FURTHER-STUDIES ON THE MECHANICS OF THE COCHLEAR PARTITION IN THE MOUSTACHED BAT .2. A 2ND COCHLEAR FREQUENCY MAP DERIVED FROM ACOUSTIC DISTORTION PRODUCTS

It has been proposed that acoustic 2f1-f2 distortions reflect the freq uency characteristics of a secondary cochlear filter mechanism (Brown et al., 1992; Allen and Fahey, 1993). This concept was used to constru ct a second cochlear frequency, f1 was varied and the frequency ratio f2/f1 determined that produced maximum levels of the 2f1-f2 distortion (best ratio). The second cochlear frequency map was derived by plotti ng the distortion frequency that corresponded to the best ratio f2/f1 against the cochlear place of f2 which was obtained from the HRP-frequ ency that corresponded to the best ratio f2/f1 against the cochlear pl ace of f2 which was obtained from the HRP-frequency map of Pteronotus (Kossl and Vater, 1985b). Minimum best ratio of 1,0005 and hence pract ically identical characteristic frequencies of the putative tuning of basilar membrane (HRP) and TM (2f1-f2) were found at about 45% distanc e from the base, a point at which 62 kHz are represented on the BM. Th is frequency is associated with strong cochlear resonance and large ev oked and spontaneous otoacoustic emissions. Between 45% and 20% distan ce from the base, the basilar membrane (BM) tuning progressively incre ases to about 70 kHz whereas the calculated TM tuning remains constant at a frequency close to 62 kHz. The range of constant TM tuning coinc ides with the sparsely innervated cochlear region of Pteronotus where BM thickness is maximal and TM mass and limbal attachment are reduced (Vater and Kossl, 1996). We suggest that here the TM oscillates strong ly at 62 kHz and may carry most of the energy of cochlear resonance wh ich is transferred into movement of the organ of Corti at and apical t o the 45% location where the Bm is tuned to 62 kHz.