"Dumping" of rebound nystagmus and optokinetic afternystagmus in humans.

Rebound nystagmus (RN) is an involuntary movement of the eyes, characterized by slow-phase eye velocity in the direction of previously maintained eccentric gaze. The purpose of this study was to clarify the neural or neuromuscular events that are responsible for the generation of RN. To do so, we examined whether a briefly presented visual stimulus during RN reduces (i.e., "dumps") subsequent eye velocity, compared with the velocity of slow-phase eye movements when no visual stimulus was presented. For comparison, "dumping" was examined also for optokinetic afternystagmus (OKAN), which is generally believed to result from eye-velocity signals stored in a central neural integrator as a consequence of optokinetic stimulation. Results obtained from ten normal observers showed that RN and OKAN both exhibit "dumping": average slow-phase eye velocities were reliably slower after fixation of a 0.6 deg stationary target than on trials when no fixation target was presented. Although RN decayed faster than OKAN in darkness, the magnitude of "dumping" increased similarly with the duration of the visual stimulus (25 ms to 4 s) for both types of eye movement. The results imply that signals from a central velocity-storage mechanism contribute to the generation of RN.