When the head is moved passively from side to side, compensatory eye movements known as the vestibulo-ocular reflex (VOR) are elicited to rotate the eyes an amount equal to but opposite the head rotation so that the eyes keep looking straight ahead. The presence of this reflex poses a problem when one moves both the eyes and head to acquire a target lying far to the left or right. If both the eyes and head turn leftward to direct gaze to a target, the VOR would defeat this objective by causing the eyes to rotate to the right. In this project, we did two experiments to see how the brain deals with the counterproductive VOR. First, we applied brief perturbations to the head in the middle of a gaze shift to ascertain whether the VOR indeed was turned on during an active gaze shift. If the perturbation of the head movement was in the direction of the gaze shift, there was a reduction of ongoing eye movements as would be predicted if the VOR were on. These experiments, i n contrast to other reports in the literature, indicate that the VOR is not disabled, even at the onset of large amplitude gaze shifts. In the second experiment, we disabled the receptors that sense head movement. Consistent with the data obtained with brief perturbations to the head, gaze initially overshot the target because the VOR was absent and there were no compensatory eye movements. In time, however, compensatory eye movements returned in all three monkeys tested. After performing a series of behavioral tests, we concluded that the compensatory eye movements were generated by enhanced activation of neck receptors and an enhanced signal from the gaze command centers. FUNDING NIH grants RR00166 and EY00745. Freedman, E. G., Ling, L. and Fuchs, A. F. Perturbing the head re-assessing the gain of reflex interactions during orienting eye-head movements. Soc. Neurosci. Abstr. 24 1412, 1998.
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