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Participants were 23 undergraduate psychology students from the University of Leuven (5 male and 18 female, mean age = 21.1 years). They received course credits for their participation. All had normal or corrected-to-normal vision, had provided informed written consent, and were naïve with respect to the purpose of the study. The study was approved by the Ethical Committee of the Faculty of Psychology of the University of Leuven and was conducted in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki.
Stimuli consisted of two point-light figures, each consisting of 13 markers indicating the centre of the major joints of the actor (head, shoulders, elbows, wrists, hips, knees, and feet). Ten point-light stimuli were employed, five belonging to the communicative condition, five belonging to the individual condition. Stimuli for the communicative condition displayed a communicative interaction between two agents, with agent A performing a communicative gesture towards a second agent (B), who responded accordingly (e.g., A asks B to squat down, B squats down). Stimuli for the communicative condition were selected from the Communicative Interaction Database (CID, [12]), and included: ‘Get down’, ‘Pick it up’, ‘Look at that ceiling’, ‘Help yourself’, and ‘Sit down’. Stimuli for the individual condition were created by substituting agent A's communicative action with a non-communicative action with the same onset and duration (‘Turn’, ‘Jump’, ‘Sneeze’, ‘Lateral step’, ‘Drink’). In both the communicative and the individual conditions, the action by agent B (e.g., ‘picking something up’) was always coupled with a fixed action by agent A (‘pointing to something to be picked up’ in the communicative condition; ‘jumping’ in the individual condition). Stimuli were constructed in accordance with the motion capture procedures described in detail by Dekeyser, Verfaillie, and Vanrie [13]. For the communicative condition, the actions of the two actors were captured at the same time, in order to guarantee that B's response matched A's communicative gesture in all respects (e.g., timing, position, and kinematics). The distance between A and B during stimulus acquisition was about two meters. A and B were always visible but the onset of A's action always preceded that of B. For the individual condition, A's action was captured while the actor was acting alone, and was then coupled with B's action, so as to maintain the same temporal structure as in the communicative interaction (i.e., A's action had the same onset and duration as in the communicative condition). Stimulus duration ranged from 3600 to 8200 ms (duration of A's actions ranged approximately from 2000 to 2600 ms; duration of B's actions ranged approximately from 2200 to 6700 ms). In both the communicative and the individual conditions, agent A and agent B remained approximately at a constant distance from the centre of the screen for the whole duration of the action and never physically touched one another. In all action stimuli (in both the individual and in the communicative conditions), they always faced each other. Recognisability of the selected stimuli: In order to assess the efficacy of stimuli included in the Communicative Interaction Database, Manera and colleagues [12] examined how well each stimulus was spontaneously recognized. Communicative stimuli and non-communicative control stimuli showing two agents acting independently of each other were presented to 54 naive observers. Participants were asked, first, to decide whether the two agents (A and B) were communicating or acting independently of each other and, second, to provide a short description of the actions of both agents. Results revealed that, on average, the stimuli were correctly recognized as communicative by more than 85% of the participants; the communicative gesture of the action stimuli was correctly identified by more than 64% of the participants. For the stimuli included in the present study, the percentage of participants who correctly classified the action stimuli as communicative varied from 72% (‘Help yourself’) to 96% (‘Get down’). The percentage of participants who also correctly identified the specific communicative gesture ranged from 37% (‘Help yourself’) to 93% (‘Get down’). Control individual stimuli included in the present study were rarely classified as communicative. The percentage of participants who erroneously classified the individual stimuli as communicative varied from 23% (‘Sneeze’) to 0% (‘Jump’). The percentage of participants who correctly identified the specific individual gesture ranged from 72% (‘Sneeze’) to 98% (‘Jump’).
Stimuli were displayed on a 21 inch CRT monitor (refresh rate = 120 Hz) using MatLab (7.1 version) software. Viewing distance was 57 cm. Dots (subtending approximately 0.14 deg each) were black against a grey background and were rendered from a three-quarter view (corresponding to the 125° reference orientation used in the CID). The visual angle between the points attached to the head and the feet was about 7.15 deg. Participants were tested individually in a dimly lit and sound attenuated room. A two-alternative forced-choice (2AFC) paradigm was employed. Each trial consisted of two intervals, a target interval (containing agent B) and a nontarget interval (not containing agent B), with a 500 ms fixation cross (black against a grey background) in between. In the target interval, B's actions were displayed using the limited lifetime technique and masked with limited lifetime noise dots [1], [12] (see Figure 1). The limited lifetime technique was used to prevent observers from using local motion or position cues to perform the task [14]. Indeed, when dots are always visible in the same location on the actors' body, it is easier for observers to rely on local cues. The limited lifetime stimulus requires more global visual processing [1], [15]. Each signal dot was presented for a fixed duration (200 ms) at one of the 13 possible locations, then disappeared, and reappeared at another randomly chosen location. Six signal dots per frame were shown. Dot appearance and disappearance were asynchronous across frames in order to avoid motion transients from simultaneous transitions of all sampling dots. Noise dots had the same trajectories, size, and duration as the signal dots, but were temporally and spatially scrambled (they appeared in an area subtending approximately a 8.6°×14.3° region). The number of noise dots was adjusted individually for each participant during a training session (see below).
Figure data removed from full text. Figure identifier and caption: 10.1371/journal.pone.0014594.g001 Example of a communicative signal trial.Agent A points to an object to be picked up; agent B bends down and picks it up. B was presented using limited-lifetime technique (6 signal dots) and masked with temporally scrambled noise dots. The noise level displayed is the minimum allowed in the experiment (5 noise dots). To provide a static depiction of the animated sequence, dots extracted from 3 different frames are superimposed and simultaneously represented; the silhouette depicting the human form was not visible in the stimulus display.
In the nontarget interval, agent B was substituted by limited lifetime scrambled dots obtained by temporally scrambling the corresponding signal action. Noise dots were also added so as to obtain the same number of dots as displayed in the signal interval. On average, positions and motions of the dots in the nontarget interval equaled those of the target interval (see also [1]). In both the target and the nontarget intervals, A was neither limited lifetime nor masked. Observers were asked to decide which interval contained agent B as opposed to no agent. Responses were given by pressing one of two keys on a keyboard. Each participant completed four blocks of 25 trials (10 repetitions of five actions in two conditions). Each block consisted of trials of both conditions presented in a randomized order. Blocks lasted approximately seven minutes each and were separated by a rest period of two minutes. Accuracy feedback was given after each block. Stimuli consisted in five actions performed by a single agent, masked with five levels of noise (5, 15, 25, 35, or 45 noise dots). The actions were different from those used in the experiment. Actions were selected from the CID and included ‘raising arms’, ‘doing aerobics’, ‘picking something up’, ‘standing up’, and ‘turning’. Each participant completed two blocks of 25 trials (five actions by five noise levels and by two repetitions). Trials in each block were presented in a randomized order. Individual noise levels were determined by fitting a cumulative Gaussian function to the proportion of correct responses and determining the 75% threshold. The minimum noise level allowed was five noise dots (M = 22.4, SD = 17.3).
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