Auditory nerve neurophonic tuning curves produced by masking of round window responses

In response to low-intensity, low-frequency, phase-locked tonal stimuli with non-alternating polarity, the time-average round window (RW) response of the gerbil is a mixture of the auditory nerve neurophonic (ANN) and cochlear microphonic (CM), with the former often being of equal or greater magnitude than the latter. Forward masking (using a conservative 25% amplitude reduction criterion) can be used to generate ANN tuning curves (TC). Most of these TCs are sharply tuned V-shaped functions. Harmonic distortion is often present in the ANN, especially in response to the lower-frequency (< or = 1 kz) or higher-intensity (> or = 50 dB) stimuli. The TCs created by forward masking of the harmonics are similar in appearance to those generated by masking the fundamental frequency of the ANN. When lower-frequency probe stimuli (< or = approximately equal to 1 kHz) are used, the frequency of the TC tip tends to be higher than that of the probe; with higher probe frequencies, the tip tends to be lower. Regardless of the frequency of the probe, the TC tip threshold occurs at an intensity level lower than that of the probe. The sharpness of these TCs generally increases as a function of the frequency of the probe stimulus and the values of Q10dB are comparable to those of FTCs of cochlear nerve fibers of the gerbil. The amplitude of the ANN is often enhanced in response to a limited intensity range of forward maskers over a restricted range of frequencies that are outside the high-frequency boundary of the forward masker TC. By alternating the polarity of the probe stimulus, the CM can be canceled, allowing the effects of simultaneous maskers to be evaluated.