Voluntary and visual control of the vestibuloocular reflex after cerebral hemidecortication.
Horizontal vestibuloocular reflex (VOR) function was studied in five patients after cerebral hemidecortication. In darkness, VOR gain contralateral to the decorticate hemisphere was slightly higher than ipsilateral gain. Voluntary modulation of the reflex by attempted fixation of imaginary targets in darkness increased this VOR asymmetry. Voluntary cancellation of the ipsilateral VOR was better than cancellation of the contralateral VOR. Voluntary enhancement of the contralateral VOR was better than enhancement of the ipsilateral VOR. Visual control of the reflex while viewing foveal targets further increased the VOR asymmetry. Defective visual modulation corresponded to paresis of ipsilateral smooth pursuit. Abnormal voluntary responses in darkness indicate that cerebral control of the reflex can be independent of the pursuit system. The hemispheres may use a corollary discharge of eye position in the orbit and a head velocity signal to direct smooth eye movements toward the perceived location of objects. One hemisphere regulates ipsilateral smooth eye movements that achieve voluntary and visual control of the VOR.