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DRB Aviation Medical Research Unit, Department of Physiology, McGill University, Montréal, Québec, Canada

1. Two sets of experiments have examined the vestibulo-ocular response (VOR) to repeated sinusoidal rotation (A) in the dark and (B) after attempting visual tracking of a mirror-reversed image of the visual surround.

2. In both A and B a horizontal sinusoidal rotational stimulus of 1/6 Hz and 60°/sec angular velocity amplitude was employed, specifically chosen to lie within the presumed range of natural stimulation of the semicircular canals.

3. In A each of seven subjects underwent ten 2-min runs of the standard stimulus in the dark on each of three consecutive days, with 3-min rest periods between runs. Using d.c. electro-oculography (EOG) the VOR gain was measured throughout as eye velocity/head velocity. Mental arousal was maintained by competitive mental arithmetic. Constancy of EOG gain was assured by 50 min dark adaptation before experimentation.

4. The results of A showed no consistent change of VOR gain over the three times scales of a run, a day and the 3-day experiment.

5. In B the same subjects underwent a similar pattern of vestibular stimulation, but during eight of the 2-min daily runs they attempted the reversed visual tracking task. VOR gain was measured during the 1st, 6th and last runs which were conducted in the dark for this purpose. Constancy of EOG gain was maintained by using red light throughout.

6. The results of B showed a substantial (approx. 25%) and highly significant (P « 0·001) reduction of VOR gain attributable solely to the 16 min of reversed visual tracking attempted during the 50 min daily experiment. In addition the pre-test control gain was lower on day 3 than on day 1 (approx. 10% attenuation, P < 0·01) indicating a small cumulative effect from beginning to end of the 3-day experiment.

7. It is concluded (A) that the repeated vestibular stimulus did not itself cause significant attenuation of VOR gain, but (B) that superposition of a reversed visual tracking task did induce retained VOR attenuation which was solely due to the antagonistic visual stimulus.

8. In conjunction with other experimental evidence it is inferred that this attenuation probably represents an adaptive change in the VOR induced at least in part by retinal image slip.

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