Dynamic properties of the responses of single neurons in the inferior colliculus of the rat.

The responses of single cells in the central nucleus of the inferior colliculus of the rat were studied with characteristic frequency tones amplitude modulated by pseudorandom noise or sinusoidal waveforms, in order to investigate the degree to which these responses can be described by a linear model. When pseudorandom noise was used as the modulating waveform, period histograms of the response locked to the periodicity of the noise were cross-correlated with a single period of the noise. The response of a model, having this cross-correlogram as its impulse response and the pseudorandom noise sequence as the input waveform, differed in appearance from the corresponding period histogram of the neural discharges, indicating that the latter contained a non-negligible, nonlinear component. Further manipulation of the data showed that the most significant nonlinearities were of even order, which indicates that the changes in neural discharge rate to increments and decrements in stimulus intensity are asymmetrical. In some units, particularly at low mean stimulus intensities, this was clearly evident as a halfwave rectification of the period histogram. The magnitude of the modulation of the period histograms increased as a function of the sound intensity for some units, while in others it decreased; in still other units the magnitude of the modulation of the neural discharges was relatively constant over a large range of stimulus intensities. When the modulation transfer functions were estimated from the responses to noise-modulated sounds they were found to be very similar to those obtained using sinusoidally modulated sound, despite large degrees of nonlinearity being present in the responses to both types of sound.