Eye movements and the motion aftereffect: alternatives to the induced motion hypothesis.

It has been previously reported that prolonged unidirectional smooth pursuit often produces a negative motion aftereffect (MAE). This was believed to be caused by retinal image motion of stationary environmental contours during pursuit which subsequently produced a primary motion aftereffect in the tracking direction. The peripheral MAE then induced motion in the stationary tracking target resulting in illusory movement in the opposite direction. We have found that a negative MAE is also produced when the adapting field is devoid of any contours. Furthermore, the presence of a moving textured background in conjunction with smooth pursuit produced an MAE whose direction was inconsistent with the induced motion hypothesis. Since all examples of motion aftereffects in this study were associated with the pursuit aspect of the experiment rather than any interactions with background contours, it was proposed that the illusory motion had an oculomotor determinant. A scheme was tentatively outlined in which fixation suppression of an unregistered ocular drift following prolonged pursuit adaptation (pursuit after-nystagmus) produced the post-adaptive motion illusions.