Plasticity of compensatory eye movements in rotatory tests. II. The effect of voluntary, visual, imaginary, auditory and proprioceptive mechanisms.

Two groups of 10 healthy volunteers each with a mean age of 28 years (17-39) were tested in low-frequency rotatory experiments (sinusoidal harmonic acceleration, SHA), at frequencies of 0.01-0.32 Hz. The purpose was to ascertain whether voluntary enhancement and reduction of gain and phase occurred with voluntary performances. The different tests were alertness in darkness, stationary and moving targets, imaginary stationary and moving targets in darkness, proprioceptive moving targets and acoustic stationary and moving targets. Alertness tests in darkness demonstrated a gain increase (0.5-0.7) at 0.01-0.32 Hz, and a decreasing phase lead (40-2 degrees) with increasing frequency. The alertness tests served as a reference for the other tests. In tests with stationary targets, the gain reached unity and the phase was almost 0 degrees. In imaginary stationary target tests, gain and phase were significantly increased compared with alertness. Stationary acoustic targets in darkness significantly enhanced the gain. An increased phase lead was also found. In visual suppression tests (moving target), the gain was near 0. In imaginary moving target tests, the gain decreased significantly. In darkness the gain was significantly more depressed with a proprioceptive moving target than during imaginary moving target. Testing with proprioceptive + acoustic moving target in darkness, displayed an additional gain depression and negative phase at 0.08 Hz. The conclusion is that the influence of non-vestibular mechanisms substantially affects low frequency sinusoidal rotatory testing.