Contribution of global and local biological motion information to speed perception and discrimination.

To respond to movements of others and understand the intention of others' actions, it is important to accurately extract motion information from body movements. Here, using original and spatially scrambled point-light biological motions in upright and inverted orientations, we investigated the effect of global and local biological motion information on speed perception and sensitivity. The speed discrimination task revealed that speed sensitivity was higher for the original than for scrambled stimuli (Experiment 1) and higher for upright than for inverted stimuli (Experiment 2). Perceived motion speed was slower for the original than for scrambled stimuli (Experiment 2), but regardless of the orientation of the display (Experiment 1). A subsequent experiment comparing different scrambled stimuli of the same actions showed that the higher speed discrimination sensitivity to upright stimuli was preserved even in the scrambled biological motions (Experiment 3). Taken together, our findings suggest that perception of the speed of biological movements emanates from both global and local biological motion signals.