Effects of social intention on movement kinematics in cooperative actions

Communicative modulations of early action components support the prediction of distal goals

The successful unfolding of many social interactions relies on our capacity to predict other people's action goals, whether these are proximal (i.e., immediate) or distal (i.e., upcoming). The present set of studies asks whether observers can predict the distal goal of two-step action sequences when presented with communicative modulations of the first movement component of the sequence. We conducted three online experiments in which we presented participants with animations of a box moving to a first target location before moving onwards to a final, either near or far, target location. The second movement component and the target locations were occluded. After observing the first movement, participants were asked to select the most likely final target location, i.e., the distal goal of the sequence. Experiment 1 showed that participants relied on the velocity modulations of the first movement to infer the distal goal. The results of Experiment 2 indicated that such predictions of distal goals are possible even when the second movement in the sequence does not contain any velocity information, thus suggesting that the information present in the first movement plays the major role in the process of linking movements to their distal goals. However, Experiment 3 showed that under some circumstances the second movement can also contribute to how observers predict a distal goal. We discuss these results in terms of the underlying simulation processes that enable observers to predict a distal goal from the observation of proximal communicative modulations.

The contribution of eye gaze and movement kinematics to the expression and identification of social intention in object-directed motor actions

The intention to include another person in an interaction (i.e., social intention) is known to influence the spatio-temporal characteristics of motor performances. However, the interplay between these kinematic variations and the social cues provided by eye gaze has not been properly assessed yet. In the present study, we tested whether limiting the access to eye gaze altered the motor-related effects of social intention on motor performances. In a dyadic interaction, the agents' task was to displace a dummy glass to a new position with the intention to fill it themselves (personal intention) or having it filled by the observers facing them (social intention). The observers performed their action only when they were able to identify a social intention in agents' action. The task was performed while having access to observers' eye gaze or not, through the manipulation of an occluder. Results showed an effect of social intention on agents' motor performances, that induced an amplification of the kinematic spatio-temporal parameters. Such amplification was smaller when the observers' eye gaze was not available. In this latter condition, the identification of the social intention in the observed actions was impaired. Altogether, the results suggest that the presence of eye gaze cues contributes significantly to the success of social interaction, by facilitating the expression and the understanding of social intentions through the kinematics of object-directed actions.

Effect of Social Context on Cognitive and Motor Behavior: A Systematic Review

Human cognitive and motor behavior is influenced by the social contexts. The aim of this systematic review is to investigate the impact of the social contexts on human behaviors. A systematic search of the literature was performed via Pub-Med/Medline, Web of sciences, Google scholar, Science direct, Springer-Link and EMBASE and 68 articles were selected. After applying all the inclusion and exclusion criteria, 16 articles were retained. The results show that the presence of other people and the social context influence motor behavior (i.e. movement duration, trajectory behavior, maximum speed) and cognitive behavior (reaction time). Studies have shown an improvement in performance in the presence of other people compared to the individual situation. However, other studies showed that the presence of other people led to deterioration in performance compared to the individual situation. The improvement of behavior is attributed to the social phenomenon of facilitation while the deterioration was explained by the conduct theory or the distraction conflict theory. These social phenomena of facilitation or inhibition could be related to the perception-action theory, which interferes with interaction with other. This, in turn, seems to be associated with neural circuits of mirror neurons and motor system.

The Kinematics of Social Action: Visual Signals Provide Cues for What Interlocutors Do in Conversation

During natural conversation, people must quickly understand the meaning of what the other speaker is saying. This concerns not just the semantic content of an utterance, but also the social action (i.e., what the utterance is doing-requesting information, offering, evaluating, checking mutual understanding, etc.) that the utterance is performing. The multimodal nature of human language raises the question of whether visual signals may contribute to the rapid processing of such social actions. However, while previous research has shown that how we move reveals the intentions underlying instrumental actions, we do not know whether the intentions underlying fine-grained social actions in conversation are also revealed in our bodily movements. Using a corpus of dyadic conversations combined with manual annotation and motion tracking, we analyzed the kinematics of the torso, head, and hands during the asking of questions. Manual annotation categorized these questions into six more fine-grained social action types (i.e., request for information, other-initiated repair, understanding check, stance or sentiment, self-directed, active participation). We demonstrate, for the first time, that the kinematics of the torso, head and hands differ between some of these different social action categories based on a 900 ms time window that captures movements starting slightly prior to or within 600 ms after utterance onset. These results provide novel insights into the extent to which our intentions shape the way that we move, and provide new avenues for understanding how this phenomenon may facilitate the fast communication of meaning in conversational interaction, social action, and conversation.

