Methods Participants. A new sample of twelve right-handed [18], na¨ıve volunteers (7 females and 5 males, age 22–25 yrs.) participated in the experiment. Apparatus and stimuli. The apparatus and stimuli were the same as those for experiment 2, except that the position of the sugar lump was closer (12 cm instead of 22 cm) to the conspecific and to human body-shape facing the participants, i.e. it was farther (26 cm distant) from the SP (Fig. 1). Moreover, the conspecific’s mouth remained closed before and during the trial. Correspondingly, a wooden plate covered the mouth-like aperture. Procedure, movement recording and data analysis. The procedure, movement recording and data analysis were the same as in experiment 2. Results and Discussion Maximal finger aperture increased and placing peak velocity decreased in the close-to-conspecific placing task as compared to the close-to-body-shape placing task (Table 1 and Fig. 2). The closeness of hand trajectory to the conspecific’s body induced an increase in maximal finger aperture. Larger maximal finger apertures allow compensation for an increase in uncertainty in the hand trajectory control [20]. When the fingers moved inside the peripersonal space during the final reaching-grasping the salience of the context probably increased. Thus, the attention of the agent focused on the conspecific’s body to a greater extent causing uncertainty and less control of hand trajectory. By comparing the results of experiment 2 with those of experiment 3 we deduce that the social request to be fed (i.e. the conspecific’s mouth aperture) is sufficient to activate a social affordance even when the giver does not actually interact with a present conspecific, and, in particular, with her mouth. This slows down the second phase of reach of the sequence not actually finalized to feed. In contrast, the closeness of hand trajectory to the conspecific’s body has a minor effect on the reach kinematics, whereas it greatly affects the grasp (see above). Experiment 4 The results of experiments 1 and 2 suggest that the effect of the social request (and of the corresponding social affordance) was stronger in experiment 1 than in experiment 2, i.e. when the social intention and social request were congruent. Indeed, in experiment 1, the social affordance affected the percentage of deceleration time, which also takes into account the first (acceleration) phase of reach, whereas in experiment 2, it affected reach peak deceleration which concerns the second (deceleration) phase of reach only. In experiment 2 a direct interaction with the conspecific was precluded and the hand trajectory during the placing directed away from the conspecific’s face. On the basis of these data we hypothesized that a greater effect of the social request could be found during direct interactions with the conspecific, even if they are not finalized to feed, and when hand trajectories were closer to the mouth. In order to test this hypothesis, in experiment 4 participants reached-grasped the sugar lump and with this in their hand they touched the conspecific’s forehead (conspecific touching task) or the ‘‘forehead’’ of the body-shape (body-shape touching task). The conspecific’s mouth was opened as in experiment 2 and the hand trajectory was closer to the conspecific’s body as in experiment 3. Methods Participants. A new sample of twelve right-handed [18], na¨ıve volunteers (8 females and 4 males, age 23–25 yrs.) participated in the experiment. Social Request and Social Affordance Apparatus and stimuli. The apparatus and stimuli were the same as those of experiment 1. However, differently from experiment 1, the position of the sugar lump was the same as that in experiment 3. Procedure, movement recording and data analysis. The participants reached-grasped the sugar lump and with this in their thumb and index finger touched the conspecific’s forehead (conspecific touching task) or the ‘‘forehead’’ of the body-shape (body-shape touching task) (Fig. 1). The conspecific’s mouth was opened as in experiment 2. The remaining procedure and movement recording were the same as in experiment 1 except that one reference marker was attached to the forehead of the human or non-human receiver, in addition to the three markers attached to the thumb, index finger, and wrist of the participant. In the ANOVAs the within-subjects factor was task (conspecific touching task vs body-shape touching task). Results and Discussion Peak velocity of finger opening, and reach peak deceleration decreased, whereas percentage of reach deceleration time showed a trend to increase, in the conspecific touching task as compared to the body-shape touching task (Table 1 and Fig. 2). Placing peak velocity decreased and placing time showed a trend to increase in the conspecific touching task (Table 1 and Fig. 2). A direct interaction with the conspecific and a hand trajectory closer to the mouth increased the effect (i.e. the accuracy demand) of the social request to be fed and of the corresponding social affordance on a sequence not finalized to feed. The social affordance also affected the grasp since peak velocity of finger opening decreased. Consequently, the effect of the hand trajectory closeness to the conspecific’s body found in experiment 3 (i.e. the increase in maximal finger aperture) was removed by the lower velocity of finger opening. Moreover, the results of both experiment 2 and 4 confirm that the social request affects the control of all the sequence rather than the final placing phase. In fact, the givers did not actually interact with the receiver’s mouth. This was suggested in experiment 1by the results concerning variability of the placing end point. Experiment 5 The results of experiment 4 do not exclude that other factors inherent in the conspecific’s face are responsible for effects on hand movements finalized to touch the conspecific’s forehead; for example, the gaze of the conspecific. Indeed, it is well known that the conspecific’s gaze can be a signal in order to initiate a communication ([15]; see also [16], concerning the structures in the social brain activated by the ‘‘eye contact effect’’). Moreover, during interactions it can be a signal to make a social request effective and, consequently, to activate a social affordance. This hypothesis was tested in experiment 5, in which the same sequence of actions as in experiment 4 was directed to a blindfolded conspecific or a ‘‘blindfolded’’ body-shape. Methods Participants. A new sample of twelve right-handed [18], na¨ıve volunteers (6 females and 6 males, age 24–26 yrs.) participated in the experiment. Apparatus and stimuli. The apparatus and stimuli were the same as those of experiment 4, except that the conspecific and the human-body shape were blindfolded (Fig. 1). Procedure, movement recording and data analysis. Procedure, movement recording and data analysis were the same as in experiment 4. In addition, a second series of ANOVAs PLoS ONE | www.plosone.org 6 January 2011 | Volume 6 | Issue 1 | e15855