VESTIBULOOCULAR REFLEX OF THE SQUIRREL-MONKEY DURING ECCENTRIC ROTATION WITH CENTRIPETAL ACCELERATION ALONG THE NASO-OCCIPITAL AXIS

The vestibule-ocular reflexes (VOR) are determined not only by angular acceleration, but also by the presence of gravity and linear accelera tion. This phenomenon was studied by measuring three-dimensional nysta gmic eye movements, with implanted search coils, in four male squirrel monkeys. Monkeys were rotated in the dark at 200 degrees/s, centrally or 79 cm off-axis, with the axis of rotation always aligned with grav ity and the spinal axis of the upright monkeys. The monkey's position relative to the centripetal acceleration (facing center or back to cen ter) had a dramatic influence on the VOR. These studies show that a to rsional response was always elicited that acted to shift the axis of e ye rotation toward alignment with gravito-inertial force. On the other hand, a slow phase downward vertical response usually existed, which shifted the axis of eye rotation away from the gravito-inertial force. These findings were consistent across all monkeys. In another set of tests, the same monkeys were rapidly tilted about their interaural (pi tch) axis. Tilt orientations of 45 degrees and 90 degrees were maintai ned for 1 min. Other than a compensatory angular VOR during the rotati on, no consistent eye velocity response was ever observed during or fo llowing the tilt. The absence of any response following tilt proves th at the observed torsional and vertical responses were not a positional nystagmus. Model simulations qualitatively predict all components of these eccentric rotation and tilt responses. These simulations support the conclusion that the VOR during eccentric rotation may consist of two components: a linear VOR and a rotational VOR. The model predicts a slow phase downward, vertical, linear VOR during eccentric rotation even though there was never a change in the force aligned with monkey' s spinal (Z) axis. The model also predicts the torsional components of the response that shift the rotation axis of the angular VOR toward a lignment with gravito-inertial force.