The influence of cooperative action intention on object affordance: evidence from the perspective-taking ability of individuals

In complex interactive scenarios, action understanding lies at the heart of social interactions. Nevertheless, the ability to understand action intention may differ among people. The current study distinguished two groups of participants with different social intention-understanding abilities (high and low) based on a perspective-taking task to investigate the influence of social intention on object affordance under conditions of individual and cooperative action intention. In the affordance perception experiment, participants were shown a video with the presenter reaching to grasp an object in different grips and asked to classify objects into kitchen or non-kitchen items by pressing the left- or right-hand button under the two intention conditions. The results showed that the object affordance effects were modulated by the participants' understanding of social intention in the interactive scenarios. Specifically, the object affordance effects were observed only in the high perspective-taking ability group under the condition of cooperative action intention. However, under the condition of individual action intention, object affordance effects were shown in both the high and low perspective-taking ability groups, and the difference between the two groups was not significant. This study suggests that processing of object affordance depends greatly on the contextual correspondence of perception and action and that the understanding of cooperative action intention can affect the activation of object affordance.

Predictable real-time constraints reveal anticipatory strategies of coupled planning in a sequential pick and place task

Planning a sequence of two motor elements is much more than concatenating two independent movements. However, very little is known about the cognitive strategies that are used to perform fluent sequences for intentional object manipulation. In this series of studies, the participants’ task was to reach for and pick to place a wooden cylinder to set it on a place pad of three different diameters, which served to modify terminal accuracy constraints. Participants were required to perform the sequences (1) at their preferred speed or (2) as fast as possible. Action kinematics were recorded with the Qualisys motion-capture system in order to implement a real-time protocol to get participants to engage in a true interactive relation. Results revealed that with low internal constraints (at preferred speed), low coupling between the two elements of the motor sequence was observed, suggesting a step-by-step planning strategy. Under high constraints (at fastest speed), an important terminal accuracy effect back propagated to modify early kinematic parameters of the first element, suggesting strong coupling of the parameters in an encapsulated planning strategy. In Studies 2 and 3, we further manipulated instructions and timing constraints to confirm the importance of time and predictability of external information for coupled planning. These findings overall sustain the hypothesis that coupled planning can take place in a pick and place task when anticipatory strategies are possible. This mode of action planning may be the key reason why motor intention can be read through the observation of micro variations in body kinematics.

What Can You See? Identifying Cues on Internal States From the Movements of Natural Social Interactions

In recent years, the field of Human-Robot Interaction (HRI) has seen an increasing demand for technologies that can recognize and adapt to human behaviors and internal states (e.g., emotions and intentions). Psychological research suggests that human movements are important for inferring internal states. There is, however, a need to better understand what kind of information can be extracted from movement data, particularly in unconstrained, natural interactions. The present study examines which internal states and social constructs humans identify from movement in naturalistic social interactions. Participants either viewed clips of the full scene or processed versions of it displaying 2D positional data. Then, they were asked to fill out questionnaires assessing their social perception of the viewed material. We analyzed whether the full scene clips were more informative than the 2D positional data clips. First, we calculated the inter-rater agreement between participants in both conditions. Then, we employed machine learning classifiers to predict the internal states of the individuals in the videos based on the ratings obtained. Although we found a higher inter-rater agreement for full scenes compared to positional data, the level of agreement in the latter case was still above chance, thus demonstrating that the internal states and social constructs under study were identifiable in both conditions. A factor analysis run on participants' responses showed that participants identified the constructs interaction imbalance, interaction valence and engagement regardless of video condition. The machine learning classifiers achieved a similar performance in both conditions, again supporting the idea that movement alone carries relevant information. Overall, our results suggest it is reasonable to expect a machine learning algorithm, and consequently a robot, to successfully decode and classify a range of internal states and social constructs using low-dimensional data (such as the movements and poses of observed individuals) as input.

