Humans live in social groups and spend much time engaging in cooperative actions (Richerson and Boyd, 1998) or interpreting observed behaviors (Barresi and Moore, 1996), even when there is no clear intention to interact with conspecifics (Frith and Frith, 2006). Motor actions have the special feature of being influenced by both the goal pursued (Marteniuk et al., 1987; Ansuini et al., 2006, 2008; Naish et al., 2013) and the social context in which they are performed (Ferri et al., 2011; Gianelli et al., 2011; Innocenti et al., 2012; Quesque et al., 2013; Scorolli et al., 2014). As a consequence, observers can detect, from kinematic variations in motor performances, the goal of an ongoing action before it is entirely executed (Orliaguet et al., 1997; Elsner et al., 2012; Stapel et al., 2012; Lewkowicz et al., 2013) and even the actor’s social intention (Sebanz and Shiffrar, 2009; Manera et al., 2011; Sartori et al., 2011). For instance, Manera et al. (2011) showed that observers could readily categorize from movement information whether an object was grasped to perform an individual action or with the intention of socially cooperating. In line with this specific sensitivity to social cues borne by action, recent neuroimaging studies highlighted the capacity of the brain to discriminate very rapidly from the optic flow information relating to human bodies (see de Gelder et al., 2010 for a review) and bodily expressions (see Blake and Shiffrar, 2007 for a review). It has been suggested that the implicit process of socially relevant motor features could optimize cooperation between agents and contribute to the selection of adapted responses depending on the social demands (Gallagher, 2008).

The role of sensorimotor cues in social interactions is a particular aspect of human communication that originates from the very early motor experiences that infants share with their parents (Brand et al., 2002; Brand and Shallcross, 2008). The so-called “motionese” strategy reflects the fact that parents exaggerate their movements when addressing their children. Although less accentuated in later life, this effect does not seem restricted to childhood since changes in kinematics have also been observed when communication occurs between adults in pointing (Cleret de Langavant et al., 2011) and grasping tasks (Sartori et al., 2009). In the latter experiment, participants were asked to reach, grasp, and lift colored spheres for an individual or cooperative purpose requiring an observer to decode a message from the alternation of colors via a simplified Morse code. Although the goal of the motor action was identical in the two conditions for the actor, the reach-to-grasp movements were performed differently when there was a social communication constraint. More precisely, the reaching movements were slower with less straight trajectories in the communicative than in the non-communicative condition. Thus, it appears that when endorsing social intention – that is, when other actors are crucial elements for satisfying the intended goals (Ciaramidaro et al., 2007) – humans tend to modify the kinematics of their motor behaviors, even when there is no explicit instruction to communicate. In agreement with this, when actors move an object to allow a partner (rather than themselves) to perform a goal-directed action, they move and place the objet using a more curved trajectory (Becchio et al., 2008b; Quesque et al., 2013) and a longer movement initiation time (Quesque et al., 2013). Such an increase in movement amplitude has been interpreted as an implicit strategy to catch the partner’s attention and communicate social intention (Quesque et al., 2013); the movements being performed with a higher amplitude due to the partner’s eye level representing a social target that influences the implementation of goal-directed action.

Supporting the assumption of an influence of eye level on cooperative tasks, several studies have pointed out the predominant role of gaze in human social interactions (Argyle and Cook, 1976; Kleinke, 1986; Langton et al., 2000; Becchio et al., 2008a) from the early days of life (Farroni et al., 2002). In comparison with other primate species, humans have especially visible eyes (Kobayashi and Kohshima, 1997) which renders their gaze direction much more salient, thus facilitating cooperative behaviors and joint actions. In studying how social context affects movement kinematics, recent research has led to the conclusion that the appropriate direction of a partner’s gaze is a prerequisite to effective social interactions (Ferri et al., 2011; Innocenti et al., 2012; Scorolli et al., 2014).

In this context, the present study aimed to evaluate the effect of a partner’s eye level on the execution of individual or cooperative voluntary reach-to-grasp movements. If hand elevation when performing an action in a social context is influenced by the height of a partner’s eyes, as suggested by previous studies, hand trajectories would be expected to be higher when a motor action was performed in the presence of a partner taking a higher seated position. Furthermore, this study investigated whether the effect of eye level on the spatio-temporal parameters of motor responses depends on the communicative context, i.e., when a social intention is endorsed, or if it depends on a more implicit influence occurring even in the absence of any social interaction (e.g., Bateson et al., 2006; Ernest-Jones et al., 2011).

Trials were excluded from the data analysis when a participant responded erroneously, when the marker was not correctly recorded during the movement, or when the reaction time was shorter than 200 ms or longer than 2.5 SDs from the median (Leys et al., 2013) computed from the Preparatory actions. 5.2% of the trials, homogenously distributed across the conditions, were thus excluded.

