Response dynamics of horizontal canal afferents in barbiturate-anesthetized cats

Abstract

The relation between discharge frequency and rotational acceleration was determined for 8th nerve vestibular neurons (94) of the barbiturate-anesthetized cat. Response gain and phase of each cell were estimated at 7-9 frequencies between 0.005-3.7 Hz. To determine whether dynamic properties were correlated with resting discharge variability, cells were classified into 3 groups according to the coefficient of variation (CV) of the resting discharge (low-variability (LV) cells had CV less than 10%, high-variability (HV) cells had CV > 30% and IV cells were of intermediate variability). The response properties of individual cells were described by Bode plots of gain and phase for each cell. Data were described mathematically with a transfer function equation: gain and phase values at any test frequency varied widely from cell to cell, demonstrating that a rotational stimulus may be encoded differently by each member of this population of afferents. The observed between-cell differences were not the result of measurement error. Each 8th nerve afferent studied may be tuned to respond maximally to certain kinds of head movements. The wide variability seen in gain and phase from 1 canal afferent to another suggest that the concept of an average transfer function is an oversimplification, since different cells in the same animal can have different dynamic properties. The quantification of the variety of dynamic response properties among the members of this population enhances the understanding of how between-cell differences might contribute to the encoding of head movements.