. After the priming, participants had to place their hand on the mouse to give their response.

Data analysis

Response accuracy and response variability were assessed for each participant.

Response accuracy corresponded to the absolute constant errors (i.e., the difference in cm between the participant's end-point mouse position and the position of the end of the movement registered by Zébris) and response variability to the standard deviation (SD) of the 20 replications. For both response accuracy and response variability, a three-way analysis of variance (ANOVA) was performed according to the priming condition (action, observation), the congruency (relevant vs irrelevant movements) and the stimulus condition (20 cm or 30 cm). Local comparisons were performed using post-hoc Newman-keuls' comparisons. Effect sizes were computed using eta-square estimates.

Performances after observation and action priming conditions were compared with performance after neutral priming by using Student t-test. Bonferroni corrections were applied for multiple comparisons.

RESULTS

With respect to response accuracy, we observed two interesting interactions. First, we had an interaction between the congruency and the stimulus condition (F(1,10)=7.14; p<0.05; η²=0.41). In both 20 cm and 30 cm stimuli conditions, relevant priming conditions affected more the performance in anticipation than irrelevant priming conditions but relevant conditions interfered with the performance in the 20 cm stimulus condition while they facilitated the performance in the 30 cm stimulus condition.

The other most important interaction appeared in Figure 2 between the priming condition, the congruency and the stimulus condition (F(1,10)=13.57; p<0.01; η²=0.58).

To clarify the last interaction we made separate 2 (type of priming: visual vs action) *2 (congruency: relevant vs irrelevant movements) ANOVA for each stimulus condition.

For the 20 cm stimuli, we observed in figure 2A better performance after the action (mean=1.20 cm, SD=0.52 cm) than the observation (mean=1.33 cm, SD=0.55 cm) priming conditions (F(1,10) = 54.43; p<0.01; η²= 0.84) and better performance for the irrelevant (mean=1.17 cm, SD=0.57 cm) than the relevant (mean=1.36 cm, SD=0.50 cm) priming conditions (F(1,10)=8.13; p<0.05; η²= 0.44). Further inspection of Figure 2A revealed also a significant interaction with a higher decrease of performance in the relevant observation priming condition than the relevant action priming condition (p<0.01). Finally, only the relevant observation priming condition (mean=1.46 cm; SD=0.52 cm) caused poorer anticipation performance than the neutral priming condition (mean= 1.16 cm; SD=0.54 cm, t 10 = 3.92; p<0.05).

For the 30 cm stimuli, Figure 2B showed better anticipation performance after relevant (mean=1.48 cm, SD=0.35 cm) than irrelevant (mean=1.67 cm, SD=0.50 cm) priming conditions (F(1,10)=4.75; p=0.05; η²= 0.32). Moreover, a significant interaction appeared in figure 2B with a higher increase of performance in the relevant observation priming condition than the relevant action priming condition (p<0.05). Finally, further analysis revealed that only the relevant observation priming condition improved significantly the performance of participants (mean=1.41; SD=0.34) in comparison with the performance observed after the neutral priming condition (mean= 1.67; SD=0.44, t 10 = 3.20; p<0.05). With respect to the response variability, we observed no main effect of the priming condition (F(1,10)<1), the congruency (F(1,10)<1) or the stimulus condition (F(1,10)=2.14; p=0.16) and no interaction between these factors (all p>0.20). In all conditions, we had a mean response variability of 0.47 cm (SD= 0.12 cm) which was not significantly different of the variability observed after the neutral priming condition (mean= 0.43 cm; SD=0.15 cm, t 10 =1.75; p=0.11).

DISCUSSION

This study compared the effect of action and observation priming on the anticipation capacity after observation of incomplete point-light pointing movements. The main finding is that relevant observation priming is more efficient than equivalent relevant action priming in anticipating the final state of a biological stimulus. In the specific context of the motor theory of biological motion perception, the superiority of the visual relevant priming over the other conditions is an intriguing finding and suggests that not only motor competencies but also specific perceptual competencies can be used to resolve perceptual anticipation tasks. This finding agrees with some experiments that show that humans can have better perceptive competencies regarding biological movements than motor competencies (e.g., [START_REF] Pavlova | Perception and production of biological movement in patients with early periventricular brain lesions[END_REF]. Moreover, it argues in the favor of the hypothesis of the use of internal action representation in anticipation task [START_REF] Pozzo | Kinematic features of movement tunes perception and action coupling[END_REF]. It is also in accordance with a recent work which showed that inferring another's goal implies more reasoning regarding action and context than pure motor simulation [START_REF] Liepelt | How do we infer others' goals from non-stereotypic actions? The outcome of context-sensitive inferential processing in right inferior parietal and posterior temporal cortex[END_REF]. One interesting similarity between observation priming and the actual stimulus is that both involved another person making a pointing movement. Some experiments have shown that perspective (self vs. other) can influence the activation of motor systems [START_REF] Fourkas | Corticospinal facilitation during first and third person imagery[END_REF]. Moreover, it is well known that the analysis of our own motions is processed differently than the analysis of the motions generated by others (e.g., [START_REF] Loula | Recognizing people from their movement[END_REF] and that this difference is manifested in behavior and in brain activity [START_REF] Chaminade | Leader or follower? Involvement of the inferior parietal lobule in agency[END_REF][START_REF] Farrer | Experiencing oneself vs another person as being the cause of an action: the neural correlates of the experience of agency[END_REF][START_REF] Shmuelof | A mirror representation of others' actions in the human anterior parietal cortex[END_REF].

