Optokinetic nystagmus in the ferret: including selected comparisons with the cat.

The aim of this study was to evaluate the functional significance of similarities observed in the anatomy and the physiology of cat and ferret visual systems. Optokinetic nystagmus (OKN) in response to movement of the entire visual field, and optokinetic after nystagmus (OKAN) were measured in 8 ferrets with binocular stimulation. A shift of the beating field in the same direction as the fast phase of eye movements was observed both in ferret and cat. The absence of a fast rise in slow phase velocity (SPV) and similarities in the time constant to reach the steady state OKN gain, using step velocity stimuli are noted. As in the cat, primary OKAN was observed with a gradual decrease in its SPV. Following termination of stimulation, no sudden fall in SPV was noted for either species. However, for the ferret, the decrease was more rapid. With monocular stimulation, small differences were observed in OKN gain when responses to temporonasal and nasotemporal directions of the stimulus were compared in the two species. In contrast, the ferret displays a OKN gain which is approximatively twice that of the cat at stimulus velocities of 100 degrees/sec. Even at 200 degrees/sec., visual movement still induces a discernable OKN response (gain .0.07). Secondary OKAN, always present in the cat, was observed in only 43% of ferret records. Taken together with other considerations, these findings recommend the ferret as an alternative to the cat for the study of OKN and of other visuo-motor capacities in carnivores.