FURTHER-STUDIES ON THE MECHANICS OF THE COCHLEAR PARTITION IN THE MOUSTACHED BAT .2. A 2ND COCHLEAR FREQUENCY MAP DERIVED FROM ACOUSTIC DISTORTION PRODUCTS
M. Kossl et M. Vater, FURTHER-STUDIES ON THE MECHANICS OF THE COCHLEAR PARTITION IN THE MOUSTACHED BAT .2. A 2ND COCHLEAR FREQUENCY MAP DERIVED FROM ACOUSTIC DISTORTION PRODUCTS, Hearing research, 94(1-2), 1996, pp. 78-86
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.