Vestibulo-ocular reflex (VOR) induced by passive head rotation and goal-directed saccadic eye movements do not simply add in man.

Additivity between vestibulo-ocular reflex (VOR) and saccadic eye movements was quantified in man by passively rotating the subject's head as he tracked a stepping target. A systematically increased gaze (i.e. eye + head) saccadic velocity was observed when the head was rotated toward the target, as compared to a head-fixed condition, indicating that VOR and saccades do not fully add. Moreover, although VOR was assumed to be inhibited during ocular saccades, mean gaze saccadic amplitude remained unchanged. This would suggest an on-line computation of gaze position to be fed back to the saccadic system in order to stop the saccade once gaze reaches its goal.