The different mechanisms of the motion direction illusion and aftereffect.

Direction repulsion is the illusory expansion of the angle between two directions of motion, and may occur when the two directions are presented simultaneously (an illusion) or successively (an aftereffect). Here we demonstrate that the motion direction illusion (DI) and aftereffect (DAE) have different mechanisms. Two experiments show that when the two interacting stimuli are presented to different eyes, the DI is greatly reduced but the DAE is obtained at near to full strength. These results suggest that different populations of cells within the visual pathway produce the DI and DAE.