Flicker-induced nystagmus in a ganzfeld situation in monkeys

The possibility that flicker-induced nystagmus (FIN) is a form of optokinetic nystagmus evoked by an apparent motion effect was explored in six rhesus monkeys. FIN was elicited by stroboscopic stimulation of one or both eyes directly (heterogeneous field condition), and through a diffuser which approximates a Ganzfeld situation (homogeneous field condition). FIN and afterresponses (FIAN) were easily obtained with monocular stimulation and, in fact, were enhanced in the homogeneous field condition as shown by a shorter latency and higher frequency of FIN, and by longer and more prevalent FIAN with additional reversals. The magnitudes of FIAN were greater following longer stimulation. The build-up period and mean peak frequency were interdependent in the heterogeneous but not in the homogeneous field condition. Binocular flicker, particularly in the latter situation, resulted in a preponderance of FIN to the right, a response characteristic of right eye stimulation. Similarly, in some animals, the response and afterresponses were stronger on flickering the right eye. The results demonstrate that apparent motion does not play a role in the initiation and/or propagation of FIN and FIAN. The enhancement obtained in the Ganzfeld situation suggests the opening of a negative feedback loop which may operate during direct stimulation. The "reafference principle" could explain the relationship of the build-up period and mean peak frequency in the two stimulus conditions. The asymmetric responses obtained in this study reflect a lateralization of function. FIN, as an unlearned form of behavior, could be useful as a model for elucidating the mechanism of such a bias.