Consequently, we can hypothesize that action priming that involves our own motor competencies is processed differently by the brain than equivalent observation priming that involves motions generated by others. In that case, judgment of the observation priming and the visual stimuli might involve a similar mechanism, which could account for the superior efficiency of observation priming. This common mechanism could be supported on a neurophysiological level by the activation of the superior temporal sulcus (STS) when participants observe actual (during the priming) or point-light (during the task) biological motions. Indeed, it is well known that the observation of biological movements (but not their production) specifically activates the STS (see [START_REF] Pavlova | Biological motion processing as a hallmark of social cognition[END_REF] for a review). Consequently, we propose that the initial activation of the STS in observation priming could affect the subsequent anticipative visual competencies. However, further investigations are necessary to examine this neurophysiological hypothesis, which will be conducted in a future study. One alternative explanation for our results could be that observation priming had a higher spatio-temporal proximity with respect to the point-light display stimulus than the pointing movement performed by the participant. Indeed, observed prime and stimulus were presented in vertical plane whereas action priming had been executed in horizontal plane. However, it is important to note that observation priming had been built from a production in the horizontal plane. Accordingly, when we asked to participants whether the observation priming showed someone who produced a pointing movement in horizontal or vertical plane, 10 out of 11 people answered that the movement had been executed in a horizontal plane. Consequently, it is few unlikely that the vertical presentation of observation priming can explain the difference obtained between action and observation priming conditions. One other possibility is that kinematics used in the observation priming condition was nearer kinematics of the stimuli than kinematics associated with action priming condition. Without kinematics analysis of primes, we cannot completely reject this assumption. However, the person used for the observation priming and the person used for the creation of the point-light stimulus were different, and individual analysis indicates that the difference between observation and action relevant priming is present in more than 80 % of the participants (100% for the 20 cm stimuli and 73% for the 30 cm stimuli). In a random sampling, it is unlikely that more than 80% of participants would produce a greater difference between their own action and the point-light stimulus than the difference between the observed prime and the point-light stimulus. Moreover, both the distance and the target, which are two major components known to influence the spatio-temporal properties of a pointing movement [START_REF] Fitts | The information capacity of the human motor system in controlling the amplitude of movement[END_REF], were different in the observation priming and the stimulus displays. Finally, previous experiments implying biological human movements have shown that there are differences between males and females in their production of movements and that observers are sensitive to these sex differences when they judge human movements [START_REF] Alaerts | Action and emotion recognition from point light displays: an investigation of gender differences[END_REF][START_REF] Bidet-Ildei | Observing or producing a motor action improves later perception of biological motion: Evidence for a gender effect[END_REF][START_REF] Kozlowski | Recognizing the sex of a walker from dynamic point-light displays[END_REF]. In the present study, the observation priming involved movements produced by a woman, whereas the stimuli involved movements produced by a man, which suggests that our findings are not due to similarities between the observation priming and the stimulus.

One other important finding is that the priming effect demonstrated in the present experiment is specific; we show an effect only with relevant priming conditions on the anticipation accuracy regarding biological stimuli. This specific effect is consistent with the idea of [START_REF] Wohlschläger | Visual motion priming by invisible actions[END_REF] which suggests that "priming only occurred if the goal of the action and the motion display shared a common cognitive dimension".

Most importantly, we obtain an interaction between the effect of relevant priming condition and the stimulus implicated in the anticipation task. While relevant movements facilitate the performance in anticipation for the 30 cm stimuli, they interfere with the performance in anticipation for the 20 cm stimuli. Given that this difference is significant in both cases at least for observation priming condition, it excludes a pure context effect where observation priming of 40 cm would have been more efficient for the 30 cm stimuli than for the 20 cm stimuli. It is more likely that this interaction results of a difficulty to disengage attentionnal resources in 20 cm stimuli.

Actually, there is 300 ms of difference between the 20 cm (500 ms) and 30 cm (900 ms) stimuli and therefore it may be possible that treatment of visual congruent priming is not finished when participants had to give their answer about the 20 cm stimuli. In contrast, with the 30 cm stimuli, participants would have the time to finish the treatment of priming condition before to give their answer. This explanation should be assessed in future experiments but it is coherent with the idea that both observation priming and anticipation task involve a similar treatment of action's goal.

