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Masahiro Hirai, Daniel R. Saunders, Nikolaus F. Troje; Local motion versus global shape in biological motion: A reflexive orientation task. Journal of Vision 2010;10(7):786. doi: 10.1167/10.7.786.
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Our visual system can extract directional information even from spatially scrambled point-light displays (Troje & Westhoff, 2006). In three experiments, we measured saccade latencies to investigate how local features in biological motion affect attentional processes. Participants made voluntary saccades to targets appearing on the left or the right of a central fixation point, which were congruent, neutral or incongruent with respect to the facing direction of a centrally presented point-light display. In Experiment 1, we presented two kinds of human point-light walker stimuli (coherent and spatially scrambled) with three different viewpoints (left-facing, frontal view, right-facing) at two different stimulus durations (200 and 500 ms) to sixteen observers. The saccade latency of the incongruent condition was significantly longer compared to that of the congruent condition for the 200-ms coherent point-light walker stimuli, but not for the spatially scrambled stimuli. In Experiment 2, a new group of observers (N = 12) were presented with two point-light walker displays. The only difference with respect to Exp. 1 was, that in the scrambled version of the stimulus, the location of the dots representing the feet, was kept constant. Different from the results of Experiment 1, the saccade latency in the incongruent condition was significantly longer than that in the congruent condition irrespective of the stimulus types. In Experiment 3, we put into conflict the facing direction indicated by the local motion of the feet and the facing direction as indicated by the global structure of the walker by presenting newly recruited observers (N = 12) with backwards walking point-light walkers. In agreement with the results of Experiment 2, the modulation of the saccade latency was dependent on the direction of feet motion, irrespective of the postural structure of the walker. These results suggest that the local motion of the feet determines reflexive orientation responses.
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