Dissecting the frog inner ear with Gaussian noise. II. Temperature dependence of inner ear function.

The temperature dependence of the response of single primary auditory nerve fibers (n = 31) was investigated in the European edible frog, Rana esculenta (seven ears). Nerve fiber responses were analyzed with Wiener kernel analysis and polynomial correlation. The responses were described with a cascade model, consisting of a linear bandpass filter, a static nonlinearity, and a linear lowpass filter. From the computed Wiener kernels and the polynomial correlation functions, the characteristics of the three model components were obtained. With increasing temperature (1) tuning of the first filter increased in the majority (n = 16) of amphibian papilla fibers (best excitatory frequency, BEF < 1 kHz, n = 21) but remained unchanged in the majority (n = 10) of basilar papilla fibers (BEF > 1 kHz, n = 11), (2) the gain of the first filter remained unchanged, (3) the shape of nonlinear IO function remained unchanged, (4) the combined gain of the static nonlinearity and the second filter usually increased, but displayed considerable scatter across fibers (from -0.7 dB/degrees C to 3 dB/degrees C), and (5) the cutoff frequency of the second lowpass filter increases, with average 0.13 oct/degrees C. The immunity of the shape of the nonlinearity is considered evidence of a temperature independent gating mechanism in the transduction channels. The temperature dependence of the second filter may have resulted from a decrease of the hair cell membrane resistance, but may also reflect changes in subsequent staging of nerve fiber excitation.