Nonlinear phenomena as observed in the ear canal and at the auditory nerve

We report here several measures of nonlinear effects in the mammalian ear made in the external auditory meatus and in single neurons of the auditory nerve. We have measured the 2f₁ − f₂ and thef₂ − f₁ distortion products and we have found that (a) the neural distortion product threshold curve for 2f₁ − f₂ mirrors the low-frequency side of the frequency threshold curve, (b) when the neural distortion product threshold curve of 2f₁ − f₂ is plotted versus log(f₂/f₁) its slope is about 50 dB/oct and its intercept is 10–20 dB above the frequency threshold at the characteristic frequency CF, (c) substantial 2f₁ − f₂ distortion was seen in all animals studied while the f₂ − f₁ distortion product was only rarely found at substantial levels, and (d) the distortion product pressure observed in the ear canal was at a level equal to that detected at threshold by the neural units under study. We have also made measurements of two-tone rate suppression thresholds using two new and consistent threshold paradigms. We find that (a) for high and intermediate characteristic frequency neural units the suppression threshold is independent of frequency and at a level of about 70 dB SPL, (b) the suppression above CF is much less than below CF, and (c) the tip of the frequency tuning curve can be suppressed by up to 40 dB by a low-frequency suppressor.