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 eve movements. Its response is reduced followin g damage to the vestibular endorgan, but recovers over time. The VOR is med iated by both direct and indirect anatomical pathways: most direct pathways include only two central synapses, both located in the brainstem. To inves tigate: the possibility that a direct pathway is modified during the recove ry of VOR gain, we measured the oculomotor response to single current pulse s delivered to the vestibular labyrinth of two alert cats after plugging th e contralateral horizontal canal. The response was also measured after moto r learning induced by continuously worn lenses (optically induced motor lea rning) in two cats. The gain of the VOR A as monitored concurrently. The ey e movement evoked by a current pulse increased more than 100% during recove ry, from a plug. The electrically evoked rye movement did not change during optically induced motor learning either before the plug or after recovery. The gain of the VOR H-as modified in both situations. We conclude that dir ect VOR pathways are modified significantly during recovery after a plug. b ut not during optically induced learning. Our results suggest that signific ant modification of direct pathways may require a change in vestibular sens ory input.