Behavioural and neural correlates of vestibular compensation.

Sudden complete loss of input from one labyrinth results in a massive change in behaviour. A vigorous horizontal ocular nystagmus occurs together with postural changes. These dramatic changes are short-lived and within about a week they have almost disappeared. This very rapid recovery has been the basis for the postulation that vestibular compensation is a textbook model for the study of neural plasticity in the central nervous system. Whilst the behavioural recovery is dramatic, quantitative testing reveals the loss and the permanent asymmetry of the system (Table 1). Recordings from single neurones show that many neurones in the ipsilesional VN are silenced by the unilateral loss, but as they start to fire again, so the spontaneous nystagmus declines. The major question which is still unanswered is the cause of the return of the firing of neurones in the ipsilesional VN. The answer may be found by studies of the neurochemistry of the VN using brain slice preparations. This review shows some of the errors which have been made by attempting to infer purely vestibular function from measurements of eye movements when other sources of ocular motor control may operate.