M. Faulstich et al., ANALYSIS OF NONLINEAR COCHLEAR MECHANICS IN THE MARSUPIAL MONODELPHIS-DOMESTICA - ANCESTRAL AND MODERN MAMMALIAN FEATURES, Hearing research, 94(1-2), 1996, pp. 47-53
The acoustic distortion product 2f1-f2 was measured in the ear canal o
f the gray short-tailed opossum, Monodelphis domestica, during stimula
tion with two pure tone stimuli of the frequencies f1 and f2 (f1<f2).
From 2f1-f2 growth functions for different primary frequencies from 4
to 85 kHz, a distortion threshold curve was obtained describing the re
lative thresholds of non-linear mechanical processes in the cochlea. D
istortion products could be measured over the entire investigated freq
uency range and the distortion audiogram proved sensitive (thresholds
<20 dB SPL) over a wide range from 8 to 55 kHz with a broad minimum ar
ound 17.5 kHz. Thresholds steeply increased for frequencies below 8 kH
z which can be regarded as an original mammalian feature (Heffner and
Masterton, 1980). To assess the tuning characteristics of the cochlea,
suppression tuning curves of 2f1-f2 were measured. The shape and tuni
ng sharpness of these curves resembled that of modern eutherian mammal
s with a shallower low-frequency and a steeper high-frequency flank an
d Q10dB values between 2.4, and 7.12. The optimum primary ratio f2/f1
(best-ratio) was on average 1.172. For f2 frequencies between 20 and 3
0 kHz the ratio curves displayed multiple peaks in periodic intervals.
The same peaks were also evident for higher-order distortions (3f1-2f
2, 4fl-3f2). It is discussed to which extent this finding is consisten
t with the second filter theory of Brown et al. (1992) and Alien and F
ahey (1993) and how in Monodelphis domestica this filter could work di
fferently, possibly representing a less evolved stage, than in eutheri
an mammals.