Visual-Vestibular Interaction Upon Nystagmus Slow Phase Velocity In Man

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Abstract
The influence of vestibular and visual (optokinetic) stimuli on the nystagmus slow phase velocity (SPV) in man was studied using different combinations of visual horizon and/or passive body rotations (velocity trapezoids). The interactions of combined stimulation were evaluated in comparison to pure optokinetic and pure vestibular reactions. The results indicate that retinal image stabilization and vestibular systems simultaneously activate ocular reflexes during passive body accelerations in the light. Results: 1. Exclusive vestibular stimulation by passive body rotation in the dark yields a rather low gain of the vestibulo-ocular reflex (VOR) with respect to acceleration. 2. With optokinetic stimulation the gain depends on pattern velocity. It is close to unity at velocities below 30°/sec and progressively decreases with increasing pattern velocity. 3. During passive body rotation in the light (a concomitant visual and vestibular stimulation occurs) the form of the response profile suggests that both visual and vestibular inputs contribute to the response. Compared with pure optokinetic stimulation this results in a better correspondence of the SPV and stimulus velocity at higher accelerations and velocities. 4. When subjects viewed a surround rotating with constant relative velocity, the amount of enhancement or depression of SPV by the additional application of passive body rotation increased with the body acceleration and with surround velocity. With small surround velocities and high enhancing body accelerations the SPV may be faster than the relative surround velocity. Depressing vestibular stimuli cause a greater SPV modulation than enhancing of the stimuli sometimes even reversing the direction of nystagmus. The results indicate an interaction between optokinetic (visual) and vestibular inputs.