Directional dependence of auditory sensitivity and frequency selectivity in the leopard frog.

Direction-dependent changes in frequency selectivity were documented in three populations of neurons in the auditory periphery of the leopard frog: low-, mid-, and high-frequency sensitive fibers. These changes were most pronounced in the mid-frequency (720-1199 Hz) sensitive fibers which exhibited positive shifts in characteristic frequencies (CFs) and concomitant narrowed bandwidths from posterior speaker presentations. Maximum sensitivities were observed for ipsilateral speaker presentations in these fibers. Low-frequency (63-500 Hz) sensitive fibers exhibited mean threshold shifts at their CFs of up to -7 dB with posterior speaker presentations. Low- and high-frequency sensitive fibers whose CFs closely correspond to the peaks of spectral energy of the species' mating call were the most sensitive (exhibited lowest thresholds) but showed little directional-dependent frequency selectivity. Directional-dependent variation in frequency selectivity and sensitivity in low- and mid-frequency sensitive fibers was attributed to pressure and phase differences impinging on the inner surface of the eardrum.