Vestibular (semicircular canal) primary neurons in bullfrog: nonlinearity of individual and population response to rotation.

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Abstract
The activity of individual semicircular canal primary neurons in the bullfrog [Rana catesbeiana] was examined during prolonged horizontal-plane angular velocity stimuli, both sinusoidal and triangular, having different amplitudes and frequencies. In most cells, the peak response in successive stimulus cycles gradually declined over a 1- to 4-min interval, and thereafter remained essentially constant. Results described here pertain to this late, nondeclining response. Nonlinearities in neural behavior were examined fusing xy plots, which for sinusoidal stimuli were constructed by aligning the phase of the stimulus with that of the response and then plotting the instantaneous magnitude of neural response (on the y axis) against the corresponding magnitude of the phase-shifted stimulus (on the x axis). When neural activity elicited during different-amplitude 0.3-Hz sinusoidal stimuli was examined using these xy plots, all but 2 of 51 neurons exhibited behavior that was clearly incompatible with any model composed of a dynamic linear element followed by a static nonlinearity. Neural responses were characterized by gains and thresholds that were highly dependent on stimulus magnitude. The shape of the threshold distribution was also strongly affected by stimulus amplitude. Estimates of the viscoelastic time constant and of high-frequency phase discrepancy were uncorrelated with estimates of the adaptation time constant. During 0.3-Hz sine-wave stimuli, the average activity of the observed sample of semicircular canal primary neurons had a waveform whose shape was remarkably unaffected by stimulus magnitude. The upper half of this waveform was sinusoidal, lacking the nonlinear characteristics exhibited by individual neurons; the lower half was nonsinusoidal.