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Effects of visual and non‐visual mechanisms on the vestibulo‐ocular reflex during pseudo‐random head movements in man.

1. The behaviour of the vestibulo‐ocular reflex (VOR) in man was examined using pseudo‐random and sinusoidal whole‐body angular‐motion stimuli applied about the yaw axis by a servo‐controlled turn‐table. 2. The VOR response was assessed in four conditions; during fixation on a head‐fixed target (HFT); during attempted fixation in the dark of an imagined head‐fixed (IHFT) or earth‐fixed target (IEFT) and in darkness (DRK) whilst performing an auditory discrimination task. 3. When the pseudo‐random stimulus was composed of four sinusoids, the three lowest frequencies (0.11, 0.24 and 0.37 Hz) were maintained constant whilst the highest frequency (F4) was varied from 0.39 to 2.08 Hz. In darkness (DRK condition) and when imagining a head‐fixed target (IHFT condition) the gain of slow‐phase eye velocity was not significantly affected by the frequency of the highest‐frequency component, although there were significant changes in the phase for the IHFT condition. 4. During fixation of a real head‐fixed target (HFT condition), both the gain and phase of eye velocity were significantly modified by the frequency (F4) of the highest‐frequency component. When F4 was 0.39 Hz, all frequency components had a low gain (mean 0.05), but as F4 was increased there was a significant (P less than 0.001) increase in gain for all three low‐frequency components which reached a maximum (mean 0.17) when F4 was 2.08 Hz. However, the gain for the highest frequency component always remained comparable to that obtained in response to a single discrete sinusoid of the same frequency. 5. When the stimulus was composed of only two sinusoids a similar increase in gain of the lower‐frequency (0.22 Hz) component was observed in the head‐fixed target condition as the frequency of the higher‐frequency component was increased from 0.39 to 2.78 Hz. However, VOR gain was not significantly modified by the frequency of the higher‐frequency component when subjects imagined a head‐fixed or earth‐fixed target in darkness. 6. The findings indicate that high levels of VOR suppression can be achieved in the head‐fixed target condition with pseudo‐random stimuli when all frequency components are below 0.4 Hz. But if the highest‐frequency component rises above 0.8 Hz, optimum suppression is confined to the highest‐frequency component, whilst suppression of the low‐frequency components is significantly reduced.(ABSTRACT TRUNCATED AT 400 WORDS)