Biological motion input to the oculomotor system

Abstract

The capacity to identify and perceive biologically relevant actions is essential for survival and social skills. We hypothesized that the perception of biological motion could induce a different behavioral response compared with stimuli devoid of biological relevance. To check this hypothesis, we analyzed the eye movement response to a moving walker, or its scrambled version. Subjects were asked to pursue a point-light walker, created using Cutting’s algorithm, or its scrambled version. The walker consisted of 11 dots; a green fixation point on the hip dot and 10 red dots. The control stimulus - scrambled version- was obtained by shuffling the mean vertical position of the dots of the walker -except the hip dot- to disrupt the global form while keeping the same local motion. The point-light walker, or its scrambled version, then appeared and began to move in the randomized heading direction for 800ms before disappearing behind an invisible occluder. The stimulus reappeared 890ms later for 800ms. The type of stimulus -biological motion or scrambled version- its direction and its velocity were selected at random for each trial. We analyzed separately 4 different phases in the responses. In the first one -the reactive smooth pursuit-, we saw that the motor response was stronger with a biological stimulus. The smooth eye velocity was significantly greater for the walker 200ms after pursuit onset. We noticed no difference in eye movement velocity between both stimuli neither during the steady-state phase nor in the occlusion phase. But after reappearance of the stimulus, we saw a stronger smooth pursuit response for the biological motion. The mean acceleration of smooth pursuit eye movements for the first 150ms after stimulus reappearance was significantly higher for biological motion than for the control stimulus. These results show that the biological relevance of an action can influence the behavioral response to the visual stimulus.