Responses of Single Neurons of the Vestibular Nuclei to Lateral Tilt and Caloric Stimulation in the Intact and Hemilabyrinthectomized Cats

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Abstract
Responses of unitary discharges of the vestibular nuclei to lateral tilt and caloric stimulation were studied in the unanesthetized encéphale isolé preparations of intact and hemilabyrinthectomized cats. The responses of firing rate of 93 units to lateral tilt were classified into 5 groups: increased type. (increase with lateral tilt to either or both directions), decreased type (decrease with lateral tilt to either or both directions), reversed type-I (increase with ipsilateral and decrease with contralateral tilt), reversed type-II (decrease with ipsilateral and increase with contralateral tilt), and refractory type (refractory with lateral tilt to both directions). The most dominant pattern of the responses to lateral tilt was the decreased type (60.2%). On the other hand, more than half of the vestibular units (61.3%) showed reverse changes in frequency to caloric stimulation with cold and warm water. Two main types of units were localized in the vestibular nuclei: one increased the frequency in response to ipsilateral tilt selectively, and another increased the frequency in response to caloric stimulation with cold water. Contralateral labyrinthectomy decreased the firing rate of spontaneous unitary discharges in the vestibular nuclei and the decreased frequency recovered to the control level in 42 days after the operation. However, the pattern of responses of the vestibular neurons to lateral tilt was not modified by the contralateral labyrinthectomy.