Velocity coding: evidence from perceived velocity shifts.

Measurements of the perceived velocities of moving patterns were made under a variety of conditions in an attempt to gain information concerning the way in which velocity is encoded in the visual system. Adaptation to a pattern moving in the same or opposite direction reduces the perceived velocity of a moving test pattern, but only if the adaptation pattern moves as fast as or faster than the test pattern. The aftereffect peaks at an adaptation velocity slightly higher than the test velocity and then remains constant at higher velocities. Similar results were obtained for several types of pattern. Perceived velocity reduction also follows adaptation to a flickering homogeneous field. The results can be explained in terms of a theory of velocity coding in which two channels are considered in terms of variations in the velocity sensitivities of the neurones they comprise.