Effect of confederate’s eye level on reach-to-grasp action

Introduction

 Humans live in social groups and spend much time engaging in cooperative actions (Richerson & Boyd, 1998) or interpreting observed behaviors (Barresi & Moore, 1996), even when there is no clear intention to interact with conspecifics (Frith & Frith, 2006). Motor actions have the special feature of being influenced by both the goal pursued (Ansuini, Giosa, Turella, Altoe & Castiello, 2008; Ansuini, Santello, Massaccesi & Castiello, 2006; Marteniuk, Mackenzie, Jeannerod, Athenes & Dugas, 1987; Naish, Reader, Houston-Price, Bremner & Holmes, 2013) and the social context in which they are performed (Ferri, Campione, Dalla Volta, Gianelli, & Gentilucci, 2011a; Gianelli, Scorolli, & Borghi, 2011; Innocenti, De Stefani, Bernardi, Campione, & Gentilucci, 2012; Quesque, Lewkowicz, Delevoye-Turrell, & Coello, 2013; Scorolli, Miatton, Wheaton & Borghi, 2014). As a consequence, observers can detect, from kinematic variations in motor performances, the goal of an ongoing action before it is entirely executed (Elsner, Falck-Ytter, & Gredebäck, 2012; Lewkowicz, DelevoyeTurrell, Bailly, Andry, & Gaussier, 2013; Orliaguet, Kandel & Boë, 1997; Stapel, Hunnius, & Bekkering, 2012) and even the actor’s social intention (Manera, Becchio, Cavallo, Sartori & Castiello, 2011; Sartori, Becchio & Castiello, 2011; Sebanz & Shiffrar, 2009). 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 & Shiffrar, 2007 for a review). It has been suggested that the implicit process of socially relevant motor features could optimize 128 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, Baldwin & Ashburn, 2002; Brand & 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, Becchio, Bara & Castiello, 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, Sartori, Bulgheroni, & Castiello, 2008; 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 129 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 & Cook, 1976; Becchio, Bertone & Castiello, 2008; Kleinke, 1986; Langton, Watt, & Bruce, 2000) from the early days of life (Farroni, Csibra, Simion & Johnson, 2002). In comparison with other primate species, humans have especially visible eyes (Kobayashi & 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., 2011a; 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 eye, 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 spatiotemporal 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, Nettle & Roberts, 2006; Ernest-Jones, Nettle, Bateson, 2011).

Material and Methods 

Participants

Twenty-one healthy, right-handed (as determined by the Edinburgh Handedness Inventory, Oldfield, 1971) adults (mean age = 21.05 years, SD =1.96 years, 4 males) were tested. They had no prior knowledge about the scientific aim of the study and provided their written informed consent before participating. The protocol followed the general ethics rules defined by the local ethics committee and was in accordance with the principles of the Declaration of Helsinki (World Medical Organization, 1996). The experimenter (the first author of this paper) was a 24-year-old man who played the role of the social partner for all participants. 

Apparatus and stimuli

 Participants and the partner sat on either side of a table (120 x 80 cm), facing each other. 2 cm x 2 cm black markings on the table indicated three specific locations, which will be hereinafter referred to as the initial, central and final positions. In addition, the starting positions used for the right hand of the participants and the partner were indicated by black markings located at each edge of the table. The object to be manipulated was a wooden dowel (diameter 2 cm, length 4 cm), which was to be moved from one spatial landmark to the next following a defined sequence, each movement in the sequence being triggered by an auditory cue (see Figure 1). 131 Figure 1. Top view picture of the experimental setup with the initial, central and final positions, the starting position of the participants, and an illustration of the Preparatory, Main and Repositioning actions. 