Figure 1 :

 1 Figure 1: A) Example of the procedure used in the experiment for the "Irrelevant

Figure 2 :

 2 Figure 2: Mean and standard error of the absolute constant errors according to the

Videos of stimuli and observation priming are visible on https://bv.univ-poitiers.fr/access/content/user/cildei/stimuli%20tokyo/

Acknowledgements

Christel Bidet-Ildei was a visiting post-doctoral student in Kazuo Hiraki's laboratory. This work was supported by the short-term program of the Japanese Society for Promotion of Sciences (JSPS). We thank all members of Hiraki's lab for their participation, particularly Goh Matsuda and Shun Itagaki for their help in the experimental design. We thank also Arnaud Badets, Lucette Toussaint, Tim Welsh and the two anonymous reviewers for their helpful comments that are contributed to improve our paper.


Arnaud Badets
email: arnaud.badets@univ-poitiers.fr

Christel Bidet-Ildei

Mauro Pesenti

Influence of biological kinematics on abstract concept processing

Keywords: number representation, random number generation, biological marker, spacenumber interaction

During a random number generation task, human beings tend to produce more small numbers than large numbers. However, this small number bias is modulated when motor behaviour, such as a turn of the head, is performed during the random number generation task. This result fits with the finding that number representation is linked to laterally-oriented actions, with small-and large-magnitude numbers generally linked to movement towards the left or the right side of space respectively. To test whether this number-space association is specific to human motor behaviours or extends to any type of laterally oriented movements, we assessed whether the presentation of biological or non-biological leftward or rightward movement affected a subsequent random number generation task. Biological and non-biological movements were obtained by varying the kinematic parameters of the movements. Biological kinematics represented the tangential velocity actually observed in a human pointing movement; non-biological kinematics represented equivalent movements but with an inverse tangential velocity along the path. The results show that only the observation of biological movements induces a space-number bias whereas observing non-biological movements do not. This finding is the first evidence of a link between a biological marker and the semantic representation of a concept as abstract as number.

Introduction

Human beings are highly sensitive to biological movements, and the kinematics of such actions is one of the most important pieces of information to them when observing others (see [START_REF] Blake | Perception of human motion[END_REF] for review; [START_REF] Pavlova | Biological motion processing as a hallmark of social cognition[END_REF]. In the seminal paradigm on biological kinematics proposed four decades ago [START_REF] Johansson | Visual perception of biological motion and a model for its analysis[END_REF], participants had to identify particular actions from just a small number of light dots placed on the main body joints of a person whom the participants could not see. In this point-light display methodology, only action kinematics characterizing living organisms are retained whereas other types of biological information, such as the form of the limbs, skin texture, or other features of movement, are removed [START_REF] Atkinson | Emotion perception from dynamic and static body expressions in point-light and full-light displays[END_REF]. Observers were able to recognize a human actor walking, running or dancing. Since then, it has been demonstrated that dynamic point-light displays afford sufficient information to identify a person (Cutting & Kozlowski, 1977) or an arm action [START_REF] Hill | Exaggerating temporal differences enhances recognition of individuals from point light displays[END_REF], the gender of an actor (Kozlowski & Cutting, 1977), and even their emotions [START_REF] Atkinson | Emotion perception from dynamic and static body expressions in point-light and full-light displays[END_REF] or intentions [START_REF] Martel | Anticipating the terminal position of an observed action: Effect of kinematic, structural, and identity information[END_REF]. In summary, the human visual system has evolved to recognize biological-based movements, and this sensitivity is largely employed when recognizing or interpreting the behaviours of living organisms.

Interestingly, the existence of a semantic link between biological movements and the processing of abstract meanings like action words or numbers has recently been documented [START_REF] Andres | Actions, Words, and Numbers: A Motor Contribution to Semantic Processing[END_REF][START_REF] Bedny | Perception, action, and word meanings in the human brain: the case from action verbs[END_REF] for reviews; [START_REF] Michaux | Let us redeploy attention to sensorimotor experience[END_REF]. For instance, when the action verb "to run" was presented to participants, the recognition of point-light running behaviour was greatly improved [START_REF] Bidet-Ildei | Reading action word affects the visual perception of biological motion[END_REF]. This line of research provides empirical evidence for theories of embodied cognition which hold that sensory-motor mechanisms are co-opted to assist with mental activities whose referents may be distant in time [START_REF] Wilson | Six views of embodied cognition[END_REF]. The co-opted mechanisms are thus decoupled from their initial function in bodily experience to support offline conceptual processing [START_REF] Anderson | Neural reuse: a fundamental organizational principle of the brain[END_REF]. A comparable interaction between biological movements and number processing has also been found: in various tasks and experimental set-ups, small-magnitude numbers were found to be associated with finger grip closing, and large-magnitude numbers with grip opening [START_REF] Andres | Number magnitude and grip aperture interaction[END_REF][START_REF] Andres | Time course of number magnitude interference during grasping[END_REF]Badets & Pesenti, 2010, 2011;[START_REF] Chiou | Relative size of numerical magnitude induces a size-contrast effect on the grip scaling of reach-tograsp movements[END_REF][START_REF] Lindemann | Getting a grip on numbers: numerical magnitude priming in object grasping[END_REF]. Interestingly, [START_REF] Badets | Finger-number interaction: an ideomotor account[END_REF] see also Badets & Pesenti, 2010;[START_REF] Badets | Number generation bias after action observation[END_REF][START_REF] Loetscher | Head turns bias the brain's internal random generator[END_REF] for head movement) have found an asymmetrical interaction for small number processing. Specifically, they found that the perception of grip closing improved the processing of small numbers only, while no effect was found for the perception of the grip opening movement and large numbers. This bias towards small numbers reveals that the perception of grip closing could evoke grasping for a small object goal.

