Functional organization of the mechanisms subserving the optokinetic nystagmus in the cat

Abstract

In the cat both crossed and uncrossed retinal fibres are able to mediate optokinetic nystagmus in both temporonasal and nasotemporal directions. There exists, however, a slight directional predominance of the nystagmus for the crossed fibre system in the temporonasal stimulus direction and for the uncrossed fibres in a nasotemporal direction. During the first 9 days following ablation of the visual cortex the optokinetic nystagmus elicited monocularly is greatly asymmetrical: the nystagmus elicited by a temporonasal stimulus is moderately affected particularly at higher stimulus velocities, whereas the nystagmus elicited by a nasotemporal stimulus is present only at stimulus velocities below 20–30 deg/s and has a low gain. Without the visual cortex, selective stimulation of the crossed retinal fibres of one eye may evoke a weak nystagmus on temporonasal stimulus motion only. In contrast, in absence of visual cortex, the uncrossed retinal fibres do not mediate any optokinetic nystagmus. The behaviour of the vestibulo-ocular reflex in light and of the visual fixation suppression of the postrotatory nystagmus in our lesioned animals provided another means to reach similar conclusions. Twenty-seven units recorded in the vestibular nuclei showed responses to optokinetic stimulations, which were in line with the behaviour of the optokinetic nystagmus. These data suggest that optokinetic nystagmus has two components: (i), a subcortical component in which the temporonasal direction of stimulation is predominant in eliciting the nystagmus and in which both the crossed and uncrossed retinal fibres are involved, although with a different weight, and (ii), a cortical component responsible for a symmetrical optomotor response, which also involves the crossed and uncrossed retinal fibres.