Head unrestrained horizontal gaze shifts after unilateral labyrinthectomy in the rhesus monkey

Following the orienting saccade of a combined eye-head gaze shift, normal monkeys exhibit a compensatory eye counterrotation that stabilizes gaze as the head movement continues. This counterrotation, which has a gain (eye velocity/head velocity) of near unity, is a manifestation of the vestibulo-ocular reflex (VOR). Acute unilateral labyrinthectomy (UL) causes severe asymmetry in the VOR during passive head rotations that recovers incompletely over time. The purpose of this investigation was to compare the recovery of the counterrotation gain during horizontal gaze shifts with that of the passive VOR after UL. During the 1st week after UL, counterrotation gains were asymmetric, being lower for head movements towards the lesion but nearly normal for head movements towards the intact side. Whereas this asymmetry in the counterrotation gain resolved within a week after UL, asymmetries in the passive VOR persisted. During the 1st week after UL, behavioral performance was generally poor, with a high incidence of inaccurate gaze shifts and larger latencies. In addition, animals used slower head movements such that peak head amplitude during the eye saccade was significantly lower during the 1st week after UL as compared to control values. Bilateral labyrinthectomy (BL) resulted in larger but symmetric deficits in counterrotation, which, contrary to the passive VOR, exhibited significant recovery over time. It is hypothesized that recovery of counterrotation gain after UL has contributions from multiple sources, including the contralateral intact labyrinth and an efference copy of the head movement.