The perception of motion during colinear eye movements.

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When the eyes are engaged in pursuit movements, the image of a stationary object shifts on the retina, but such a target is either perceived as stationary or seems to move only little. This is the result of a compensation process called position constancy, which takes the eye movements into account. Becklen, Wallach, and Nitzberg (1984) reported that position constancy does not operate when the target undergoes a motion of its own, in a direction that differs from the direction of the eye movements. Other findings have indicated that position constancy has an effect when the target motion is colinear with the eye movements, but the accuracy with which it then operates has not been known. We measured how correctly motions that were colinear with eye movements were perceived and found that the extents of target motions were accurately perceived when they were in the same direction as the eye movement, but that position constancy showed a small, but distinct, lag when eye-movement and target motions were in opposite directions.