When asked to generate numbers randomly, participants prefer to produce small rather than large numbers (for a review, see Loetcher & Brugger, 2007). This is termed the small number bias (SNB) and reflects the fact that, whereas no specific instructions to do so are given, an "internal random generator" biases random generation towards small numbers (Loetcher, Schwarz, Schubiger, & Brugger, 2008). This natural tendency to produce more small numbers can be modulated by various internal or external cues. For instance, observing pictures mimicking grip closing or opening influences number production (more small numbers than large numbers being produced after observation of a grip closing), whereas observing the closing or opening movements of non-biological geometric shapes has no effect [START_REF] Badets | Number generation bias after action observation[END_REF]. This indicates that the effect does not result from the general processing of motion amplitude. In a same vein, random number generation (RNG) is affected by the performance of (passive whole body motion [START_REF] Hartmann | Moving along the mental number line: Interactions between whole-body motion and numerical cognition[END_REF]head turns: e.g., Loetscher & Brugger, 2007), or observation of (e.g., gaze shifs: [START_REF] Grade | Influence of gaze observation on random number generation[END_REF] motor actions [START_REF] Badets | Number generation bias after action observation[END_REF]. The kinematics of hand movements in space was also found to be affected by number magnitude: when participants had to aim with their hand at a visual target presented to the left or to the right of space after the processing of an Arabic digit, the hand movement was biased towards the left for smallmagnitude numbers and toward the right for large-magnitude numbers [START_REF] Song | Numeric comparison in a visually-guided manual reaching task[END_REF]; see also Wiemers, Bekkering & Lindemann, in press, for a "motion-arithmetic compatibility" effect wherein the performance in addition and subtraction problem solving was modulated by the direction -upward vs. downward and leftward vs. downward respectively -of hand movements the participants had to perform while calculating). These latter results and the SNB finding have been taken as evidence of the mental number line concept, where small quantities are represented on the left and large quantities on the right side of the line. This number-space association possibly stems from reading and writing habits [START_REF] Dehaene | Varieties of numerical abilities[END_REF].

On the whole, all these findings support the idea that at least part of number semantics rests upon the interaction between the body and its environment, thus providing new evidence supporting embodied cognition theories. These findings indicate that processing a number influences the perception or the production of biological kinematics through a semantic-tomotor link (Badets & Pesenti, 2010). However, to our knowledge, the possible influence of specific kinematic parameters on numerical semantic processing has never been investigated.