Pupils say more than a thousand words: Pupil size reflects how observed actions are interpreted

Humans attend to others' facial expressions and body language to better understand their emotions and predict goals and intentions. The eyes and its pupils reveal important social information. Because pupil size is beyond voluntary control yet reflective of a range of cognitive and affective processes, pupils in principal have the potential to convey whether others' actions are interpreted correctly or not. Here, we measured pupil size while participants observed video-clips showing reach-to-grasp arm movements. Expressors in the video-clips were playing a board game and moved a dowel to a new position. Participants' task was to decide whether the dowel was repositioned with the intention to be followed up by another move of the same expressor (personal intention) or whether the arm movement carried the implicit message that expressor's turn was over (social intention). Replicating earlier findings, results showed that participants recognized expressors' intentions on the basis of their arm kinematics. Results further showed that participants' pupil size was larger when observing actions reflecting personal compared to social intentions. Most interestingly, before participants indicated how they interpreted the observed actions by choosing to press one of two keys (corresponding to the personal or social intention), their pupils within a split second, had already given away how they interpreted the expressor's movement. In sum, this study underscores the importance of nonverbal behavior in helping social messages get across quickly. Revealing how actions are interpreted, pupils may provide additional feedback for effective social interactions.

Adults with autism spectrum disorder are sensitive to the kinematic features defining natural human motion

It has been hypothesized that individuals with Autism Spectrum Disorder (hereafter "autism") have problems perceiving biological motion, which contributes to their social difficulties. However, the ability to perceive the kinematic profile characteristic of biological motion has not been systematically examined in autism. To examine this basic perceptual ability we conducted two experiments comparing adults with autism with matched typical adults. In Experiment 1, participants indicated whether two movements-which differed in the quantity of formula-generated biological motion-were the same or different. In Experiment 2, they judged which of two movements was "less natural," where the stimuli varied in the degree to which they were a product of real movement data produced by autistic and typical models. There were no group differences in perceptual sensitivity in either experiment, with null effects supported by Bayesian analyses. The findings from these two experiments demonstrate that adults with autism are sensitive to the kinematic information defining biological motion to a typical degree-they are both able to detect the perceptual information in a same-different judgment, and as inclined to categorize biological motion derived from real models as natural. These findings therefore provide evidence against the hypothesis that individuals with autism exhibit low-level difficulties in perceiving the kinematics of others' actions, suggesting that atypicalities arise either when integrating this kinematic information with other perceptual input, or in the interpretation of kinematic information. Autism Res 2019, 12: 284-294 © 2018 International Society for Autism Research, Wiley Periodicals, Inc. LAY SUMMARY: It has previously been suggested that autistic children and adults have problems perceiving the detailed manner in which others move-that is, the subtle changes in speed as we move from point to point-which may impact on their ability to learn from, and about, others in a typical fashion. However, the results from the present two studies demonstrate that adults with autism can perceive this information, suggesting that atypicalities in processing others' movement may arise mainly as a consequence of atypical interpretation rather than perception.

Contribution of Developmental Psychology to the Study of Social Interactions: Some Factors in Play, Joint Attention and Joint Action and Implications for Robotics

Children exchange information through multiple modalities, including verbal communication, gestures and social gaze and they gradually learn to plan their behavior and coordinate successfully with their partners. The development of joint attention and joint action, especially in the context of social play, provides rich opportunities for describing the characteristics of interactions that can lead to shared outcomes. In the present work, we argue that human-robot interactions (HRI) can benefit from these developmental studies, through influencing the human's perception and interpretation of the robot's behavior. We thus endeavor to describe some components that could be implemented in the robot to strengthen the feeling of dealing with a social agent, and therefore improve the success of collaborative tasks. Focusing in particular on motor precision, coordination, and anticipatory planning, we discuss the question of complexity in HRI. In the context of joint activities, we highlight the necessity of (1) considering multiple speech acts involving multimodal communication (both verbal and non-verbal signals), and (2) analyzing separately the forms and functions of communication. Finally, we examine some challenges related to robot competencies, such as the issue of language and symbol grounding, which might be tackled by bringing together expertise of researchers in developmental psychology and robotics.

