Further Studies On the Peripheral Auditory System of ‘CF-FM’ Bats Specialized for Fine Frequency Analysis of Doppler-Shifted Echoes

Abstract

In the mustache bat (Pteronotus parnellii rubiginosus), the cochlear microphonic (CM) recorded from the round window was sharply tuned at 61 kHz and showed a prminent transient response to a tone burst at about 61 kHz, (i.e., its amplitude increased exponentially at the onset of the stimulus and decreased at its cessation). In terms of the time constant (1.1 .+-. 0.3 ms) and the resonance frequency (61.1 .+-. 0.43 kHz) of this transient response, the Q of this system, assumed to correspond to a 2nd-order filter, was 204 .+-. 57. Peripheral neurons sensitive to 61 kHz had a very sharp excitatory area (or tuning curve). The Q of a tuning curve markedly increased with the rise in best frequency up to 61 kHz and decreased beyond 61 kHz. The Q value of a single neuron with best frequencies between 60.76-61.75 kHz was 210 .+-. 89. If the assumption that the CM is directly related to the mechanical motion of the basilar membrane is correct, the very sharp tuning curves of single neurons at about 61 kHz could be simply due to the mechanical tuning of the basilar membrane. Since this animal predominantly uses a 61 kHz sound for echolocation and peripheral auditory neurons show a low threshold and extremely sharp tuning at about 61 kHz, its peripheral auditory system is specialized for the reception and fine-frequency analysis of the principle component of orientation sounds and echoes. Sharply tuned neurons can code a frequency modulation as small as 0.01%, so that the wing beat of an insect would be easily coded. Unlike the CM, N1 was tuned at 64 kHz. This difference in best frequency is simply due to the properties of a sharply tuned resonator and N1, and not due to a mechanism comparable to lateral inhibition.