In previous studies the stimuli used were either static, or, when human or non-human movements were presented, kinematic parameters were kept constant. Some theories of embodiment proposed that movements that can possibly be self-experienced through body experience are more strongly embodied than others (see Meteyard, Cuadrado, Bahrami & Vigliocco, 2012 for review). In this context, biological kinematics, which correspond to natural human dynamics, are more strongly embodied than movements with non-biological kinematics parameters, which represent movements impossible to reproduce (e.g., when participants have to follow a non-biological elliptic trajectory that does not correspond to the two-third power law, their performance is greatly reduced with an increase in variability and in shape distortions; [START_REF] Viviani | Perceiving and tracking kinesthetic stimuli: further evidence of motor-perceptual interactions[END_REF]. Therefore, as our non-biological stimuli used inverse dynamics compatible neither with human movements nor with physical movements in a real environment (indeed, inverse velocity profile do not exist in the physical world and can just be produced artificially in laboratory), a plausible prediction relating to embodied theories of numerical cognition is that number semantics should be more strongly linked to movements with biological kinematic parameters than with non-biological kinematic parameters. The present study explored this prediction thanks to an RNG task coupled with a priming paradigm which used lateralised pointing movements to assess whether and if so how, the SNB was moderated by the type and direction of the movement. Participants were required to randomly produce a number after observing a single dot representing a pointing movement. A pointing movement was used because of its implicit link with numbers through numerosity processing. Indeed, in the context of object counting, pointing and touching gestures are known to help keep track of the count and to match the objects with the verbal string of number names in order to establish one-to-one correspondence. This has been documented in non-human primates who spontaneously exhibit pointing and touching when trained to count an array of items [START_REF] Boysen | Indicating acts during counting by a chimpanzee (Pan troglodytes)[END_REF], in human children who count more accurately when they perform pointing and touching actions themselves, compared to when the pointing and touching actions are performed by a puppet manipulated by the experimenter [START_REF] Alibali | The function of gesture in learning to count: More than keeping track[END_REF][START_REF] Saxe | Gesture in early counting: A developmental analysis[END_REF], and brain-damaged patients, most of whom (60% and 65% respectively) use pointing and touching while counting dots, starting with the leftmost dot and thus implying a left-to-right movement [START_REF] Seron | Dot counting by brain damaged subjects[END_REF]. This has also recently been observed in healthy children and adults, where the direction of pointing during counting related to reading and writing habits (Shaki et al., 2012). Here, two movement conditions were created by varying the kinematics of the movement, thus providing the observers with the same spatial feature information but differing kinematic information: a biological condition used specific biological kinematic features with acceleration and deceleration of the movement during pointing, while the nonbiological condition used inverse biological kinematics. If spatial information is sufficient to induce a space-number bias, possibly by inducing an attentional shift on a mental number line, both biological and non-biological movements should influence number production and moderate the SNB the same way: the proportion of small numbers produced should be higher with a leftward movement and, with a rightward movement, should be lower than (or, at least, not different to) the proportion of large numbers produced, regardless of the type of kinematics. In contrast, if number semantics are only linked to biological kinematics, the moderation of the SNB should only be observed after the biological movement. Moreover, as the probability of producing a number smaller or larger than the previous number differs depending upon the magnitude of that previous number [START_REF] Costermans | An analysis of the association rates between numbers from 1 to 20[END_REF], a further interesting prediction could be made. The influence of movement direction could be measured by the proportional decrease or increase in magnitude of the new number relative to the previous number: a leftward movement might produce a number smaller than the previous one, and a rightward movement might produce a number larger than the previous one. Again, this should be observed in the case of both biological and non-biological kinematics if spatial information is sufficient to induce SNB, but only observed in the case of biological kinematics if it is the result of a specific embodied link.

Method

Participants

Sixteen undergraduate students (5 female; 2 left-handed, aged between 19 and 22 years, mean= 18.2 ± 0.7 years) from the University of Poitiers gave their informed consent to participate in the study for partial course credits. None of them reported any sensory or motor deficits and all had normal or corrected-to-normal vision.

Stimuli and apparatus

The stimuli were displayed on a 17-inch CRT computer screen (Nyama, spatial resolution: 1024*768 pixels, sampling rate: 60 Hz) positioned on a table at a viewing distance of 50 cm. The visual angle of the screen subtended at eye-level was 27.5° vertical (V) * 37° horizontal (H). Primes were point-light biological or non-biological pointing movements directed to the right or to the left side of space. Each point-light display consisted of a white dot (97 cd/m2, Ø: 0.6 of visual angle) presented on a dark background (0.14 cd/m2). The biological movements came from a previous experiment [START_REF] Martel | Anticipating the terminal position of an observed action: Effect of kinematic, structural, and identity information[END_REF] where the spontaneous production by several participants of pointing movements with the forefinger toward a target 1 cm in diameter placed 10 cm away in a sagittal plane were recorded with a three dimensional capture system (Zebris ultrasound system, http://www.zebris.de, spatial resolution: 0.5 mm; temporal resolution :100 Hz). It is important to note that with this stimuli configuration, the participants observed the pointing movements as if they were looking at a model in a back position that is as if the movements were performed from their own subjective perspective. Consequently, an action from the model would engage the same effectors in the participants if they were to perform a similar action (see [START_REF] Badets | Error detection processes during observational learning[END_REF]Shea, 2006, andBadets, Blandin, &Shea, 2006, for a similar observational set-up within different paradigms). From this previous recording, two-dimensional trajectories were reconstructed using Matlab software routines (http://www.mathworks.com/), so as to obtain an avi format movie of 640 * 512 pixels size with a frame rate of 60 frames/s. At this point, one typical movement was rotated to 20° and 160° to obtain a pointing movement directed to the right side and the left side respectively. These video sequences represented the biological priming condition insofar as the spatial path and kinematics of the registered dots were similar to the natural movements performed by humans. From these biological point-light displays, we built non-biological equivalent pointing movements with the same trajectory and duration but with modified kinematics along the path using an inverse profile. For this, we inversed the biological pointing kinematics point by point along the path so as to obtain a decrease in tangential velocity followed by an increase of tangential velocity (see Figure 1B). Each priming movement had a duration of 440 ms and a trajectory length of 9.8 cm. Each priming movement started at 18.5° (H) * 9.1° (V) and ended at 7.8° (H) *3.8° (V) on the left movement and at 28.9° (H) * 3.8° (V) on the right movement. The imperative stimulus to which the participants responded was a low sound (audio frequency =1000 Hz, intensity = 2.5 Watt) played for 150 ms through computer speakers (multi-media speaker system, frequency response: 22 Hz ~ 20 kHz, maximum output = 2 * 5 Watts) placed on each side of the computer screen.

