The response of vestibulo-ocular reflex pathways to electrical stimulation after canal plugging.

The vestibulo-ocular reflex (VOR) allows clear vision during head movements by generating compensatory eye movements. Its response to horizontal rotation is reduced after one horizontal semicircular canal is plugged, but recovers partially over time. The majority of VOR interneurons contribute to the shortest VOR pathway, the so-called three-neuron arc, which includes only two synapses in the brainstem. After a semicircular canal is plugged, transmission of signals by the three-neuron arc originating from the undamaged side may be altered during recovery. We measured the oculomotor response to single current pulses delivered to the vestibular labyrinth of alert cats between 9 h and 1 month after plugging the contralateral horizontal canal. The same response was also measured after motor learning induced by continuously-worn telescopes (optically induced motor learning). Optically induced learning did not change the peak velocity of the evoked eye movement (PEEV) significantly but, after a canal plug, the PEEV increased significantly, reaching a maximum during the first few post-plug days and then decreasing. VOR gain also showed transient changes during recovery. Because the PEEV occurred early in the eye movement evoked by a current pulse, we think the observed increase in PEEV represented changes in transmission by the three-neuron arc. Sham surgery did not result in significant changes in the response to electrical stimulation or in VOR gain. Our data suggest that different pathways and processes may underlie optically induced motor learning and recovery from plugging of the semicircular canals.