The way others move can influence what we choose

Whether pointing at a menu item or rifling through a clothes rack, when we choose we often move. We investigated whether people's tendency to copy the movements of others could influence their choices. Participants saw pairs of pictures in private and indicated which one they preferred. They then entered a virtual art gallery and saw the same picture pairs in the presence of a virtual character. Having observed the virtual character point to indicate her preference with either a high or low movement trajectory, participants indicated their preference. There was either an anatomical (same movement, same choice) or spatial correspondence (same movement, different choice) between the participant's pictures and those of the virtual character. We found that participants copied the movement made by the virtual character rather than her action goal (i.e., her choice of picture). This resulted in a shift towards the virtual character's preferences in the anatomical condition but away from her preferences in the spatial condition. This effect was driven by the observation of the virtual character's high pointing movements. In a further experiment, we did not find any significant differences in imitation behaviour in autism, although autistic participants were less consistent in their choices. Our findings demonstrate that we are not only influenced by other's choices but also the types of movements others make to indicate those choices.

Your error in my hand: An investigation of observational posterror slowing

For human beings, monitoring others' errors is essential for efficient goal-directed behavior. Indeed, the mere observation of other individuals' errors provides a rich source of information that can be used to avoid potential errors and improve our performance without direct experience. Recent studies have outlined that vicarious experience of errors influences the observer's overt behavior. This observational posterror slowing (oPES) is supposed to reflect a strategic increase in control aimed at reducing the probability of an error. Because the consequences of error observation have been exclusively investigated by means of arbitrary button-press responses, which limit the investigation to premovement processes, it is unclear whether the observation of an error also influences the online control of goal-directed actions. In the present study, for the first time, we investigated the effect of error observation on the reach-to-grasp movement by means of kinematical analysis. The results revealed that error-observation effects are not confined to premovement processes-they also strategically affect spatial movement trajectories. Our findings add substantially to previous literature, showing that the oPES spreads to movement execution when a more realistic, ecologically valid task is employed.

Behaviour versus performance: The veiled commitment of experimental psychology

The use and teaching of experimentation in psychology ought to accompany a discussion of what is within and what is beyond the reach of the method. I address this question by outlining the necessary restrictions that are prerequisite for conducting an experiment. The restrictions include establishing a fixed goal, the fulfillment of which represents successful participation in the experiment, a finite set of possible expressions for satisfying the goal, and a fixed assignment between each possible expression and how it should be performed. These restrictions entail rules that determine what counts as good participation (evaluation), and rules that determine whether a behaviour counts as participation at all (inclusion). Participants’ conformity to the rules makes experimentation possible and, more importantly, maintains the experimenter’s attention on features of performance. This selective attention, in turn, neglects participants’ capacities for adopting and violating rules, and possible alterations in their goals and interpretations. While these capacities fall beyond the scope of experimental psychology, their recognition is necessary for encountering what is not already understood at the start of a research project.

Cooperative and competitive contexts do not modify the effect of social intention on motor action

In social interactions, the movements performed by others can be used to anticipate their intention. The present paper investigates whether cooperative vs competitive contexts influence the kinematics of object-directed motor actions and whether they modulate the effect of social intention on motor actions. An "Actor" and a "Partner" participated in a task consisting in displacing a wooden dowel under time constraint. Before this Main action, the Actor performed a Preparatory action which consisted in placing the dowel at the center of the table. Information about who would make the forthcoming Main action was provided only to the Actor through headphones. Results demonstrate an exaggeration of spatial and temporal actions' parameters when acting for the Partner, in cooperative, as well as in competitive context. This finding suggests that the motor manifestation of social intention is largely determined by non-conscious implicit processes that seem little influenced by the context of social interaction.

The role of perspective in discriminating between social and non-social intentions from reach-to-grasp kinematics

Making correct inferences regarding social and individual intentions may be crucial for successful interactions, especially when we are required to discriminate between cooperative and competitive behaviors. The results of previous studies indicate that reach-to-grasp kinematic parameters may be used to infer the social or individual outcome of a movement. However, the majority of the studies investigated this ability by presenting reach-to-grasp movements from a third-person perspective only. The aim of the present study was to assess whether the ability to recognize the intent associated to a reach-to-grasp movement varies as a function of perspective by manipulating the perspective of observation (second- and third-perspective) within participants. To this end, we presented participants with video clips of models performing a reach-to-grasp movement with different intents. The video clips were recorded both from a lateral view (third-person perspective) and from a frontal view (second-person perspective). After viewing the clips, in two subsequent tasks participants were asked to distinguish between social and non-social intentions by observing the initial phase of the same action recorded from the two different views. Results showed that, when a fast-speed movement was presented from a lateral view, participants were able to predict its social intention. In contrast, when the same movement was observed from a frontal view, performance was impaired. These results indicate that the ability to detect social intentions from motor cues can be biased by the visual perspective of the observer, specifically for fast-speed movements.

