The plasticity of compensatory eye movements in rotatory tests. I. The effect of alertness and eye closure.

Fifteen voluntary subjects with a mean age of 26 years (17-39), participated in two rotatory experiments. The rotatory test used was sinusoidal harmonic acceleration (SHA) test at frequencies of 0.01-0.32 Hz. Compensatory eye movements were measured by means of EOG. Experiment A included rotation in darkness during alerting tasks (day 1), rotation in darkness with no tasks (day 2) and rotation in darkness during alerting tasks (day 3). In the alertness tests, the gain varied between 0.5 and 0.77. Phase values decreased with increasing frequency. The alertness results between days 1 and 3 did not differ in gain or phase. The gain of day 2 (no tasks) displayed significantly reduced gain at all frequencies, and a more pronounced decrease in phase than in the alertness tests. Experiment B comprised testing with eyes open (day 1) and eyes closed (day 2) during alerting tasks. These tests displayed significantly decreased gain at all frequencies with eyes closed compared with eyes open and with more negative phase at 0.08-0.32 Hz with eyes closed. The results indicate a central effect of the alertness level on compensatory eye movements, and the effect of eye closure might be explained by bio-mechanical and possibly even central effects on the eye movements. In low-frequency testing, as short a test time as possible should be used, and due to different alertness levels, the results should be evaluated with great caution.