Perceiving human locomotion: priming effects in direction discrimination

During the perception of biological motion, the available stimulus information is confined to a small number of lights attached to the major joints of a moving actor. Despite this drastic impoverishment of the stimulus, the human visual apparatus organizes the swarm of moving dots in a vivid percept of a human figure. In addition, observers effortlessly identify the action the figure is involved in. After a historical introduction and a short walk through the literature, data from a priming experiment are presented. In a serial two-choice reaction-time task, participants were presented with a point-light walker, facing either to the right or to the left and walking either forward or backward on a treadmill. Subjects had to identify the direction of articulatory movements. Reliable priming effects were established in consecutive trials, but these effects were tempered by the relation between priming and primed walker. The reaction time to a walker was shorter when the walker in the preceding trial moved in the same direction and was facing in the same direction. The findings are discussed in relation to recent data from neuropsychological case studies, neuroimaging, and single-cell recording.