Sensorimotor Coarticulation in the Execution and Recognition of Intentional Actions

Humans excel at recognizing (or inferring) another's distal intentions, and recent experiments suggest that this may be possible using only subtle kinematic cues elicited during early phases of movement. Still, the cognitive and computational mechanisms underlying the recognition of intentional (sequential) actions are incompletely known and it is unclear whether kinematic cues alone are sufficient for this task, or if it instead requires additional mechanisms (e.g., prior information) that may be more difficult to fully characterize in empirical studies. Here we present a computationally-guided analysis of the execution and recognition of intentional actions that is rooted in theories of motor control and the coarticulation of sequential actions. In our simulations, when a performer agent coarticulates two successive actions in an action sequence (e.g., "reach-to-grasp" a bottle and "grasp-to-pour"), he automatically produces kinematic cues that an observer agent can reliably use to recognize the performer's intention early on, during the execution of the first part of the sequence. This analysis lends computational-level support for the idea that kinematic cues may be sufficiently informative for early intention recognition. Furthermore, it suggests that the social benefits of coarticulation may be a byproduct of a fundamental imperative to optimize sequential actions. Finally, we discuss possible ways a performer agent may combine automatic (coarticulation) and strategic (signaling) ways to facilitate, or hinder, an observer's action recognition processes.

Examining ecological validity in social interaction: problems of visual fidelity, gaze, and social potential

Social interaction is an essential part of the human experience, and much work has been done to study it. However, several common approaches to examining social interactions in psychological research may inadvertently either unnaturally constrain the observed behaviour by causing it to deviate from naturalistic performance, or introduce unwanted sources of variance. In particular, these sources are the differences between naturalistic and experimental behaviour that occur from changes in visual fidelity (quality of the observed stimuli), gaze (whether it is controlled for in the stimuli), and social potential (potential for the stimuli to provide actual interaction). We expand on these possible sources of extraneous variance and why they may be important. We review the ways in which experimenters have developed novel designs to remove these sources of extraneous variance. New experimental designs using a 'two-person' approach are argued to be one of the most effective ways to develop more ecologically valid measures of social interaction, and we suggest that future work on social interaction should use these designs wherever possible.

Faster but Less Careful Prehension in Presence of High, Rather than Low, Social Status Attendees

Ample evidence attests that social intention, elicited through gestures explicitly signaling a request of communicative intention, affects the patterning of hand movement kinematics. The current study goes beyond the effect of social intention and addresses whether the same action of reaching to grasp an object for placing it in an end target position within or without a monitoring attendee's peripersonal space, can be moulded by pure social factors in general, and by social facilitation in particular. A motion tracking system (Optotrak Certus) was used to record motor acts. We carefully avoided the usage of communicative intention by keeping constant both the visual information and the positional uncertainty of the end target position, while we systematically varied the social status of the attendee (a high, or a low social status) in separated blocks. Only thirty acts performed in the presence of a different social status attendee, revealed a significant change of kinematic parameterization of hand movement, independently of the attendee's distance. The amplitude of peak velocity reached by the hand during the reach-to-grasp and the lift-to-place phase of the movement was larger in the high rather than in the low social status condition. By contrast, the deceleration time of the reach-to-grasp phase and the maximum grasp aperture was smaller in the high rather than in the low social status condition. These results indicated that the hand movement was faster but less carefully shaped in presence of a high, but not of a low social status attendee. This kinematic patterning suggests that being monitored by a high rather than a low social status attendee might lead participants to experience evaluation apprehension that informs the control of motor execution. Motor execution would rely more on feedforward motor control in the presence of a high social status human attendee, vs. feedback motor control, in the presence of a low social status attendee.

