Horizontal vestibuloocular reflex evoked by high-acceleration rotations inthe squirrel monkey. IV. Responses after spectacle-induced adaptation

The horizontal angular vestibuloocular ref . (VOR) evoked by sinusoidal rot ations from 0.5 to 15 Hz and acceleration steps up to 3,000 degrees /s(2) t o 150 degrees /s was studied in six squirrel monkeys following adaptation w ith X2.2 magnifying and X0.45 minimizing spectacles. For sinusoidal rotatio ns with peak velocities of 20 degrees /s, there were significant changes in gain at all frequencies; however, the greatest gain changes occurred at th e lower frequencies. The frequency- and velocity-dependent gain enhancement seen in normal monkeys was accentuated following adaptation to magnifying spectacles and diminished with adaptation to minimizing spectacles. A diffe rential increase in gain for the steps of acceleration was noted after adap tation to the magnifying spectacles. The gain during the acceleration porti on, G(A), of a step of acceleration (3000 degrees /s(2) to 150 degrees /s) increased from preadaptation values of 1.05 +/-0.08 to 1.96 +/-0.16, while the gain during the velocity plateau, G(V), only increased from 0.93 +/-0.0 4 to 1.36 +/-0.08. Polynomial fits to the trajectory of the response during the acceleration step revealed a greater increase in the cubic than the li near term following adaptation with the magnifying lenses. Following adapta tion to the minimizing lenses, the value of G(A) decreased to 0.61 +/-0.08, and the value of G(V) decreased to 0.59 +/-0.09 for the 3 000 degrees /s(2 ) steps of acceleration. Polynomial fits to the trajectory of the response during the acceleration step revealed that there was a significantly greate r reduction in the cubic term than in the linear term following adaptation with the minimizing lenses. These findings indicate that there is greater m odification of the nonlinear as compared with the linear component of the V OR with spectacle-induced adaptation. In addition, the latency to the onset of the adapted response varied with the dynamics of the stimulus. The find ings were modeled with a bilateral model of the VOR containing linear and n onlinear pathways that describe the normal behavior and adaptive processes. Adaptation for the linear pathway is described by a transfer function that shows the dependence of adaptation on the frequency of the head movement. The adaptive process for the nonlinear pathway is a gain enhancement elemen t that provides for the accentuated gain with rising head velocity and the increased cubic component of the responses to steps of acceleration. While this model is substantially different from earlier models of VOR adaptation , it accounts for the data in the present experiments and also predicts the findings observed in the earlier studies.