Level dependence of AC and DC magnitude transfer functions in the cochlea: Implications for intensity and frequency coding

AC and DC magnitude transfer functions were recorded from cells in the organ of Corti in the middle turn of the gerbil cochlea. It was found that a healthier cochlea shows more compression around the BF, a greater intensity-dependent BF shift, and a stronger DC response. Both AC and DC magnitude transfer functions show a shift in the BF. The shift exceeds an octave as intensity is raised from 40 to 100 dB SPL. Both the DC and the AC transfer functions exhibit an identical high-frequency cutoff that remains constant up to 80 kB SPL. These results further support the conclusion that the changing BF cannot encode the pitch of a pure tone, but that the high-frequency response cutoff (the apical cutoff in the spatial excitation pattern) could. At the same time, the strong compression in both the AC and DC I/O functions measured around the frequency of greatest sensitivity confirms that, for pure tone inputs, the growth of the response at one location in the cochlea cannot encode the entire range of audible intensities. [Work supported by NIDCD.]