The overall goal of this application is to evaluate assumptions behind the Allen-Fahey analysis framework for estimating the gain of the cochlear amplifier. Allen and Fahey estimated gain based on response to the 2f1-f2 distortion products generated by the cochlea and concluded that cochlea mechanics are essentially passive: that is, there is no cochlear amplifier. This finding is in conflict with prevailing cochlear models that require 30-40 dB of gain to account for basilar-membrane motion. To explain this discrepanc the investigator proposes to evaluate a set of assumptions that underlie the Allen-Fahey analysis by carrying out experiments that are variations on the Allen-Fahey design. Allen and Fahey measured the firing rate of a fiber to a tone at the fiber's characteristic frequency, and then determined the amplitud of primary tones f1 and f2 that would generate a distortion product at the fiber's characteristic frequency that would elicit the same firing rate. Unde circumstances in which both a single tone and the primary tones caused the sam response in the fiber, amplitude of the distortion product measured in the ear canal (2f1-f2 otoacoustic emission) and the level of the single tone in the ea canal were very nearly the same. This suggests that no gain is provided by th cochlea. The same experimental paradigm will be used here, except that there will be various manipulations to examine (Aim 1): (1) the possible influences of suppression of the distortion product cause by the primaries; (2) whether measurements made at one location (single unit) can be used to deduce cochlear gain accurately; (3) whether emissions evoked by the primaries contribute to t distortion product; and (4) the impact of middle ear and ear canal impedance. Results will be compared with those collected using a noninvasive paradigm already developed by the investigator's group (Aim 2). In this paradigm, a third external tone (f3) will be introduced that will serve as a primary for t generation of a secondary distortion product at frequency 2f3-(2f1-f2). The phase and amplitude of the 2f1-f2 distortion product is deduced from its interaction with the third tone, and this procedure is reported by the investigator to improve sensitivity of emission measurements. Finally, experimental results will be compared with predictions of several prevailing cochlear models (Aim 3).
| Shera, C A; Guinan Jr, J J (1999) Evoked otoacoustic emissions arise by two fundamentally different mechanisms: a taxonomy for mammalian OAEs. J Acoust Soc Am 105:782-98 |