Bodily action penetrates affective perception

Fantoni & Gerbino (2014) showed that subtle postural shifts associated with reaching can have a strong hedonic impact and affect how actors experience facial expressions of emotion. Using a novel Motor Action Mood Induction Procedure (MAMIP), they found consistent congruency effects in participants who performed a facial emotion identification task after a sequence of visually-guided reaches: a face perceived as neutral in a baseline condition appeared slightly happy after comfortable actions and slightly angry after uncomfortable actions. However, skeptics about the penetrability of perception (Zeimbekis & Raftopoulos, 2015) would consider such evidence insufficient to demonstrate that observer's internal states induced by action comfort/discomfort affect perception in a top-down fashion. The action-modulated mood might have produced a back-end memory effect capable of affecting post-perceptual and decision processing, but not front-end perception. Here, we present evidence that performing a facial emotion detection (not identification) task after MAMIP exhibits systematic mood-congruent sensitivity changes, rather than response bias changes attributable to cognitive set shifts; i.e., we show that observer's internal states induced by bodily action can modulate affective perception. The detection threshold for happiness was lower after fifty comfortable than uncomfortable reaches; while the detection threshold for anger was lower after fifty uncomfortable than comfortable reaches. Action valence induced an overall sensitivity improvement in detecting subtle variations of congruent facial expressions (happiness after positive comfortable actions, anger after negative uncomfortable actions), in the absence of significant response bias shifts. Notably, both comfortable and uncomfortable reaches impact sensitivity in an approximately symmetric way relative to a baseline inaction condition. All of these constitute compelling evidence of a genuine top-down effect on perception: specifically, facial expressions of emotion are penetrable by action-induced mood. Affective priming by action valence is a candidate mechanism for the influence of observer's internal states on properties experienced as phenomenally objective and yet loaded with meaning.

Facilitation effect of observed motor deviants in a cooperative motor task: Evidence for direct perception of social intention in action

Spatiotemporal parameters of voluntary motor action may help optimize human social interactions. Yet it is unknown whether individuals performing a cooperative task spontaneously perceive subtly informative social cues emerging through voluntary actions. In the present study, an auditory cue was provided through headphones to an actor and a partner who faced each other. Depending on the pitch of the auditory cue, either the actor or the partner were required to grasp and move a wooden dowel under time constraints from a central to a lateral position. Before this main action, the actor performed a preparatory action under no time constraint, consisting in placing the wooden dowel on the central location when receiving either a neutral ("prêt"-ready) or an informative auditory cue relative to who will be asked to perform the main action (the actor: "moi"-me, or the partner: "lui"-him). Although the task focused on the main action, analysis of motor performances revealed that actors performed the preparatory action with longer reaction times and higher trajectories when informed that the partner would be performing the main action. In this same condition, partners executed the main actions with shorter reaction times and lower velocities, despite having received no previous informative cues. These results demonstrate that the mere observation of socially driven motor actions spontaneously influences the low-level kinematics of voluntary motor actions performed by the observer during a cooperative motor task. These findings indicate that social intention can be anticipated from the mere observation of action patterns.

Individual differences in reading social intentions from motor deviants

As social animals, it is crucial to understand others’ intention. But is it possible to detect social intention in two actions that have the exact same motor goal? In the present study, we presented participants with video clips of an individual reaching for and grasping an object to either use it (personal trial) or to give his partner the opportunity to use it (social trial). In Experiment 1, the ability of naïve participants to classify correctly social trials through simple observation of short video clips was tested. In addition, detection levels were analyzed as a function of individual scores in psychological questionnaires of motor imagery, visual imagery, and social cognition. Results revealed that the between-participant heterogeneity in the ability to distinguish social from personal actions was predicted by the social skill abilities. A second experiment was then conducted to assess what predictive mechanism could contribute to the detection of social intention. Video clips were sliced and normalized to control for either the reaction times (RTs) or/and the movement times (MTs) of the grasping action. Tested in a second group of participants, results showed that the detection of social intention relies on the variation of both RT and MT that are implicitly perceived in the grasping action. The ability to use implicitly these motor deviants for action-outcome understanding would be the key to intuitive social interaction.

Perceiving what you intend to do from what you do: evidence for embodiment in social interactions

Although action and perception are central components of our interactions with the external world, the most recent experimental investigations also support their implications in the emotional, decision-making, and goal ascription processes in social context. In this article, we review the existing literature supporting this view and highlighting a link between reach-to-grasp motor actions and social communicative processes. First, we discuss the most recent experimental findings showing how the social context subtly influences the execution of object-oriented motor actions. Then, we show that the kinematic characteristics of object-oriented motor actions are modulated by the actor's social intention. Finally, we demonstrate that naïve observers can implicitly take advantage of these kinematic effects for their own motor productions. Considered together, these data are compatible with the embodied cognition framework stating that cognition, and in our case social cognition, is grounded in knowledge associated with past sensory and motor experiences.

