Social Request and Social Affordance Figure 1. Experimental set-up, stimuli and examples of the action sequences performed by the participants in experiments 1–6. White lines represent examples of wrist trajectories. The actor and participant have seen this manuscript and figure and have provided written consent for publication. doi:10.1371/journal.pone.0015855.g001 diameter) at a sampling rate of 120 Hz. Spatial resolution of the system is 0.3 mm. Recorded data were filtered using a linear smoothing low pass filter, i.e. a triangular filter where each value was the weighted mean computed over 5 samples (window duration: 33.3 ms). We used three markers attached to the tip of the index finger, the thumb, and to the wrist of the participant’s right hand. Other two markers were attached one to the upper and one to the lower lip of the human receiver, or in the case of the non-human receiver to the upper and lower edges of the mouth-like aperture, and were used as reference points. We analyzed the time course of the distance between the two markers placed on the two fingertips to study the grasp. The grasp time course starts with the hand in pinch position, and is constituted by a finger opening phase until a maximum (maximal finger aperture) followed by a phase of finger closing on the object [19]. We analyzed peak velocity of finger opening, and maximal finger aperture. The kinematics of the marker placed on the wrist was used to study the reaching and placing. We analyzed the following reach parameters: reach peak velocity, reach peak deceleration, and percentage of reach deceleration time (duration of the deceleration with respect to reach time). Peak velocity is a parameter related to the control of the first (acceleration) phase of reach, whereas percentage of deceleration time and peak deceleration are parameters related to the control of the second (deceleration) phase of reach. Percentage of deceleration time also takes into account the first phase of reach, whereas peak deceleration concerns the control of the second phase of reach only. The placing parameters analyzed were placing time, placing peak velocity and variability (SD) of placing end point along participants’ Y and Z axes (YSD and ZSD), i.e. the variability on the receiver’s coronal plane. The methods for calculating the beginning and end of reach and grasp is described elsewhere [10]. The frame successive to the end of reach was considered to be the time of placing beginning. In order to determine the placing end we searched for the frames when, along the longitudinal, vertical and transverse axes of the participant, the displacement of the marker was smaller than 0.3 mm (spatial resolution of the system) with respect to the previous frame. The last frame was then selected as time of placing end. Data analysis. ANOVAs were carried out on the mean values of the reaching-grasping and placing parameters. The within- subjects factor was task (conspecific feeding task vs body-shape placing task). 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 As compared to the body-shape placing task, the conspecific feeding task showed an increase in percentage of reach PLoS ONE | www.plosone.org 3 January 2011 | Volume 6 | Issue 1 | e15855