Effects of vestibulocerebellar lesions upon dynamic characteristics and adaptation of vestibulo-ocular and optokinetic responses in pigmented rabbits.

The vestibulo-ocular reflex (VOR) and optokinetic response (OKR) in the horizontal plane were examined in pigmented rabbits, using sinusoidal whole-body rotation and sinusoidal rotation of a striped screen. Sustained rotation of the animal (5 degrees peak-to-peak, 0.1 Hz) for 4 h, under different optokinetic stimulus conditions, induced the following adaptive changes in the VOR: (1) outphase rotation of the screen (5 degrees) increased the VOR gain by 0.3 (on average); (2) with the screen fixed in space, VOR gain increased by 0.2. (3) in-phase rotation of the screen (5 degrees) decreased the VOR gain by 0.16. However, (4) in-phase rotation of the screen at twice (10 degrees) the amplitude of whole-body rotation did not affect the gain. Instead, it induced a significant phase lead (23 degrees) in the VOR, which did not occur in other stimulus conditions. Adaptive increases of the OKR gain occurred under sustained rotation of the screen alone (2.5 degrees, 0.33 Hz). After bilateral destruction of floccular Purkinje cells with kainic acid the VOR gain and phase were affected only very slightly, but adaptive changes in the VOR were abolished. By contrast, the OKR gain was reduced and the OKR phase delayed. OKR adaptation was also affected in such a way that a gain increase initially produced could not be maintained during sustained screen rotation. Ablation of nodulus-uvala caused a gain increase and phase lead in both VOR and OKR, and its only effect on adaptability of the VOR or the OKR was observed for the VOR under stimulus condition (4).