FOVEATION PERIOD STABILITY AND OSCILLOPSIA SUPPRESSION IN CONGENITAL NYSTAGMUS - AN HYPOTHESIS

Congenital nystagmus (CN) waveforms contain foveation periods that occur just before the eyes accelerate away from the target and occur when the eye is in position to place the target image, most of the time, on the fovea. Using phase-plane analysis, the authors studied the waveform changes in an individual with hereditary CN who experienced intermittent oscillopsia (OSOP) following an episode of loss of consciousness six years previously. His nystagmus showed two waveforms that alternated approximately every 2 sec. One waveform (with leftward fast phases) provided repeatable, well-developed foveation periods lasting typically 114 msec, during which the image of an object of regard was within 0.5-degrees of the center of the fovea and image drift was less-than-or-equal-to 4-degrees/sec; these values defined a `foveation window'. During this waveform, the subject did not report OSOP. During the second waveform (with rightward fast phases), eye velocity (and hence image velocity) was typically > 20-degrees/sec as the image of the object of regard swept across the fovea (i.e., there were no well-developed foveation periods); during this waveform, the subject reported OSOP. Artificial (electronic) retinal image stabilization (RIS) failed to abolish OSOP during the second waveform. Efference copy of the CN waveform has been postulated as the mechanism by which individuals with CN suppress OSOP. This mechanism is tenable only if the source of the subject's acquired oscillation occurred at a point beyond where the efference-copy signal is fed back. RIS by itself is insufficient to suppress OSOP since RIS causes OSOP in most CN subjects (who do not ordinarily have OSOP) and resulted in no change in this subject. The authors conclude that the necessary condition for stable (i.e., no OSOP) vision in our subject was the ocular motor stability provided by repeatable, well-developed CN foveation periods (i.e., an efference copy of it) and without it, OSOP is not suppressed even during RIS. The authors speculate on the application of these findings to the suppression of OSOP in individuals with acquired nystagmus.