VELOCITY STORAGE, NYSTAGMUS, AND VISUAL‐VESTIBULAR INTERACTIONS IN HUMANS *

A mechanism that stores activity related to slow-phase eye velocity plays an important role in producing vestibular nystagmus, optokinetic nystagrnus (OKN) and visual-vestibular interactions.'.' This mechanism is present in a wide range of species [see Reference 3 for review] and has been studied extensively in monkeys. Stored activity promotes ocular following during OKN and is responsible for optokinetic after-nystagmus (OKAN).' During vestibular nystagmus, stored activity lengthens the time over which compensatory eye movements are rnaintained.2s4 Modeling of the vestibulo-ocular reflex [VOR) has shown that a single storage element is capable of mediating these responses and can reproduce many phenomena of visual-vestibular interactions.' Manifestations of stored activity also are found in neurons in the vestibular nuclei of the monkey. Canal afferents in the vestibular nerve have a decay time constant of about 5-6 seconds after pulses of a~celeration.~ The decay time constant of neurons in the vestibular nuclei always is longer than that of canal afferents, usually varying between 10 and 30 seconds depending on the state of habit~ation.'.~ These same central neurons also are activated during OKN and OKAN. This demonstrates stored activity in firing rates of vestibular nuclei neurons and supports the theory of a common storage element. While this storage mechanism has been studied in animals, it is not known whether it plays a role in generating or modulating nystagmus in humans. Because the dominant time constant of the cupula-endolymph system and of activity in semicircular canal afferents cannot be determined directly in man, other ways have to be found to demonstrate the presence of stored activity in the oculomotor response. One way is by the presence of OKAN. OKAN represents the steady discharge of activity related to slow-phase velocity stored during exposure to a moving Storage of activity also is manifest in visual-vestibular interactions. Mowrer and Ter Braak were the first to show that postrotatory nystagmus is weaker after rotation in light than in darkness. They inferred that OKAN had summated with the vestibular after-nystagmus to reduce or abolish it. In similar experiments on humans, after-nystagrnus was reduced

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