EVIDENCE FOR A MECHANICAL FILTER IN THE COCHLEA OF THE CONSTANT FREQUENCY BATS, RHINOLOPHUS-ROUXI AND PTERONOTUS-PARNELLII

To investigate the function of basilar membrane (BM) thickenings in the cochlea of bats which use constant frequency (CF) echolocation calls, acoustic distortion products were measured while placing the primary stimuli f(1) and f(2) at frequencies which are represented in the thickened BM regions. In Rhinolophus, for primary stimuli between about 80-100 kHz, pronounced maxima of the level of distortion products (2f(1)-f(2), 3f(1)-2f(2), 4f(1)-3f(2)) Can be measured if the frequency separation between the two primary tones is chosen so that the resulting distortion frequency matches the dominant CF frequency (resting frequency, RF). The distortion maxima extend from the individual RF down to frequencies which are 2-4 kHz lower. The data indicate that the thickened BM region in the basal halfturn of the cochlea strongly oscillates at the bats' RF and slightly below. The hearing threshold, however, is at a maximum at the RF (see Kossl 1993). Therefore, the RF oscillations are thought to be involved in absorbing the respective frequency such that the more apically located frequency place of the RF is insensitive and a mechanical notch filter is established. In Pteronotus, there are maxima of the level of 2f(1)-f(2) at distortion frequencies around the RF of about 61.5 kHz up to the frequency of a strong stimulus-frequency otoacoustic emission (SFOAE) which is a few hundred Hz higher. Pronounced distortions in the RF range can only be elicited when the stimulus frequencies are between about 62 to 72 kHz. Similar to the situation in Rhinolophus, this frequency band is represented on a stretch of thickened BM.