A limitation of position constancy.

When the eyes track a moving object, the image of a stationary target shifts on the retina colinearly with the eye movement. A compensation process called position constancy prevents this image shift from causing perceived target motion commensurate with the image shift. The target either appears stationary or seems to move in the direction opposite to the eye movement, but much less than the image shift would warrant. Our work is concerned with the question of whether position constancy operates when the image shift and the eye movement are not colinear. That can occur when, during the eye movement, the target undergoes a motion of its own. Evidence is reported that position constancy fails to operate when the direction of the target motion forms an angle with the direction of the eye movement.