Increased transmission by direct vestibulo-ocular reflex pathways after peripheral vestibular damage: a preliminary report.

The vestibulo-ocular reflex (VOR) allows clear vision during head movements by generating compensatory eye movements. Its response is reduced following damage to the vestibular endorgan, but recovers over time. The VOR is mediated by both direct and indirect anatomical pathways; most direct pathways include only two central synapses, both located in the brainstem. To investigate the possibility that a direct pathway is modified during the recovery of VOR gain, we measured the oculomotor response to single current pulses delivered to the vestibular labyrinth of two alert cats after plugging the contralateral horizontal canal. The response was also measured after motor learning induced by continuously worn lenses (optically induced motor learning) in two cats. The gain of the VOR was monitored concurrently. The eye movement evoked by a current pulse increased more than 100% during recovery from a plug. The electrically evoked eye movement did not change during optically induced motor learning either before the plug or after recovery. The gain of the VOR was modified in both situations. We conclude that direct VOR pathways are modified significantly during recovery after a plug, but not during optically induced learning. Our results suggest that significant modification of direct pathways may require a change in vestibular sensory input.