Visual suppression of vestibular nystagmus in rhesus monkeys.

Summary Visual suppression of vestibular nystagmus was studied in rhesus monkeys. In the presence of vision the velocity of slow phases of caloric nystagmus was reduced by about 50%. The percent reduction in slow phase velocity was independent of the intensity of the nystagmus except when the nystagmus was weak. Visual suppression was similar when tested several times on the same day but tended to increase if there was spontaneous nystagmus or if testing was done over a more prolonged period of time. Percent reduction of slow phase velocity of caloric nystagmus in light as compared with that in darkness would appear to be a convenient and stable method for assessing the strength of visual fixation. Visual fixation also suppressed spontaneous nystagmus and positional alcohol nystagmus (PAN). Higher levels of alcohol were shown to reduce or abolish visual suppression of PAN and of caloric nystagmus.

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