SEEING BIOLOGICAL MOTION

One of the more stunning examples of the resoucefulness of human visio n is the ability to see 'biological motion', which was first shown(1) with an adaptation of earlier cinematic work(2): illumination of only the joints of a walking person is enough to convey a vivid, compelling impression of human animation, although the percept collapses to a ju mble of meaningless lights when the walker stands still. The informati on is sufficient to discriminate the sex and other details of the walk er(3,4), and can be interpreted by young infants(5). Here we measure t he ability of the visual system to integrate this type of motion infor mation over space and time, and compare this capacity with that for vi ewing simple translational motion. Sensitivity to biological motion in creases rapidly with the number of illuminated joints, far more rapidl y than for simple motion. Furthermore, this information is summed over extended temporal intervals of up to 3 seconds (eight times longer th an for simple motion). The steepness of the summation curves indicates that the mechanisms that analyse biological motion do not integrate l inearly over space and time with constant efficiency, as may occur for other forms of complex motion(6), but instead adapt to the nature of the stimulus.