Interaural time sensitivity dominated by cochlea-induced envelope patterns.

To localize sounds in space, humans heavily depend on minute interaural time differences (ITDs) generated by path-length differences to the two ears. Physiological studies of ITD sensitivity have mostly used deterministic, periodic sounds, in which either the waveform fine structure or a sinusoidal envelope is delayed interaurally. For natural broadband stimuli, however, auditory frequency selectivity causes individual channels to have their own envelopes; the temporal code in these channels is thus a mixture of fine structure and envelope. This study introduces a method to disentangle the contributions of fine structure and envelope in both binaural and monaural responses to broadband noise. In the inferior colliculus (IC) of the cat, a population of neurons was found in which envelope fluctuations dominate ITD sensitivity. This population extends over a surprisingly wide range of frequencies, including low frequencies for which fine-structure information is also available. A comparison with the auditory nerve suggests that an elaboration of envelope coding occurs between the nerve and the IC. These results suggest that internally generated envelopes play a more important role in binaural hearing than is commonly thought.