Ocular counterrolling in response to static and dynamic tilting: implications for human otolith function

Nineteen subjects underwent rotation about the naso-occipital axis to examine ocular counterrolling (OCR) responses in both dynamic and static conditions. Dynamic rotation consisted of tilt to 90 degrees right and left at constant velocity of 3 degrees /s, with acceleration at 0.2 degrees /s(2). Static rotation (stepwise tilting) consisted of one minute at steps 30 degrees, 60 degrees, 90 degrees, 60 degrees, 30 degrees to each side. OCR amplitudes at each of the above head tilts in dynamic vs. static tilt showed highly significant differences, with dynamic tilt exceeding that of static. Although OCR disconjugacy was greater in static than dynamic in most subjects, that difference was not statistically significant. Possible explanations for the disparity in the responses to dynamic and static tilt in humans lie in vestibular experiments leading to the conclusion that the otolith membrane in cats moves not as a unit, but rather in patches (28). This is supported by work showing the otolith membrane in bullfrogs does not move en bloc (1). In addition, hair cell responses are found to be amplified during motion as compared to the responses during static positioning (20). Functionally, it is suggested that the otolith system may perform better in moving than in static conditions.