ACOUSTIC DISTORTION PRODUCTS FROM THE COCHLEA OF THE BLIND AFRICAN MOLE-RAT, CRYPTOMYS SPEC

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.