Eye-Head Coordination in Patients with Chronic Loss of Vestibular Function
In healthy subjects sacccadic gaze shifts of more than 20° or so are achieved by moving both the head and the eyes. The coordination between head and eyes during such gaze shifts depends critically on vestibular information. A vestibulo-saccadic reflex (VSR) is thought to reduce the eye-in-head saccade in proportion to current head displacement (Laurutis and Robinson 1986), and following the saccade the vestibulo-ocular reflex (VOR) stabilises gaze if the head movement is still going on and visual mechanisms (smooth pursuit) are not yet effective. In monkey it has been shown that acute bilateral loss of vestibular function after labyrinthectomy causes overshoot and postsaccadic instability (Bizzi et al. 1972, Morasso et al. 1973, Dichgans et al. 1973). However, during the weeks and months following labyrinthectomy, these authors observed a conspicuous remittance of the deficits which they attributed to several compensatory mechanisms, noticeably a cervico-ocular reflex. The same is true for humans with chronic loss of vestibular function. These patients are able to use head movements for large gaze shifts and, in doing so, produce apparently normal gaze saccades without the overshoot or postsaccadic gaze instability one would expect to result from the lack of vestibular signals. There are few quantitative analyses of patients’ behaviour, though. The most detailed study, to our knowledge, is that of KaSci and Zee (1978). However the small number of patients in this study (3) and their different, apparently idiosyncratic, compensatory mechanisms make it difficult to discern a general pattern. Therefore, in the present study we set out to determine the average behaviour of patients with chronic loss of vestibular function during large gaze shifts and to compare it to that of normal subjects.
KeywordsHead Movement Smooth Pursuit Vestibular Function Vestibular Signal Target Step
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