COCHLEAR MECHANISMS OF FREQUENCY AND INTENSITY CODING .1. THE PLACE CODE FOR PITCH

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. Evid ence for such peak shifts has been found in the responses of auditory nerve fibers, cochlear microphonics, and the responses of outer hair c ells and supporting cells in the cochlea, as well as in basilar membra ne vibration measurements, and indirectly, in psychophysical data. How ever, direct evidence for such a peak shift in inner hair cell (IHC) r esponses has been relatively sparse. If the peak shift is preserved in the information conveyed to the auditory nerve fibers by the IHCs, th e classical 'place theory' for frequency coding in the cochlea require s modification. In this study, the nature and extent of the SPL-depend ent 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 Mo ngolian gerbil cochlea. It is demonstrated that the peak shift is a un iversal phenomenon in the diverse cell types in this region of the coc hlea. Most importantly, a large SPL-dependent peak shift is demonstrat ed in IHC responses. On the other hand, the recordings indicate that t he apical cutoff of the spatial excitation pattern is SPL-independent. We conclude, therefore, that the place theory of pitch perception mus t be abandoned or at least modified.