Visual suppression of vestibular nystagmus

The realisation of combined movements of the head and eyes requires the capacity to modulate the vestibulo-ocular reflex (VOR) but the absence of sufficiently rapid visual feed back results in an open loop function. This reflex must be modulated by other mechanisms. Two options are discussed. A modulation of the gain of the reflex loop is not satisfactory for small combined movements of the head and eyes as it would no longer permit the effective compensation for unexpected perturbations of movement of the head. An additive mechanism modifying the VOR through the addition of an other ocular movement is limited by the constrictions of latency, speed and amplitude of such movement. According to experimental conditions, varying complementary effect of the two mechanisms seems to result in maximum efficiency. This paper attempts to include the mechanism of the ocular fixation index (OFI) among the models of study. The OFI applied to a pendular test is, by the nature of its stimulus, more physiologic and more capable of being integrated into existing models. In the usual conditions of the test, the additive model is probably dominant with a preponderant role in the ocular pursuit system. But whatever may be the part of each mechanism, the OFI is not only dependent on the integrity of the floccular inhibition of the VOR but on the numerous cortical and subcortical structures involved in the realisation of voluntary ocular movements. This information should encourage clinicians to investigate the ocular movements of patients presenting vertigo, particularly those with visual instability on head movements.