The Role of Compression and Traveling Wave Pressures in the Transmission of Sound out of the Gerbil Cochlea

Cochlear emissions provide a noninvasive probe of cochlear mechanics, but their utility is hindered by incomplete understanding of their relationship to intracochlear activity. In particular, recent work has uncovered a question about the mode by which emissions travel out of the cochlea -whether they emerge via a “fast” compression pressure or a “slow” traveling-wave pressure. We further probed this question with simultaneous measurements of intracochlear distortion products (DPs) at two well-separated locations and DP oto-acoustic emissions (DPOAEs). In the broad frequency range of the local best frequency (BF), the DP responses demonstrate the now well-known forward-traveling-wave character. However, at frequencies substantially lower than the BF, comparisons of both DPOAEs to DPs and of DPs at two locations support a reverse-traveling-wave. Finally, a compression pressure DP was observed when stimulating at high levels (90 dB) with frequencies that were well above the BF. Therefore, the compression / reverse-traveling-wave question appears to be a quantitative question of the relative size of these different pressure modes. In previous and present results we find that the reverse-traveling-wave mode can be dominant both within the cochlea and in the production of DPOAEs.