Auditory cortex responses to interaural time differences in the envelope of low-frequency sound, recorded with MEG in young and older listeners

Interaural time and intensity differences (ITD and IID) are important cues in binaural hearing and allow for sound localization, improving speech understanding in noise and reverberation, and integrating sound sources in the auditory scene. Whereas previous research showed that the upper-frequency limit for ITD detection in the fine structure of sound declines in aging, the processing of envelope ITD in low-frequency amplitude modulated (AM) sound and the related brain responses are less understood. This study investigated the cortical processing of envelope ITD and compared the results with previous findings about the fine-structure ITD. In two experiments, participants listened to 40-Hz AM tones containing sudden changes in the envelope ITD. Multiple MEG responses were analyzed, including the auditory evoked N1 responses, elicited both by sound onsets and ITD changes, and 40-Hz responses, elicited by the AM. The first experiment with healthy young adults revealed a substantial decline in the magnitudes of the ITD change N1 response, and the 40-Hz phase resets at higher carrier frequencies, suggesting a similar frequency characteristic as observed for fine structure ITD. The amplitude of the 40-Hz ASSR declined only gradually with increasing carrier frequency, and it was excluded as a confounding factor in the decline in the ITD response. Larger responses to outward ITD changes than inward changes, here first reported for envelope ITD, were another characteristics that were similar to fine-structure ITD. A second experiment with groups of young and older listeners examined the effects of aging and concurrent noise on the cortical envelope ITD responses. One important research question was, whether binaural cues are accessible in noise. Behavioural tests showed an age-related hearing loss in the older group and decreased performance in envelope ITD detection and speech-in-noise (SIN) understanding. Binaural hearing and SIN performance were correlated with one other, but not with hearing loss. The frequency limit for envelope ITD was reduced in older listeners similarly as previously found for fine structure ITD, and older listeners were more susceptible to concurrent multi-talker noise. The similarities between responses to envelope ITD and to fine structure ITD suggest that a common cortical code exists for the envelope and fine structure ITD. The dependency on the carrier frequency suggests that envelope ITD processing at the subcortical level requires stimulus phase locking, which might be reduced in aging.