Ocular counterrolling differs in dynamic and static stimulation

In the past, the majority of ocular counterrolling (OCR) studies were performed with subjects tilted and held statically. Studies in our laboratory have focused on dynamic rotation below the threshold of the semicircular canals. The present study compares OCR in both static and dynamic modes. Ten normal subjects, mean age 50.9 years (SD 16.2 years), underwent rotation about their naso-occipital axis to 90 degrees to the right and left, at a constant velocity of 3 degrees/s and an acceleration of 0.2 degree/s2. Subsequently, they were tilted at the same acceleration and velocity to 30 degrees, 60 degrees, 90 degrees, 60 degrees, 30 degrees and 0 degree to both sides and held in each position for 1 min. The results showed that OCR varied substantially in the two protocols. The most dramatic difference was disconjugacy in the static mode, with the two eyes differing by as much as 4 degrees, in contrast to the generally conjugate OCR in the dynamic mode. Amplitudes also tended to differ, some subjects having greater and others lesser OCR in one mode vis-à-vis the other. Possible explanations for these differences may be found in the work of Hudspeth and colleagues, who found that mechanical deflection of the bullfrog saccula resulted in gradated responses in the underlying hair cells. Further, hair cells in the process of active bending led to different responses than those in a fixed position. Possibly in humans, too, the otoconia do not maintain a fixed relation to the underlying hair cells. Additionally, this study confirms our earlier finding of independent control in the two eyes.