Responses of inferior colliculus neurons to harmonic and mistuned complex tones

Responses of inferior colliculus neurons to simplified stimuli that may engage mechanisms that contribute to auditory scene analysis were obtained. The stimuli were harmonic complex tones, which are heard by human listeners as single sounds, and the same tones with one component 'mistuned', which are heard as two separate sounds. The temporal discharge pattern elicited by a harmonic complex tone usually resembled the same neuron's response to a pure tone. In contrast, tones with a mistuned component elicited responses with distinctive, stereotypical temporal patterns that were not obviously related to the stimulus waveform. For a particular stimulus configuration, the discharge pattern was similar across neurons with different pure-tone frequency selectivity. A computational model that compared response envelopes across multiple narrow bands successfully reproduced the stereotypical response patterns elicited by different stimulus configurations. The results suggest that mistuning created a temporally synchronous distributed representation of the mistuned component that could be identified by higher auditory centers in the presence of the ongoing response produced by the remaining components; this kind of representation might facilitate the identification of individual sound sources in complex acoustic environments.