THE VESTIBULOOCULAR REFLEX OF THE SQUIRREL-MONKEY DURING ECCENTRIC ROTATION AND ROLL TILT

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 six male squirrel monkeys during eccentric rotation. Monkeys were rotated in the dark at a constant velocity of 200 degrees/s (centrally or 79 cm off axis) wi th the axis of rotation always aligned with gravity and the spinal axi s of the upright monkeys. The monkey's orientation (facing-motion or b ack-to-motion) had a dramatic influence on the VOR. These experiments show that: (a) the axis of eye rotation always shifted toward alignmen t with gravito-inertial force; (b) the peak value of horizontal slow p hase eye velocity was greater with the monkey facing-motion than with back-to-motion; and (c) the time constant of horizontal eye movement d ecay was smaller with the monkey facing-motion than with back-to-motio n. All of these findings were statistically significant and consistent across monkeys. In another set of tests, the same monkeys were rapidl y tilted about their naso-occipital (roll) axis. Tilted orientations o f 45 degrees and 90 degrees were maintained for 1 min. Other than a co mpensatory angular VOR during the angular rotation, no consistent eye velocity response was observed during or following the tilt for any of the six monkeys. The absence of any eye movement response following t ilt weighs against the possibility that translational linear VOR respo nses are due to simple high-pass filtering of the otoIith signals. The VOR response during eccentric rotation was divided into the more fami liar angular VOR and linear VOR components. The angular component is k nown to depend upon semicircular canal dynamics and central influences . The linear component of the response decays rapidly with a mean dura tion of only 6.6 a, while the axis of eye rotation rapidly aligns (<10 s) with gravito-inertial force. These results are consistent with the hypothesis that the measurement of gravito-inertial force by the otol ith organs is resolved into central estimates of linear acceleration a nd gravity, such that the central estimate of gravitational force minu s the central estimate of linear acceleration approximately equals the otolith measurement of gravito-inertial force.