THE HORIZONTAL VESTIBULOOCULAR REFLEX IN THE HEMILABYRINTHECTOMIZED GUINEA-PIG

The horizontal vestibulo-ocular reflex (HVOR) in the alert guinea-pig elicited by sinusoidal rotations and by velocity steps was studied wit h scleral search coil measurement between 3 and 7 days (short term) an d between 35 and 160 days (long term) after hemilabyrinthectomy. Anima ls of the short-term group were always tested after spontaneous nystag mus in darkness had disappeared. The HVOR gain in response to sinusoid al rotations (peak angular velocity: 40 deg/s) in the short-term group was bilaterally depressed compared to normal animals. The HVOR phase showed a shift towards larger phase leads over the whole frequency ran ge tested (from 0.05 to 3 Hz). In addition, both the mean number of fa st phases per half-cycle of sinusoidal rotation and the mean amplitude were reduced. HVOR responses to velocity steps at a constant accelera tion of 300 deg/s2 up to final velocity (0 to 100 deg/s) and of 1000 d eg/s2 up to final velocity (0 to 300 deg/s) were depressed bilaterally and asymmetrically such that the gain for rotation towards the intact side greatly exceeded that obtained for rotation towards the lesioned side. Finally, the latency of the vestibular responses was increased and the time constant reduced for both sides of rotation. The HVOR gai n values for sinusoidal rotations in the long-term group were lower th an normal but higher than in the short-term group: they were asymmetri c as a result of a greater compensation for rotation towards the intac t side. Neither the phase lead nor the HVOR latency and time constant recovered values close to normal. Finally, the mean number of fast pha ses per half-cycle remained depressed although the mean amplitude reco vered. These results demonstrate that in the guinea-pig, the dynamic d eficits show a certain degree of recovery after unilateral labyrinthec tomy. However, compared to the compensation of the static deficits pre viously quantified, the rate of recovery is much lower. This suggests that different processes may be involved in the compensation of the st atic and dynamic deficits.