attributed these effects to the social affordance of the sequence, since placing an object in the conspecific’s hand is performed in order to ‘‘give’’ and is characteristic of joint actions. Ferri, Campione, Dalla Volta, Gianelli and Gentilucci [13] found that when a giver reaches to grasp and places a piece of food into the mouth of a human receiver in order to feed her, the final phase of reaching and the placing slow down. This was relative to the execution of the same sequence directed to a mouth-like aperture on the ‘‘face’’ of a human body shape (non-human receiver). In this study the interaction with a conspecific likely activated a social affordance, too. The results of these studies [12,13] suggest that the social affordances guiding approaching action sequences increases the accuracy demand during the execution of these sequences. Alternatively, the increasing demand of accuracy may be explained by the fact that such actions are executed inside the conspecific’s peripersonal space, where the probability of touching the receiver’s body is higher [14]. In addition, if actions approaching a conspecific activate social affordances that increase accuracy demand, the question arises if and how the correspond- ing social requests (i.e. the request gestures) play a role in activating social affordances and consequently in modifying the kinematics of the sequences. We addressed these problems in the present kinematic study through six experiments in which we compared sequences guided by social affordances related to approaching a conspecific (human receiver), with sequences guided by affordances related to approaching an inanimate target (non-human receiver). In baseline experiment 1, participants (the givers) reached-grasped a sugar lump and placed it into either the mouth of a conspecific (i.e. fed her) or a mouth-like aperture in a human body shape (i.e. placed it). Distances and size of the two final targets, i.e. their indices of difficulty according to the Fitts’ law [1], were the same. Consequently, if the interaction with conspecifics increases the accuracy demand, a specific social affordance is likely activated, and the kinematics of reaching-grasping as well as of placing should be slowed down. If the hypothesis is incorrect, no slowing down of movement should be observed. In experiments 2, 3 and 4 we tested the role of the peripersonal space in affecting the accuracy requirements of the sequence (experiment 3) and the role of the social request to be fed (experiments 2 and 4) in activating the corresponding social affordances. Specifically, in experiments 2 and 4, we verified whether a social affordance is activated by the social request to be fed even when the sequence is directed to the conspecific in order to place (without any direct interaction with the conspecific, experiment 2) and to touch (experiment 4) rather than to feed. Previous studies showed that during social interactions the receiver’s gaze can be a signal in order to initiate a communication and even a joint action [15–17]. Thus, we aimed at verifying whether the receiver’s gaze plays a role in making a social request effective at activating a social affordance (experiments 5 and 6). Experiment 1 In experiment 1, we tested whether and to what extent the interaction with a conspecific guided by the social intention of feeding modifies the kinematics of a sequence constituted by reaching-grasping and placing. In other words, we aimed at verifying whether a social affordance was activated. The activation of a social affordance can concern the interaction either with a specific part of the conspecific’s body (in the present experiment, the mouth) or with the entire conspecific’s body. Indeed, in the present experiment, the reaching and the initial placing were dire Social Request and Social Affordance cted towards the conspecific’s chest (Fig. 1). If the first hypothesis is correct the slowing down of movement should produce a decrease in the variability of the placing end point, since the givers reduced the effective target width (size disposable to introduce the food into the mouth) in order to avoid touching the receiver’s lips. If the second hypothesis is correct, a decrease in the variability in the placing end point is unlikely to be found. Methods Participants. Twelve right-handed [18], na¨ıve volunteers (4 females and 8 males, age 22–25 yrs.) participated in the experiment. The Ethics Committee of the Medical Faculty at the University of Parma approved the study. The experiments were conducted according to the principles expressed in the Declaration of Helsinki. We obtained written informed consent from all participants in the present study. Apparatus and stimuli. The participants (the givers) sat in front of a table on which they placed their right hand with the thumb and index finger in pinch position (Starting Position, SP). Depending upon the task condition, either an experimenter (the human receiver) sat, or a human body shape (the non-human receiver) was placed in front of them. The same female experi- menter participated as human receiver in all experiments. The human receiver’s chest, or the body shape, was 38 cm distant from the SP. The body shape was a wooden panel, the outline of which resembled the head, and the upper trunk of a human body. The ‘‘face’’ of the human-like shape had an ellipse-shaped slit resembling a mouth (mouth-like aperture). The size of this mouth- like aperture was approximately the same as that of the human receiver’s mouth (when it was opened) and the distance of its center from the table plane (42.5 cm) was equivalent to that for the human mouth. Behind the mouth-like aperture, a support allowed an easy placing of a small object. The target of the reach-grasp action (see below) was a sugar lump (cube of 16161cm) placed on the table plane in front of the participant at a distance of 16 cm from SP. Procedure. The participants (the givers) executed the following two tasks depending on whether either the conspecific (i.e. the human receiver) or the body shape (i.e. the non-human receiver) was present: 1) reaching-grasping and placing the sugar lump into the conspecific’s mouth (conspecific feeding task), 2) reaching-grasping and placing the sugar lump into the mouth-like aperture (body-shape placing task). The participants grasped the sugar lump using their right thumb and index finger (i.e. with a precision grasp). In both tasks, the participant was required to move with a natural velocity as during spontaneous movements and to put carefully the sugar lump into the mouth or the mouth- like aperture. Figure 1 shows the apparatus, stimuli, and tasks of the experiment. In the conspecific feeding task, the receiver’s mouth was opened, before and during the trial. In all experiments her gaze was directed at a point just beyond the participants’ left face. The receiver never came into eye contact with the giver in order to avoid that a mutual gaze interfered with the visual control of the execution of the sequence. The participants were requested to look at the opened mouth or the mouth-like aperture in front of them before starting the motor sequence; once the ‘‘GO’’ signal was given, they were free to look at the scene as during natural interactions with objects and people. The two tasks were executed in blocks of 8 trials with counterbalanced order across the participants. Data recording. Movements of the participants’ right hand were recorded using the 3D-optoelectronic SMART system (BTS Bioengineering, Milano, Italy). This system consists of six video cameras detecting infrared reflecting markers (spheres of 5-mm PLoS ONE | www.plosone.org 2 January 2011 | Volume 6 | Issue 1 | e15855