Compensation of the human vertical vestibulo-ocular reflex following occlusion of one vertical semicircular canal is incomplete

The vestibulo-ocular reflex (VOR) was studied in nine human subjects 2–15 months after permanent surgical occlusion of one posterior semicircular canal. The stimuli used were rapid, passive, unpredictable, low-amplitude (10–20°), high-acceleration (3000–4000°/s²) head rotations in pitch and yaw planes. The responses measured were vertical and horizontal eye rotations, and the results were compared with those from 19 normal subjects. After unilateral occlusion of the posterior semi-circular canal, the gain of the head-up pitch vertical VOR — the vertical VOR generated by excitation from only one and disfacilitation from two vertical semicircular canals — was reduced to 0.61±0.06 (normal 0.92±0.06) at a head velocity of 200°/s. In contrast the gain of the head-down pitch vertical VOR — the VOR still generated by excitation from two, but disfacilitation from only one vertical semicircular canal — was within normal limits: 0.86±0.11 (normal 0.96±0.04). The gain of the horizontal VOR in response to yaw head rotations — ipsilesion 0.81±0.06 (normal 0.88±0.05) and contralesion 0.80±0.11 (normal 0.92±0.11) — was within normal limits in both directions (group means ± two-tailed 95% confidence intervals given in each case). These results show that occlusion of just one vertical semicircular canal produces a permanent deficit of about 30% in the vertical VOR gain in response to rapid pitch head rotations in the excitatory direction of the occluded canal. This observation indicates that, in response to a stimulus in the higher dynamic range, compensation of the human VOR for the loss of excitatory input from even one vertical semicircular canal is incomplete.