Procedure 

The task for the participants was to reach and grasp the wooden dowel using their thumb and index finger and move it from one position to the next in a sequence of three successive actions. Before performing each action, both the participants and the partner were requested to remain stationary with their thumb and index finger pinched together and resting upon the starting position. Each sequence started with the wooden dowel placed at the initial position. The first action was the Preparatory action, which consisted of moving the wooden dowel from the initial to the central position with no specific time constraints. The second action was the Main action, in which the wooden dowel was moved horizontally from the central to the final position as fast as possible. The third action was the Repositioning action, in which the wooden dowel was moved from the final to the initial position with no specific 132 time constraints, thus setting up for the next sequence. Time constraints were thus only applied to the Main action, in which the velocity of the participant’s wrist had to be more than 80% of the maximum reachable velocity (computed from the peak velocity recorded in a previous practice session, see below and Quesque et al., 2013 for a detailed description). Each movement was triggered by a specific auditory cue, always broadcast in the same order (cue 1 initiated the Preparatory action; cue 2 the Main action; cue 3 the Repositioning action). Thus, participants and the partner had their right hand on the starting positions before initiating any of the movements in the sequence, while their left hand remained in their lap. When the participant or the partner was acting, the other person had to keep motionless. Furthermore, participants were not allowed to communicate and were asked to fix their gaze on the table during the course of the experiment in all sessions. In order to prevent participants from anticipating the time of movement initiation, between-sequences intervals varied randomly between 3 and 3.5 seconds. In addition, the interval between the 1st and 2nd auditory cue was varied randomly between 3.5 and 4 s while the interval between the 2nd and 3rd auditory cue was fixed at 2 s in order to provide feedback on the participant’s performance immediately after they had completed the Main action. Participants performed four successive sessions of 25 sequences of action. In these sessions, the Main action was carried out by either the participant or the partner (block trials), with the seat of the partner being either at the same height as or higher than that of the participant (block trials). The eye level of the partner was manipulated using an adjustable seat, which was either at the same height as that of the participant (0 cm condition) or 5 cm higher (5 cm condition, counterbalanced order). In order to minimize the risk that participants detected this manipulation, the two height conditions were performed on different days separated by one week and the height of the seat was adjusted before the arrival of the 133 participants. In each of these conditions, the participants and the partner performed the Main action in two block sessions presented in a counterbalanced order on the same day. Then, depending on the session, when performing the Preparatory action, participants could place the wooden dowel for themselves (personal intention) or for their partner (social intention). 

Practice sessions

 Before the experimental session started, all participants underwent two practice blocks, each containing 15 sequences of action. The first practice block was done to obtain an estimate of the maximum speed at which participants could grasp the wooden dowel from the central position and place it on the final position. An adjustment procedure similar to the one used in Quesque et al. (2013) was selected. The second practice block was done to check that instructions were understood and that the different cues were accurately identified and the appropriate motor responses provided. 

Data recording and analysis

 Participants’ motor performances were recorded using Qualisys 4 Oqus infrared cameras (Qualisys AB, Gothenburg, Sweden). Infrared reflective markers were placed on the forefinger (base and tip), thumb (tip) and wrist (scaphoid) of the right hand of participants. An additional marker was placed on the wooden dowel. Cameras were calibrated before each session, enabling the system to reach standard deviation accuracies of less than 0.2 mm, at a 200 Hz sampling rate. Only the Preparatory action data were analyzed, because the social influence on motor performances can be estimated only from this action. The Preparatory action was characterized by a reaching phase (reach-to-grasp action) and a transport phase (moving the wooden dowel from the initial to the central position). The focus was on 134 movement parameters that are known to be affected by social intentionality, namely reaction time, movement time, peak wrist velocity, and height of the trajectory in the reaching and transport phases (Becchio, Sartori, Bulgheroni, & Castiello, 2008; Quesque, Lewkowicz, Delevoye-Turrell, & Coello, 2013; Quesque, Delevoye-Turrell, & Coello, submitted). Reaction time, movement time and trajectory elevation were computed from the 3D coordinates of the reflective marker placed on the wrist of participants. Temporal and kinematic parameters of the (x,y,z) coordinates of the wrist marker were computed from tangential velocity profiles after filtering the data using a second-order Butterworth dual pass filter (cutoff frequency: 15 Hz). Movement onset was defined as when the first velocity value reached 20 mm.s-1. Movement end was defined as the time the velocity profile reached the minimum value following the peak velocity of the transport phase. Reaction time corresponded to the time separating the Preparatory action auditory cue from movement onset. Movement time corresponded to the time separating movement onset from movement end. Peak wrist velocity corresponded to the maximum velocity reached by the wrist during the grasping and transport phase, respectively. The maximum height of trajectory was defined as the maximum z coordinate of the wrist measured in the grasping and transport phases. Concerning reaction time, a 2 (Intention: Social vs. Personal) x 2 (Partner’s eye level: 0 cm vs. 5 cm condition) ANOVA was conducted. Concerning movement time and kinematic parameter analysis, 2 (Intention: Social vs. Personal) x 2 (Partner’s eye level: 0 cm vs. 5 cm condition) x 2 (Movement phase: Grasping vs. Transport) ANOVAs were conducted, all variables being associated with within-participants measures. The significance level was set at .