Responses to step and repeated impulse accelerations in second order vestibular neurons of the cat

The characteristics of response of 98 vestibular nuclei neurons were investigated in the barbiturate anesthetized cat. All type I and type II neurons (tonic and kinetic) responded to stimulation of the horizontal canals with step and impulse accelerations. Seventy-seven (79 per cent) neurons of all types demonstrated adaptation, while the remaining 21 (21 per cent) neurons showed either reverse adaptation or no adaptation. When the interval between impulses with impulse accelerations reached a critical minimum, the averaged response was remarkably like the response to step acceleration, including the rate of adaptation. A previously undescribed response decline to repetitive impulse acceleration was seen, which, while sharing some characteristics with habituation, appears to be more closely related to adaptation. Furthermore, the data suggest that adaptation seen at the vestibular nuclear level is more likely a central phenomenon rather than a peripheral one.