Neural processing of biological motion in the macaque temporal cortex

Cells have been found in the superior temporal polysensory area (STPa) of the macaque temporal cortex which are selectively responsive to the sight of particular whole body movements (e.g., walking) under normal lighting. These cells typically discriminate the direction of walking and the view of the body (e.g., left profile walking left). We investigated the extent to which these cells are responsive under `biological motion' conditions where the form of the body is defined only by the movement of light patches attached to the points of limb articulation. One third of the cells (25/72) selective for the form and motion of waling bodies, showed sensitivity to the moving light displays. Seven of these cells showed only partial sensitivity to form from motion, in so far as the cells responded more to moving light displays than to moving controls but failed to discriminate body view. These seven cells exhibited directional selectivity. Eighteen cells showed statistical discrimination for both direction of movement and body view under biological motion conditions. Most of these cells showed reduced responses to the impoverished moving light stimuli compared to full light conditions. The 18 cells were thus sensitive to detailed form information (body view) from the pattern of articulating motion. Cellular processing of the global pattern of articulation was indicated by the observations that none of the cells were found sensitive to movement of individual limbs and that jumbling the pattern of moving limbs reduced response magnitude. The cell responses thus provide direct evidence for neural mechanisms computing form from non-rigid motion. The selectivity of the cells was for body view, specific direction and specific type of body motion presented by moving light displays and is not predicted by many current computational approaches to the extraction of form from motion.