Auditory evoked potentials to continuous amplitude-modulated sounds: can they be described by linear models?

The responses from the round window of the cochlea and the surface of the cochlear nucleus to continuous tones and noise that were amplitude-modulated with pseudorandom noise were studied. The responses were averaged with the averager locked to the periodicity of the pseudorandom noise, and the cross-correlation between the averaged responses and one period of the pseudorandom noise was computed. The degrees of non-linearities in the responses were estimated by comparing the response of a linear model that had this cross-correlogram as its impulse response and the physiological response. The root mean square (RMS) value of the histograms increases as a function of the stimulus intensity and reaches a peak at 40-60 dB above threshold, above which intensity it decreases. The non-linear component of the response increases monotonically with sound intensity in the range from threshold to 60-70 dB above threshold. The pseudorandom noise was of the inverse-repeat type, and when even-order non-linearities were canceled by subtracting the latter half of the responses to one period of the pseudorandom noise from the first half a much closer agreement with the model responses was obtained, indicating that the non-linearities were mainly of even order. It was concluded that the non-linearities of the responses may have been caused by an unequal response to increases and decreases in stimulus intensity.