Procedure

The participants were seated in a dimly lit room in front of the computer screen, with their chest in contact with the table supporting the screen in order to limit their movements.

They had to generate numbers between 1 and 10 as randomly as possible (i.e., wholly jumbled sequence, no immediate repetition of the same number, no systematic ascending, descending or somehow ordered sequence, etc.; for a similar method, see [START_REF] Badets | Number generation bias after action observation[END_REF] and their responses were paced by a sound (imperative stimulus). Each participant performed three blocks: one training block with no prime to become familiar with the RNG task and two experimental blocks, one with biological and one with non-biological point-light movements to the left or to the right of the screen for half of the stimuli respectively. The order of the two experimental blocks was counterbalanced across participants. In the training block, the imperative stimulus was played 750 ms after the previous response. In the experimental blocks, the imperative stimulus was played after a black screen had been shown for 110 ms, the visual presentation of primes shown for 440 ms and the black screen for another 50 ms (see Figure 1). The presentation of primes and stimuli was controlled by E-prime 2.0

Data analysis

We used the percentage of small numbers (1 to 5) and the percentage increase (see [START_REF] Loetscher | Eye position predicts what number you have in mind[END_REF][START_REF] Grade | Influence of gaze observation on random number generation[END_REF], for a similar analysis) as dependent variables.

For the percentage increase, the data produced after numbers 1 and 10 had been produced (19.6% of the data) were excluded, because the increase or decrease was totally constrained in these cases. Only the percentage of small numbers and the increase in magnitude relative to the previous number produced were analysed because, as immediate repetitions of the same number were not allowed, small numbers and increase are strictly inversely related to large numbers and decrease, respectively. For each dependent variable, the numbers produced in the training performance were assessed and compared to 50 as the chance level with onetailed Student t-tests. For both variables, we presented general results and corrected results. In the corrected results, the percentages obtained in the training condition were used as movement-unbiased baselines and subtracted from those observed in each experimental condition (left and right movements for biological and non-biological primes). Accordingly, the differences obtained represent the pure effect of experimental conditions, independent of the natural tendency to produce small (or large) numbers. An analysis of variance (ANOVA) with prime kinematic (biological vs. non-biological) and prime direction (left vs. right) as within-subject factors was carried out on both general and corrected results. Effect sizes were computed using eta-square estimates. Post-hoc Newman-Keuls comparisons were used to assess the significance of left-right differences in biological and non-biological conditions. Moreover, in the general analysis, each variable (percentage of small number and percentage increase) was compared to chance level (i.e., 50) with one-tailed Student t-tests. Where multiple comparisons were performed for the post-tests analyses, a Bonferonni correction was applied.

Results

Missed trials (i.e., trials without responses) and errors (e.g., out-of-interval numbers) were excluded from the analyses (1.84% of the data).

Percentage of small numbers

General analysis

In the training condition, a t-test with 50 as the test value revealed that participants produced significantly more small numbers than chance level (M= 52.9 ± 5.7 %, t 15 =2.04; p<0.05). In the movement conditions, the ANOVA revealed no main effect of prime kinematic (% small biological: 53.8 ± 5.4 %; non-biological: 52.6 ± 4.5 %, F(1,15) < 1), but a significant main effect of prime direction (F(1,15)=8.19; p<0.05, η² =0.4) with more small numbers produced when movements were directed to the left (54.8 ± 4.6 %) than to the right (51.6 ± 4.8 %). More importantly, the results revealed a significant interaction between these factors (F(1,15) = 7.72; p <0.05, η² =0.3). Newman-Keuls post-hoc analyses indicated that the difference between left and right was significant in the biological condition (biological left: 57.6 ± 5.9 %; right: 49.9± 6.8 %, p<0.05) but not in non-biological condition (non-biological left: 52 ± 6.2 %; right: 53.2 ± 6.4 %, p=0.6). Moreover, only the percentage of small numbers in the left biological condition was significantly higher than chance level (t 15 =5.15; p<0.003).

Corrected analysis

When the percentage of small numbers produced in the baseline condition was subtracted from the percentages of small numbers produced in the movement conditions, the ANOVA revealed no main effect of prime kinematic (F(1,15) < 1), but a significant main effect of prime direction (% small left: 4.5 ± 12.7 %; right: -1.7 ± 13 %; F(1,15) = 7.98; p < 0.05; η² =0.3) and, critically, a significant interaction between these two factors (F(1,15) = 7.09; p <0.05; η² =0.3; see Figure 2A). Newman-Keuls post-hoc analyses indicated that the difference between left and right was significant in the biological condition (left: M= 9.8 ± 14.5%, right: M= -4.9 ± 15.2 %, p<0.05) but not in the non-biological condition (left: M= -0.8 ± 15.2%, right: M= 1.6 ± 16.7 %, p=0.6).

