Neural mechanisms for spectral segregation

Abstract

Humans exhibit a remarkable ability to segregate sounds produced by multiple sources that overlap in frequency and in time. It is likely that other species that use hearing also have this ability. To investigate the neural mechanisms that contribute to spectral segregation, responses of auditory nerve fibers (ANFs) and inferior colliculus (IC) neurons to harmonic complex tones and to the same tones with a mistuned component were measured. Mistuning leads to the perception of a new sound source and also produces dramatic qualitative changes in the temporal discharge patterns of IC neurons. In contrast, the same stimulus manipulation produces only modest quantitative changes in the responses of ANFs. These results indicate that the processing of complex tones undergoes a major transformation in the lower brainstem that is likely to contribute to perceptual segregation based on harmonicity. A computational model has been developed to investigate integrative mechanisms that may underlie this transformation. The model reproduces the distinctive discharge patterns of IC neurons, and suggests that these patterns arise as a result of narrow-band envelope extraction, followed by broadband excitatory–inhibitory interactions. Specific predictions of the model have been confirmed in subsequent electrophysiological experiments, providing further support for this interpretation. [Work supported by NIDCD.]