Effects of smooth pursuit eye movement on ocular responses to sudden background motion in humans

We studied the effects of horizontal smooth pursuit on the ocular tracking responses to brief perturbations of a textured background in humans. When the subject was fixating a stationary spot, a brief perturbation (60 degrees/s, 40 ms) of the background in any one of four directions (right, left, up, down) elicited a small tracking response. When the subject was pursuing a target moving against the stationary background, the same background perturbation elicited a larger response when in the same direction as the pursuit, but a smaller response when its direction was opposite to the pursuit; the response to vertical background perturbations was also enhanced during pursuit. When the subject was pursuing while the target and background were moving together, the same background perturbations elicited the larger responses regardless of their direction. These results indicate that the sensitivity to background motion is increased during smooth pursuit. However, when pursuit is executed against a stationary background--the usual situation in everyday life--the system is selectively insensitive to the reafferent visual input associated with pursuit, thereby reducing the potentially adverse effect of the background on pursuit performance.