The influence of bilateral labyrinthectomy on horizontal and vertical optokinetic reflexes in the rabbit

Abstract

Vestibular, visual and proprioceptive information converge onto the vestibular nuclei 11-13 and these nuclei participate in the control of reflexive eye movements evoked by each of these sensory inputs. Damage to the labyrinth or to the vestibular nerve not only impairs vestibulo-ocular reflexes, but also reduces the gain of optokinetic reflexes and the duration of optokinetic afternystagmus in cats 5, monkeys 6, humans a4 and rabbits 2,a. Destruction of the vestibular nuclei abolishes optokinetic nystagmus in guinea pigs 1. Previously, we have demonstrated that the horizontal vestibulo-ocular reflex has a lower gain (eye velocity/head velocity) than the vertical vestibulo-ocular reflex at low frequencies of sinusoidal stimulation, 0.005-0.050 Hz 3. This difference in low frequency gain can be ascribed to an otolithic contribution to the vertical vestibuloocular reflex. Conversely, the horizontal optokinetic reflex and the horizontal cervicoocular reflex have higher gains than the vertical optokinetic reflex and the vertical cervico-ocular reflex4, ~0. It appears that the higher gains of the horizontal optokinetic and cervico-ocular reflexes, both of which are maximal at lower stimulus frequencies, compensate in part for the absence of a low frequency otolithic contribution to the horizontal vestibulo-ocular reflex. Since the horizontal and vertical optokinetic reflexes have different gains, it seemed possible that the functional role of the vestibular nuclei in these orthogonal reflexes would also be different. If so, then these reflexes would be affected differentially by bilateral labyrinthectomies. Therefore, we have compared the open-loop gains of the monocularly evoked horizontal and vertical optokinetic reflexes before and after bilateral labyrinthectomies in rabbits. The horizontal and vertical optokinetic reflexes evoked by monocular open-loop stimulation were measured in 5 albino rabbits. During optokinetic reflex testing, the head of the rabbit was fixed with previously implanted bolts to a restraint rod at the