Processing Interaural Cues in Sound Segregation by Young and Middle-Aged Brains

Abstract

When listening to one speaker while another conversation is occurring simultaneously, we separate the competing sounds by processing physical cues such as common onset time, intensity, frequency harmonicity, and spatial location of the sound sources. Spatial location is determined in large part by differences in arrival of a sound at one ear versus the other ear, otherwise known as interaural time difference (ITD) or interaural phase difference (IPD). There is ample anecdotal evidence that middle-aged adults experience greater difficulty listening to speech in noise, even when their audiological evaluation does not reveal abnormal results. Furthermore, it has been shown that the frequency range for IPD processing is reduced in middle-aged adults compared to young adults, even though morphological changes in the auditory evoked potential (AEP) response were only observed in older adults. The purpose of the current study was to examine early aging effects (< 60 years) on IPD processing in concurrent sound segregation. We examined the change AEP evoked by detection of a mistuned and/or phase-shifted second harmonic during the last 1500 msec of a 3000 msec amplitude-modulated harmonic complex. A passive listening paradigm was used. Ten young (21-35 years) and 11 middle-aged (48-57 years) adults with normal hearing were included in the study. Scalp electroencephalographic activity was recorded from 63 electrodes. A temporospatial principal component analysis was conducted. Spatial factor scores of individual spatial factors were the dependent variable in separate mixed-design ANOVAs for each temporal factor of interest. Stimulus type was the within-subject independent variable, and age group was the between-subject independent variable. Results indicated a delay in the upward P2 slope and the P2 peak latency to a sudden phase shift in the second harmonic of a harmonic complex in middle-aged adults compared to young adults. This AEP difference increased as mistuning (as a second grouping cue) decreased and remained evident when the IPD was the only grouping cue. We conclude that our findings reflect neurophysiologic differences between young and middle-aged adults for IPD processing in concurrent sound segregation.