Neural correlates of compensation after hemilabyrinthectomy.

Vestibular brain stem neurons responsive to angular acceleration in the plane of the horizontal canal were examined in cats 30 to 52 days after contralateral labyrinthectomy and compared with similar units in cats with intact labyrinths and in other cats immediately after transection of the contralateral eighth nerve. In the compensated state, the mean spontaneous firing rate of type I neurons was 24 spikes/s, in contrast to the mean of 45/s observed immediately after contralateral labyrinthectomy. In intact cats, mean firing rate was 19 spikes/s. Sensitivity, as measured in spikes/s/deg/s2, was significantly lower immediately after labyrinthectomy than in intact controls and remained so in compensated cats. On the other hand, time constants and the ratio of adapting:nonadapting units was unchanged. Ablation of the midline cerebellum including vermis and fastigial nuclei did not materially affect these results. We concluded that (i) the main defects in static posture and nystagmus in the uncompensated state were due to the striking difference in resting firing rates between the ipsi- and contralateral vestibular nuclei; (ii) compensation in the static posture was the result of a tendency to equalize the resting firing rates in the two vestibular nuclei; and (iii) recovery in the dynamic, head-turning situation was due to partial recovery of sensitivity on the ipsilateral side, bringing it to the relatively constant, unchanging depressed sensitivity on the contralateral side.