In the present study, we examined the role of the eye level of a confederate in the execution of individual or cooperative voluntary reach-to-grasp movements. First of all, our data confirm previous findings concerning the effect on an actor of endorsing social intention. Analyses of the Preparatory action revealed that participants took more time to initiate their action, which was performed at a slower speed and with a higher hand trajectory, when they placed the wooden dowel knowing that the Main action would be performed by the partner (Becchio et al., 2008b; Quesque et al., 2013). It is also worth noting that, although spatio-temporal variations relating to social intention were quite subtle (around 80 ms for reaction time, 20 ms for movement duration and 2 mm for wrist elevation), they were significant and consistent across different studies (Quesque et al., 2013). Taken together, these effects support the hypothesis that when endorsing a social intention, humans tend to exaggerate the spatio-temporal parameters of their movements, probably in order to facilitate the confederate’s detection of the motor and social goals of the planned action, and thus improve cooperative situations. This interpretation is supported by the findings showing that humans tend to increase the amplitude of their actions when performing explicit intentional communicative (pantomime) compared to non-communicative (actual use) object-related movements (Hermsdörfer et al., 2006, 2012).

The novelty of this study is that the unnoticed modification of the body characteristics of a partner had a sharp effect on the spatio-temporal parameters of object-oriented voluntary action. In particular, modifying the partner’s eye level had an effect on the Preparatory action with participants producing movements with a higher amplitude when the partner’s seat was 5 cm higher than when it was at the same height as their own, suggesting that the properties of the other person’s body are implicitly taken into account when producing a motor action in a social context. These modulations of hand trajectory when acting in a social context might reflect specific attention allocation to several sources of information, as requested by the task (Howard and Tipper, 1997; Diedrichsen et al., 2004). For example, one may speculate that when a person performs a voluntary motor action in the presence of a partner, the latter’s eye level represents a spatial target influencing the movement parameters specified to reach a particular object in the environment. The fact that social context influences object-oriented motor actions has already been suggested in previous studies (e.g., Cleret de Langavant et al., 2011; Gianelli et al., 2011; Quesque et al., 2013). In particular, Cleret de Langavant et al. (2011) showed that trajectories of pointing movements performed after giving a verbal instruction to a confederate slightly shifted in the direction of the confederate compared to pointing movements performed in a non-communicative context. The new result here is that the height of the confederate is also considered when planning and executing object-oriented motor actions. Though the present study focuses on eye level, it is worth noting that other cues might have contributed to the observed effect, as for instance body, shoulder or head height, the bending of the head or even a change in arm posture. However, previous works have shown that gaze is a crucial component of social interactions (Argyle and Cook, 1976; Kleinke, 1986; Langton et al., 2000; Becchio et al., 2008a) and that gaze direction influences motor kinematics in a social context (Ferri et al., 2011; Innocenti et al., 2012; Scorolli et al., 2014). No previous study has shown that body height in itself or a change in arm posture influences motor kinematics in a social context. Taken together, these data suggest that eye level contributes to the social effect observed in the present study, though whether other information contributes to the effect remains an open question.

Strikingly, the effect of the partner’s eye level appeared both when social intention was endorsed and when participants followed personal goals. In fact, no interaction was found between the factors Social intention and Partner’s eye level, suggesting that even in the absence of communicative instructions, the gaze characteristics of a conspecific are taken into account when planning an object-oriented motor action. These results confirm the special importance of human bodies in motor performances in a social context (Cleret de Langavant et al., 2012). They also corroborate previous findings that the presence of conspecifics automatically leads to considering their perspectives (Mainwaring et al., 2003; Tversky and Martin Hard, 2009; Qureshi et al., 2010; Samson et al., 2010) and to processing objects in the environment with reference to them (Becchio et al., 2011). It remains possible that changing the eye level of the partner influenced the head-eye coordination strategies of participants, resulting in a change in movement control. However, although head-eye movements were not recorded, this could hardly account for the observed effect of the partner’s eye level on movement amplitude since the participants and the partner had to keep their gaze on the wooden dowel when either was acting. It is worth noting that, in our study, the interactions between participants and the partner occurred in a pre-specified cooperative context. It thus remains to be established whether the influence of conspecific gaze characteristics on motor performances is still effective when the conspecifics are no longer partners but competitors. It would also be interesting to evaluate in future research whether the influence of body characteristics of conspecifics arises in a non-predefined communicative context and in a multi-agent social context.

In conclusion, although further investigations are necessary to unravel the effect of social intention on voluntary motor action, the present study demonstrates that the body characteristics of a partner, in particular their eye level, are implicitly taken into account when performing a motor action in cooperative and non-cooperative tasks. This suggests that the conspecific’s body represents one of the crucial variables that constrain motor planning and execution.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.