EYE-HEAD COORDINATION IN LABYRINTHINE-DEFECTIVE HUMANS

Eye-head coordination during saccadic gaze shifts normally relies on v estibular information. A vestibule-saccadic reflex (VSR) is thought to reduce the eye-in-head saccade to account for current head movement, and the vestibule-ocular reflex (VOR) stabilizes postsaccadic gaze whi le the head movement is still going on. Acute bilateral loss of vestib ular function is known to cause overshoot of gaze saccades and postsac cadic instability. We asked how patients suffering from chronic vestib ular loss adapt to this situation. Eye and head movements were recorde d from six patients and six normal control subjects. Subjects tracked a random sequence of horizontal target steps, with their heads (1) fix ed in primary position, (2) free to move, or (3) preadjusted to differ ent head-to-target offsets (to provoke head movements of different amp litudes). Patients made later and smaller head movements than normals and accepted correspondingly larger eye eccentricities. Targeting accu racy, in terms of the mean of the signed gaze error, was better in pat ients than in normals. However, unlike in normals, the errors of patie nts exhibited a large scatter and included many overshoots. These over shoots cannot be attributed to the loss of VSR because they also occur red when the head was not moving and were diminished when large head m ovements were provoked. Patients' postsaccadic stability was, on avera ge, almost as good as that of normals, but the individual responses ag ain showed a large scatter. Also, there were many cases of inappropria te postsaccadic slow eye movements, e.g., in the absence of concurrent head movements, and correction saccades, e.g., although gaze was alre ady on target. Performance in patients was affected only marginally wh en large head movements were provoked. Except for the larger lag of th e head upon the eye, the temporal coupling of eye and head movements i n patients was similar to that in normals. Our findings show that pati ents with chronic vestibular loss regain the ability to make functiona lly appropriate gaze saccades. We assume, in line with previous work, three main compensatory mechanisms: a head movement efference copy, an active cervico-ocular reflex (COR), and a preprogrammed backsliding o f the eyes. However, the large trial-to-trial variability of targeting accuracy and postsaccadic stability indicates that the saccadic gaze system of patients does not regain the high precision that is observed in normals and which appears to require a vestibular head-in-space si gnal. Moreover, this variability also permeates their gaze performance in the absence of head movements.