M. Kossl et al., ACOUSTIC DISTORTION PRODUCTS FROM THE COCHLEA OF THE BLIND AFRICAN MOLE-RAT, CRYPTOMYS SPEC, Journal of comparative physiology. A, Sensory, neural, and behavioral physiology, 178(3), 1996, pp. 427-434
The measurement of distortion-product otoacoustic emissions is a nonin
vasive method that can be used for assessing the sensitivity and the f
requency tuning of nonlinear cochlear mechanics. During stimulation wi
th two pure tones f1 and f2, the acoustic 2f1-f2 distortion was record
ed in the ear canal of Cryptomys spec. to study specializations in coc
hlear mechanics that could be associated with the presence of a freque
ncy expanded cochlear region between 0.8-1 kHz. In addition, a distort
ion threshold curve was obtained which describes relative threshold of
nonlinear cochlear mechanics. Sensitive distortion thresholds could b
e measured for stimulus frequencies between 0.4 to 18 kHz with a broad
minimum between 0.75 to 2.5 kHz. The distortion threshold curve exten
ds to higher frequencies than previous neuronal data indicated. As a m
easure of mechanical tuning sharpness in the cochlea, suppression tuni
ng curves of 2f1-f2 were recorded. The tuning curves reflected the typ
ical mammalian pattern with shallow low frequency and steep high frequ
ency slopes. Their tuning sharpness was poor with Q10dB values between
0.3 and 1.88. In the range of the frequency expanded region, the Q10d
B values were below 0.5. This finding emphasizes that the presence of
frequency expansion does not necessarily lead to enhanced mechanical t
uning in the cochlea and one has to consider if in certain bat species
with cochlear frequency expansion and particularly sharp cochlear tun
ing, the two phenomena may not be interlinked.