The role of vestibular and neck afferents during eye-head coordination in the monkey.

Abstract Since during active eye-head turning the eyes move first and with higher velocity than the head, the lines of sight reach the target while the head is still moving. Then for the remaining duration of the head movement, the eyes maintain their fixation by performing a movement which is counter to that of the head and perfectly compensates for it. It is agreed that this compensatory eye movement is critically influenced by visual, vestibular and neck afferents and that it is not initiated centrally. We have investigated (a) the relative contribution of the vestibular and neck afferents to the compensatory eye movement made during active and passive head turning in monkeys and (b) the mechanisms underlying the recovery of compensatory eye movements following either the removal of the vestibular or neck loop or both. Our results have shown that (1) normal monkeys display perfect ocular stability in darkness, (2) at least 95% of ocular stability is due to the vestibular loop, and (3) the contribution of the neck loop is negligible. Following bilateral vestibulectomy the recovery of compensatory eye movements occurs gradually and reaches 90% within 7 weeks but only during active head movements. We have shown that there are at least three mechanisms underlying this recovery: (1) an increase in gain of the neck loop, (2) the occurrence of a centrally programmed compensatory eye movement, and (3) a recalibration of the saccadic and head motor system.