THE SHAPE OF 2F(1)-F(2) SUPPRESSION TUNING CURVES REFLECTS BASILAR-MEMBRANE SPECIALIZATIONS IN THE MOUSTACHED BAT, PTERONOTUS-PARNELLII

Iso-suppression tuning curves (STCs) of the 2f(1)-f(2) distortion prod uct (dp) were measured over a primary frequency range of 20 to 93 kHz in mustached bats, Pteronotus pamellii pamellii. Primary levels were c hosen to produce dp levels between 0 and 7 dB SPL. At frequencies outs ide the ranges of 60 - 72 kHz and 90 - 93 kHz the shapes of the STCs w ere symmetrical or asymmetrical with a steep high frequency slope. In the vicinity of 61 kHz where a strong stimulus frequency otoacoustic e mission (SFOAE) is present, the asymmetry of the STCs was inverted wit h a very steep low frequency slope(max. -89 dB/kHz) and a shallow high frequency slope. The inverted STCs resemble neuronal tuning curves of the same species with best frequencies at about 61 kHz. Close to 61 k Hz the STCs were sharply tuned with Q(10dB) values up to 177. The STC- thresholds were about 20 dB above the neuronal thresholds. Thickenings of the basilar membrane located just basal to the cochlear place of t he SFOAE frequency are probably involved in creating the asymmetric ST Cs. Cochlear resonance at the SFOAE frequency and an increased longitu dinal coupling within the thickened basilar membrane region are though t to contribute to the specialized STC shape. In the range of 40 - 93 kHz, the STCs are also sharply tuned with inverted asymmetry which is probably not due to an harmonic effect of the specialized cochlear mec hanics in the 60 kHz region but may be caused by an independent mechan ism.