A local mechanism for differential velocity detection

Differential velocity detection in the fovea was used to probe the properties of a local motion mechanism. Human observers can detect differences in velocity of less than 5% even for a 200 msec target duration. This precision is not based on variations in the distance traversed by the target or in the total target duration. The relevant timing signal for velocity is the detection of a difference in stimulus onset time at spatially separate points. This onset asynchrony detection is shown to be very precise for small spatial separations. Discontinuous stimuli (apparent motion) are adequate substitutes for continuous motion in velocity judgments provided that the spatial interval between target presentations is less than 20 min arc.