Percentage increase

General analysis

In the training condition, a t-test with 50 as test value revealed that the participants had no specific tendency to produce numbers smaller or larger than the previous number produced (M= 51.4% ± 10.6%, t 15 =0.54; p=0.29). In the movement conditions, the ANOVA revealed no significant main effect of prime kinematics (% increase biological: 50.7 ± 5.9 %; nonbiological: 49.7 ± 6.6 %; F(1,15) =1.31; p=0.27), but a significant main effect of prime direction (F(1,15)=5.13; p<0.05, η² =0.3) with the rightward movement more frequently leading participants to produce a larger number than the previous number produced (% increase left: 48.3 ± 8.01 %; right: 52.2 ± 5.5 %). More importantly, the results revealed a significant interaction between these factors (F(1,15) = 7.84; p <0.05, η² =0.3). For this last interaction, post-hoc analyses indicated that the percentage increase was higher when the participants were primed by the biological right movement (M= 54.1 ± 16.01%) than by the biological left movement (M= 47.5 ± 18.7%, p<0.01), whereas no such difference appeared in the case of the non-biological movement (left: M= 49.2 ± 8.4%, right: M= 50.2 ± 6 %, p=0.48). Moreover, only the percentage increase after a right movement in the biological condition was significantly higher than chance level (t 15 =2.70; p<0.04).

Corrected analysis

When the percentage increase in the baseline condition was subtracted from the percentage increase in the movement conditions, the ANOVA revealed no significant main effect of prime kinematics (F(1,15) =2.05; p=0.2), but it did reveal a significant main effect of prime direction (F(1,15)=5.72; p<0.05, η² =0.3) with the rightward movement leading participants to more frequently produce numbers larger than the previous number produced (% increase difference left: -4.17 ± 15 %; right: 4.8 ± 19.6 %). More importantly, there was a significant interaction between these factors (F(1,15) = 6.02; p <0.05, η² =0.3; see Figure 2B).

For this last interaction, post-hoc analyses indicated that the percentage increase was higher when the participants were primed by the biological right movement (M= 8.8 ± 21%) than by the biological left movement (M= -5.5 ± 19.3%, p<0.01), whereas no such difference appeared in the case of the non-biological movement (left: M= -2.8 ± 12.2%, right: M= 0.9 ± 19.6 %, p=0.26).

Discussion

This study tested whether or not kinematic biological parameters constitute core information evoking sensorimotor representations [START_REF] Martel | Anticipating the terminal position of an observed action: Effect of kinematic, structural, and identity information[END_REF]. This was done to assess, in the numerical domain, the strength of theories of embodied cognition sustaining that implicit processing of the sensorimotor features of stimuli leads to semantic elaboration [START_REF] Barsalou | Perceptual symbol systems[END_REF][START_REF] Glenberg | Grounding language in action[END_REF]. When simply asked to randomly produce numbers, participants generally produced more small numbers than large numbers. That result fits with the so-called SNB. After seeing a dot moving leftward or rightward, they produced more small numbers after a leftward than a rightward movement with biological kinematics, while such a left-right difference was not observed for a movement with non-biological kinematics. This was true even when the productions were corrected for the natural SNB.

Importantly, observing a biological movement towards the right side of space led participants to produce numbers larger than the numbers they had produced just before; again, this was not observed with a non-biological movement. In other words, the observation of a pointing movement only influenced the participants' mental selection of a number if the pointing movement had biological kinematics.

Although several studies have previously found a link between human movements and the selection of numbers during an RNG task [START_REF] Badets | Number generation bias after action observation[END_REF][START_REF] Grade | Influence of gaze observation on random number generation[END_REF][START_REF] Hartmann | Moving along the mental number line: Interactions between whole-body motion and numerical cognition[END_REF][START_REF] Loetscher | Eye position predicts what number you have in mind[END_REF][START_REF] Loetscher | Head turns bias the brain's internal random generator[END_REF], the present study is the first to demonstrate that it is the biological nature of the kinematic marker which interacts with the representation of an abstract semantic concept such as numbers. This interaction is better accounted for by the idea of an embodied representation of numbers than by a mere spatial-numerical association stemming from a left-to-right oriented continuum. This is the case because the latter would not predict an effect restricted to biological kinematics but rather an effect that should extend to any kind of lateralised movement, and this was not observed here. In the present context, biological kinematics therefore provides core information that influences semantic access. As suggested by an anonymous reviewer, one might wonder whether this difference between biological and non-biological kinematics is not merely due to attentional differences.

