High frequency distortion products from the ears of two bat species, Megaderma lyra and Carollia perspicillata

Abstract

In two echolocating bat species, Megaderma lyra and Carollia perspicillata 2f1−f2 distortion products were measured acoustically in the outer ear canal for f1 frequencies between 5 and 95 kHz in an attempt to study nonlinear cochlear processes at high frequencies. Similar to other mammals, the input/output (I/O) functions of 2f1−f2 show non-monotonicities and notches at f1 levels between 40–70 dB SPL. The slope of the initial rise of the 2f1−f2 I/O functions increased with ft frequency from 0.73 (5 kHz) to 1.41 (95 kHz) in Megaderma and from 0.66 (7.5 kHz) to 1.47 (95 kHz) in Carollia. With increasing f1 frequency the optimum frequency ratio f2/f1 in order to evoke maximum distortion level decreases from 1.18 (5 kHz) to 1.09 (95 kHz) in Megaderma and from 1.21 (75 kHz) to 1.11 (95 kHz) in Carollia. This is taken as indication of a general increase of the quality of tuning of the distortion generating mechanism with frequency. The f1 levels that were sufficient to elicit distortion levels of − 10 dB SPL were used to construct iso-distortion threshold curves which lay on average 37.2 dB (Megaderma) and 33.9 dB (Carollia) above the neuronal threshold curves [Rübsamen et al., J. Comp. Physiol. A 163 (1988); Sterbing et al., Proc. 18th Göttingen Neurobiol. Conf. Thieme Verlag, Stuttgart (1990)] and roughly parallel these. Highest distortion levels (40–50 dB SPL with f1 levels of 80 dB SPL) and lowest distortion thresholds were measured in the f1 frequency range of 10–30 kHz (Megaderma) and 15–30 kHz (Carollia). For both bat species these respective frequency ranges correlate with broad minima in the neuronal audiogram. Thus in these animals distortion measurement can be used as noninvasive means to assess relative hearing threshold. Systemic application of salicylate (100–400 mg/kg) which is known to block outer hair cell (OHC) motility (Shehata et al., 1991) reduced the level of distortion by up to 25 dB (ft frequency: 20 kHz) and up to 36 dB (f1 frequency: 70 kHz).