Horizontal vestibuloocular reflex evoked by high-acceleration rotations inthe squirrel monkey. III. Responses after labyrinthectomy

The horizontal angular vestibuloocular reflex (VOR) evoked by high-frequenc y, high-acceleration rotations war studied in four squirrel monkeys after u nilateral labyrinthectomy. Spontaneous nystagmus was measured at the beginn ing and end of each testing session. During the period that animals were ke pt in darkness (4 days), the nystagmus at each of these times measured simi lar to 20 degrees/s. Within 18-24 h after return to the light, the nystagmu s (measured in darkness) decreased to 2.8 +/- 1.5 degrees/s (mean +/- SD) w hen recorded at the beginning but was 20.3 +/- 3.9 degrees/s at the end of the testing session. The latency of the VOR measured from responses to step s of acceleration (3,000 degrees/s(2) reaching a velocity of 150 degrees/s) was 8.4 +/- 0.3 ms for responses to ipsilesional rotations and 7.7 +/- 0.4 ms for contralesional rotations. During the period that animals were kept in darkness after the labyrinthectomy, the gain of the VOR measured during the steps of acceleration was 0.67 +/- 0.12 for contralesional rotations an d 0.39 +/- 0.04 for ipsilesional rotations. Within 18-24 h after return to light, the VOR gain for contralesional rotations increased to 0.87 +/- 0.08 , whereas there was only a slight increase for ipsilesional rotations to 0. 41 +/- 0.06. A symmetrical increase in the gain measured at the plateau of head velocity was noted after the animals were returned to light. The VOR e voked by sinusoidal rotations of 2-15 Hz, +/-20 degrees/s, showed a better recovery of gain at lower (2-4 Hz) than at higher (6-15 Hz) frequencies. At 0.5 Hz, gain decreased symmetrically when the peak amplitude was increased from 20 to 100 degrees/s. At 10 Hz, gain was decreased for ipsilesional ha lf-cycles and increased for contralesional half-cycles when velocity was ra ised from 20 to 50 degrees/s. A model incorporating linear and nonlinear pa thways was used to simulate the data. Selective increases in the gain for t he linear pathway accounted for the recovery in VOR gain for responses at t he velocity plateau of the steps of acceleration and for the sinusoidal rot ations at lower peak velocities. The increase in gain for contralesional re sponses to steps of acceleration and sinusoidal rotations at higher frequen cies and velocities was due to an increase in the contribution of the nonli near pathway. This pathway was driven into cutoff and therefore did not aff ect responses for rotations toward the lesioned side.