Acoustic input-admittance of the alligator-lizard ear: nonlinear features.

The acoustic input-admittance at the alligator lizard's tympanic membrane varies with stimulus level; the magnitude of the variation can be as much as a factor of three. At 1.6 kHz, the frequency of maximum admittance magnitude, the admittance varies when the stimulus level exceeds 65 dB SPL. At frequencies above or below 1.6 kHz, larger SPLs are needed to produce admittance changes. With stimulus frequencies below 0.3 kHz or above 4.0 kHz the admittance is virtually constant for stimulus levels up to at least 100 dB SPL. The nonlinear behavior (a) is greatly reduced when the cochlear partition is destroyed, (b) does not return when the mechanical load of the partition is replaced, (c) is decreased by the introduction of proteolytic enzymes into the inner ear, and (d) is not affected by some manipulations that greatly reduce cochlear potentials. The results suggest that the mechanical properties of the cochlear partition are the source of the nonlinear admittance. Parallels between this phenomenon and two-tone distortion products in the ear canal (Rosowski et al. (1984): Hearing Res. 13, 141-158) suggest that the same nonlinear mechanical source that generates the level-dependent admittance also produces two-tone distortion products in the lizard ear canal. Published demonstrations of level-dependent admittance in mammalian ears, although rather different from these results, do not rule out the presence of a similar mechanism in the mammalian cochlea.