was carried out on mean values of the reaching-grasping and placing parameters of experiments 4 and 5. They included the within-subjects factor task (conspecific touching task vs body-shape touching task) and the between-subjects factor experiment (4 vs 5). In all analyses post-hoc comparisons were performed using the Newman-Keuls procedure. The significance level was fixed at P,0.05.When the factor was significant, we also calculated the effect size [g2p(artial)]. Results and Discussion In the first series of ANOVAs no parameter was affected by factor task except placing peak velocity, which decreased in the conspecific touching task (Table 1 and Fig. 2). Concerning the second series of ANOVAs, peak velocity of finger opening decreased when the participants interacted with the conspecific, but only in experiment 4, i.e. when the receiver’s gaze was available (interaction between task and experiment, (F(1, 22) = 5.0, p,0.05, g2p = 0.2, post-hoc test p,0.05, Fig. 2). Similarly, reach peak deceleration decreased in conspecific touching task, but in experiment 4 only (interaction between task and experiment, F(1, 22) = 5.9, p,0.05, g2p = 0.2, post-hoc test p,0.05, Fig. 2). In contrast, percentage of reach deceleration time increased in conspecific touching task in both the experiments (factor task: F(1, 22) = 4.5, p,0.05, g2p = 0.2, Fig. 2). Placing peak velocity decreased in conspecific touching task (F(1, 22) = 29.8, p,0.001, g2p = 0.6, Fig. 2). The results of experiment 5 confirm that the gaze of the human receiver plays a role in activating a social affordance. This is mainly shown by the finding that reach peak deceleration and peak velocity of finger opening decreased in conspecific touching task only when the receiver’s gaze was available. However, percentage of reach deceleration time increased even when the receiver’s gaze was not available. This result may depend on an effect of the social intention of touching in experiments 4 and 5. Since the giver’s social intention of touching was not coupled with any social request, it was less affected by the preclusion of the receiver’s gaze. Indeed, we propose that the receiver’s gaze makes effective a social request (e.g. to be fed) for the activation of a social affordance. Experiment 6 The results of experiment 5 do not completely solve the problem of whether the receiver’s gaze plays a primary role in making the social request to be fed effective to activate a social affordance. In other words, can the social intention of feeding activate a social affordance independently of the effects of the receiver’s gaze on the social request? Experiment 6 aimed at solving this problem: we compared a conspecific feeding task with a body-shape placing task, as in experiment 1, during which the (human and non- human) receiver was blindfolded. If a social affordance is only activated by a social request coupled with the gaze of the receiver, we should find no effect of the conspecific feeding task on the reach. Methods Participants. A new sample of twelve right-handed [18], na¨ıve volunteers (8 females and 4 males, age 23–27 yrs.) participated in the experiment. Apparatus and stimuli. The apparatus and stimuli were the same as those of experiment 5. Procedure, movement recording and data analysis. The participants executed a conspecific feeding task and a body-shape placing task as in experiment 1, during which the conspecific and Social Request and Social Affordance the human-body shape were blindfolded (Fig. 1). Movement recording and data analysis were the same as in experiment 1. In a second series of ANOVAs we compared experiment 6 with experiment 1; the within-subjects factor was task (conspecific feeding task vs a body-shape placing task) and the between- subjects factor was experiment (1 vs 6). Results and Discussion In the first series of ANOVAs no parameter was affected by the factor task except placing peak velocity, which decreased in the conspecific feeding task (Table 1 and Fig. 2). In the second series of ANOVAs percentage of reach deceleration time was affected by the interaction between factors task and experiment (F(1, 22) = 4.8, p,0.05, g2p = 0.2). This parameter increased during the conspe- cific feeding task as compared to body-shape placing task, in experiment 1 only (post-hoc test, p,0.05, Fig. 2). Placing time increased (F(1, 22) = 4.2, p = 0.05, g2p = 0.2) and placing peak velocity decreased (F(1, 22) = 83.04, p,0.001, g2p = 0.8) in conspecific feeding task (Fig. 2). Factor experiment affected placing time and placing peak velocity (F(1, 22) = 6.6; p,0.05, g2p = 0.2; F(1, 22) = 9.9, p,0.01, g2p = 0.3): placing time increased and placing peak velocity decreased in experiment 6 (Fig. 2). The receiver’s gaze makes the social request to be fed (mouth aperture) effective at activating a social affordance. In other words, the social intention of feeding alone is unable to activate a social affordance if it is not coupled with an effective social request. General Discussion In experiment 1, we compared the social intention of feeding a conspecific with the intention of placing a piece of food into a mouth-like aperture in a human body shape: both the intentions guided the same action sequence constituted by reaching-grasping and placing. The feeding intention increased the accuracy requirement of the overall sequence: indeed, the reaching as well as the placing slowed down. These results confirm the data of the study by Ferri and colleagues [13]. The increasing accuracy demand due to the social intention affected the action of reaching in line also with kinematic data showing that actions in a chain are related to each other [4,6–8]. The increasing accuracy demand during the control of reaching and placing might depend on the final contact with the receiver’s mouth because the participants may want to avoid touching the receiver’s lips. The results concerning variability of placing end point in experiment 1 rule out this possibility. Moreover, the results of experiment 6 showed that the final contact with the receiver’s mouth induced a decrease in placing peak velocity only, as for the case in experiment 3 when the final contact was with a mouth-like aperture (placed beside the conspecific’s face). In addition, the results of experiment 3 exclude that the increasing demand of accuracy observed in reaching and placing was due to the closeness of the hand trajectory to the conspecific’s body, which the participants might avoid touching. In fact, it was the grasp, rather than the reach, which was influenced by closer trajectories. However, in all the experiments the presence of the conspecific induced a decrease in placing peak velocity even when the sequence was directed to another final target (experiments 2 and 3). It is possible that, when planning the sequence, the maximal velocity of the placing (during which the hand trajectory was closer to the conspecific’s body) was reduced in order to facilitate a quick reaction in response to unexpected movements of the conspecific. A similar explanation may be offered for the trend of slowing down of the placing when the receiver was blindfolded. In fact, since the possibility of trunk PLoS ONE | www.plosone.org 7 January 2011 | Volume 6 | Issue 1 | e15855