The otolithic origin of the vertical vestibuloocular reflex following bilateral blockage of the vertical semicircular canals in the rabbit

Abstract

The influence of bilateral plugs of the anterior and posterior semicircular canals (ASCs, PSCs) on the vertical vestibuloocular reflex (VVOR-0) of the rabbit, oscillated about the longitudinal axis maintained in a horizontal orientation, was studied. Bilateral plugs of either the ASCs or PSCs reduced the gain of the VVOR-0 evoked by stimulus frequencies above 0.005 Hz, but left a residual gain of the VVOR at higher stimulus frequencies that could be attributed to the remaining vertical canals and utricular otoliths. Bilateral plugs of both the ASCs and PSCs totally eliminated the canal component of the VVOR-0, as determined from measurements made when the animal was oriented at 90° ('nose up') with respect to the earth horizontal axis, eliminating changes in the gravitational vector acting on the utricular maculae (VVOR-90). Bilateral plugs of both the ASCs and PSCs also eliminated the nonlinear increase in gain observed at intermediate frequencies in intact rabbits when oscillated about a supine orientation (VVOR-180). The contribution of the utricular otolith to the VVOR-0 in rabbits with both the ASCs and PSCs plugged was compared with the predicted otolith contribution based on the assumption of linear summation of semicircular canal and utricular signals. The assumption of linear summation appears to be incorrect. Bilateral plugs of the ASCs and PSCs increased the latency and the time constant of vertical eye movements evoked by step-roll stimulation about the longitudinal axis maintained in a horizontal orientation.