Cochlear mechanisms of frequency and intensity coding. I. The place code for pitch

Abstract

In the past, several researchers have reported a substantial shift in the peak of the tone-evoked excitation pattern toward the base of the cochlea following an increase in the SPL of the stimulating tone. Evidence for such peak shifts has been found in the responses of auditory nerve fibers, cochlear microphonics, and the responses of outer hair cells and supporting cells in the cochlea, as well as in basilar membrane vibration measurements, and indirectly, in psychophysical data. However, direct evidence for such a peak shift in inner hair cell (IHC) responses has been relatively sparse. If the peak shift is preserved in the information conveyed to the auditory nerve fibers by the IHCs, the classical `place theory' for frequency coding in the cochlea requires modification. In this study, the nature and extent of the SPL-dependent peak shift is examined with the help of recordings in the IHCs and other cells of the organ of Corti in the 0.5–2.5 kHz region of the Mongolian gerbil cochlea. It is demonstrated that the peak shift is a universal phenomenon in the diverse cell types in this region of the cochlea. Most importantly, a large SPL-dependent peak shift is demonstrated in IHC responses. On the other hand, the recordings indicate that the apical cutoff of the spatial excitation pattern is SPL-independent. We conclude, therefore, that the place theory of pitch perception must be abandoned or at least modified.