Responses of auditory nerve fibres to noise stimuli show cochlear nonlinearities

The frequency selectivity of the peripheral auditory frequency analyser was determined on the basis of recordings from single auditory nerve fibres in the rat, using pseudorandom noise as stimulus. The impulse response function was arrived at by cross-correlating period histograms of the recorded activity with the noise. Frequency transfer functions were computed by Fourier transformation of the derived impulse response function. The impulse response function was observed to become shorter with increasing stimulus intensity. A concomitant widening of the frequency transfer function and a downward shift in the centre frequency were seen. Since the method used is insensitive to nonlinearities, the derived impulse and frequency respone functions are assumed to be valid approximations of the basilar membrane properties. The results, then, support the hypothesis that the basilar membrane is a nonlinear frequency analyser whose selectivity decreases as sound intensity increases.