Neural indices of spoken word processing in background multi-talker babble.

OBJECTIVE: To evaluate the impact of multi-talker babble on cortical event-related potentials (ERPs), specifically the N400, in a spoken semantic priming paradigm. DESIGN: Participants listened in quiet and with background babble to word triplets, evaluating whether the third word was related to the preceding words. A temporo-spatial principal component analysis was conducted on ERPs to the first and second words (S1 and S2), processed without an overt behavioral response. One factor corresponded to the N400 and revealed greater processing negativity for unrelated as compared to related S2s in quiet and in babble. STUDY SAMPLE: Twelve young adults with normal hearing. RESULTS: Background babble had no significant impact on the N400 in the posterior region but increased neural processing negativity at anterior and central regions during the same timeframe. This differential processing negativity in babble occurred in response to S2 but not S1. Furthermore, background babble impacted processing negativity for related S2s more than unrelated S2s. CONCLUSIONS: Results suggest that speech processing in a modestly degraded listening environment alters neural activity associated with auditory working memory, attention, and semantic processing in anterior and central scalp regions.

Processing interaural cues in sound segregation by young and middle-aged brains.

BACKGROUND: 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. PURPOSE: The purpose of the current study was to examine early aging effects (< 60 years) on IPD processing in concurrent sound segregation. RESEARCH DESIGN: 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. STUDY SAMPLE: Ten young (21-35 years) and 11 middle-aged (48-57 years) adults with normal hearing were included in the study. DATA COLLECTION AND ANALYSIS: 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: 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. CONCLUSIONS: We conclude that our findings reflect neurophysiologic differences between young and middle-aged adults for IPD processing in concurrent sound segregation.