Motor interference in interactive contexts

Action observation and execution share overlapping neural substrates, so that simultaneous activation by observation and execution modulates motor performance. Previous literature on simple prehension tasks has revealed that motor influence can be two-sided: facilitation for observed and performed congruent actions and interference for incongruent actions. But little is known of the specific modulations of motor performance in complex forms of interaction. Is it possible that the very same observed movement can lead either to interference or facilitation effects on a temporally overlapping congruent executed action, depending on the context? To answer this question participants were asked to perform a reach-to-grasp movement adopting a precision grip (PG) while: (i) observing a fixation cross, (ii) observing an actor performing a PG with interactive purposes, (iii) observing an actor performing a PG without interactive purposes. In particular, in the interactive condition the actor was shown trying to pour some sugar on a large cup located out of her reach but close to the participant watching the video, thus eliciting in reaction a complementary whole-hand grasp. Notably, fine-grained kinematic analysis for this condition revealed a specific delay in the grasping and reaching components and an increased trajectory deviation despite the observed and executed movement’s congruency. Moreover, early peaks of trajectory deviation seem to indicate that socially relevant stimuli are acknowledged by the motor system very early. These data suggest that interactive contexts can determine a prompt modulation of stimulus–response compatibility effects.

For your eyes only: effect of confederate's eye level on reach-to-grasp action

Previous studies have shown that the spatio-temporal parameters of reach-to-grasp movement are influenced by the social context in which the motor action is performed. In particular, when interacting with a confederate, movements are slower, with longer initiation times and more ample trajectories, which has been interpreted as implicit communicative information emerging through voluntary movement to catch the partner’s attention and optimize cooperation (Quesque et al., 2013). Because gaze is a crucial component of social interactions, the present study evaluated the role of a confederate’s eye level on the social modulation of trajectory curvature. An actor and a partner facing each other took part in a cooperative task consisting, for one of them, of grasping and moving a wooden dowel under time constraints. Before this Main action, the actor performed a Preparatory action, which consisted of placing the wooden dowel on a central marking. The partner’s eye level was unnoticeably varied using an adjustable seat that matched or was higher than the actor’s seat. Our data confirmed the previous effects of social intention on motor responses. Furthermore, we observed an effect of the partner’s eye level on the Preparatory action, leading the actors to exaggerate unconsciously the trajectory curvature in relation to their partner’s eye level. No interaction was found between the actor’s social intention and their partner’s eye level. These results suggest that other bodies are implicitly taken into account when a reach-to-grasp movement is produced in a social context.

Body space in social interactions: a comparison of reaching and comfort distance in immersive virtual reality

Background

Do peripersonal space for acting on objects and interpersonal space for interacting with con-specifics share common mechanisms and reflect the social valence of stimuli? To answer this question, we investigated whether these spaces refer to a similar or different physical distance.

Methodology

Participants provided reachability-distance (for potential action) and comfort-distance (for social processing) judgments towards human and non-human virtual stimuli while standing still (passive) or walking toward stimuli (active).

Principal Findings

Comfort-distance was larger than other conditions when participants were passive, but reachability and comfort distances were similar when participants were active. Both spaces were modulated by the social valence of stimuli (reduction with virtual females vs males, expansion with cylinder vs robot) and the gender of participants.

Conclusions

These findings reveal that peripersonal reaching and interpersonal comfort spaces share a common motor nature and are sensitive, at different degrees, to social modulation. Therefore, social processing seems embodied and grounded in the body acting in space.

The visible face of intention: why kinematics matters

A key component of social understanding is the ability to read intentions from movements. But how do we discern intentions in others' actions? What kind of intention information is actually available in the features of others' movements? Based on the assumption that intentions are hidden away in the other person's mind, standard theories of social cognition have mainly focused on the contribution of higher level processes. Here, we delineate an alternative approach to the problem of intention-from-movement understanding. We argue that intentions become "visible" in the surface flow of agents' motions. Consequently, the ability to understand others' intentions cannot be divorced from the capability to detect essential kinematics. This hypothesis has far reaching implications for how we know other minds and predict others' behavior.