According to this account, biological kinematics would be simply more salient hence would attract more attention than non-biological ones, which, in turn, would induce a greater effect on number production. Were this to be the case, excluding that differences in the salience and/or the modulation of visual attention drive the observed effect would be needed before interpreting the present finding as an evidence for embodied numerical semantics. However, although interesting, this attentional account appears in fact little plausible when one considers the current state of knowledge concerning biological and non-biological movement processing. Indeed, previous studies that have specifically compared biological and nonbiological movement processing with a single dot showed no response latency difference between the two types of movement (e.g., [START_REF] Bidet-Ildei | Perception of elliptic biological motion[END_REF]. This indicates that processing biological and non-biological movements takes the same time, which does not support the idea of a differential involvement of attentional mechanisms. Moreover, when a difference is observed between biological and non-biological stimuli, this difference goes in fact in the direction opposite to the one anticipated from this attentional account: participants are more visually attracted by non-biological than by biological movements. It is, for instance, the case when newborns process biological and non-biological single dot movements [START_REF] Meary | Four-day-old human neonates look longer at non-biological motions of a single point-of-light[END_REF]. Finally, this also fits with the classical findings of a greater visual attraction for non-usual than usual stimuli (e.g., [START_REF] Hespos | Reasoning about containment events in very young infants[END_REF].

Interestingly, the link between biological kinematics and number production appeared more pronounced in the rightward than the leftward movement when the increase relative to the number previously produced is considered. This could be due to some pragmatic aspects of the pointing movement used in our experiment. Indeed, in western cultures, pointing and touching while counting objects displayed in external space usually goes from left to right, probably because of reading and writing habits (Shaki et al., 2012). We argue that the effect observed here reflects this preference for left-to-right pointing and touching while counting objects in that the rightward biological movement evoked this object counting gesture hence it induced an increase in magnitude in the verbal counting sequence. By comparison, the leftward biological movement did not evoke such an object counting context. In essence, the non-biological movement does not of course evoke the counting context, and this is the reason why it simply does not moderate the production of numbers in the current set-up.

More broadly, our findings suggest that the biological kinematics of human movement constitute raw information needed to understand action semantics, be it actions observed from the point of view of the observer or of others. It is well known that humans' sensitivity to each other's movements is essential to their social interactions. At a neurophysiological level, this sensitivity is classically related to mirror neurons (i.e., neurons in the premotor cortex that discharge during the observation or the execution of an action; for a review, see [START_REF] Rizzolatti | The mirror-neuron system[END_REF]. Recently, it has been suggested that this mirror system could represent the neuronal basis for the semantic link between numbers and actions (Badets & Pesenti, 2010).

Moreover, given that mirror neurons are viewed as the neuronal substrate onto which communication skills evolve [START_REF] Rizzolatti | Language within our grasp[END_REF] and in line with previous findings which demonstrate that biological kinematics are also processed through the mirror system [START_REF] Saygin | Point-light biological motion perception activates human premotor cortex[END_REF][START_REF] Ulloa | Recognition of point-light biological motion: mu rhythms and mirror neuron activity[END_REF], we can speculate that the sensitivity to biological kinematics could be directly related to human social competencies. However, further investigations are required to examine this theoretical perspective and make firm conclusions.

Conclusion

Embodied views of numerical cognition suggest a link between number representation and different biological actions [START_REF] Andres | Number magnitude and grip aperture interaction[END_REF](Andres et al. , 2008;;Badets & Pesenti, 2010;2011;[START_REF] Chiou | Relative size of numerical magnitude induces a size-contrast effect on the grip scaling of reach-tograsp movements[END_REF][START_REF] Grade | Influence of gaze observation on random number generation[END_REF][START_REF] Lindemann | Getting a grip on numbers: numerical magnitude priming in object grasping[END_REF][START_REF] Loetscher | Head turns bias the brain's internal random generator[END_REF]. The lowest common denominator for these actions is movement kinematics, which represents a key biological signature of human behaviours [START_REF] Johansson | Visual perception of biological motion and a model for its analysis[END_REF]. In the context of the actionto-semantic relationship, when asked to randomly produce numbers after observing pointlight displays representing biological or non-biological leftward or rightward movements, participants are only affected after the observation of biological movements. This shows that the action perception context and the related increase in the number name string are only evoked by biological kinematics, thus providing critical support to the idea of embodiment of at least some aspects of numerical cognition.

(

  http://www.pstnet.com/); responses were recorded on-line by the experimenter. The experiment comprised 300 trials (60 in training block, 60 * 2 directions in biological condition, 60 * 2 directions in non-biological condition; leftward and rightward movements randomly mixed in each block) resulting in a total duration of approximately 30 minutes.

Figure 1 :

 1 Figure 1: A) Time course of events in the experimental blocks. Visual primes consisted of a

Figure 2 :

 2 Figure 2: Mean percentage of (A) small numbers and (B) increase as a function of prime type

Acknowledgements

AB is a tenured researcher at the National Centre for Scientific Research (CNRS, France).

CBI is associate professor at the University of Poitiers (France). MP is a research associate at the National Fund for Scientific Research (Belgium).