Two mechanisms are believed to underlie the perception and discrimination of pitch: spectrally-based place mechanisms and temporally-based periodically mechanisms. Nonspectral pitch can be studied by using special acoustic or electrical signals which lack spectral cues such as sinusoidally amplitude modulated (SAM) broadband noise, or periodic electrical signals delivered to a deaf ear lacking hair cells. A basic assumption underlying the use of these signals to study nonspectral pitch discrimination is that the signal periodically dominates the discrimination, and other aspects of the signal are relatively unimportant. We will test this hypothesis by comparing, in the same subject, frequency difference limens (measured with careful control to eliminate loudness cues) for different stimulus types which are expected to produce different but predictable neural responses, and by measuring the effects of stimulus levels and the effects of nerve survival patterns on these difference limens. The subjects will be psychophysically trained nonhuman primates with one normal ear and one ear deafened by local perfusion of neomycin and implanted with a scala tympani electrode array. After comparisons between the normal and the implanted ear are completed, effects of a partial lesion of the auditory nerve will be studied on the unimplanted side. We will also compare the performance of the nonhuman primates on the SAM noise discrimination task with performance of human subjects tested under the same conditions to determine if human-animal differences seen with pure tone frequency discrimination also apply to nonspectral frequency discrimination. These studies are directly relevant to the design of processors for cochlear implants and to audiological assessment of auditory nerve fiber damage. They will also lead to a better understanding of stimuli used in psychophysical and neurophysiological investigations related to nonspectral pitch and the applicability of these studies to humans.
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