Electrophysiological Evidence of Augmented Interaural Asymmetry in Middle-Aged Listeners

Abstract

Various dimensions of auditory processing, especially the perception of speech in the presence of background competition, have been shown to deteriorate with age. A persistent problem in the assessment of these age-related changes has been the high prevalence of age-related high-frequency hearing loss in elderly persons. Some investigators have suggested that a more fruitful approach to the study of age-related decline might be to study middle-aged, rather than elderly, persons, where confounding high-frequency hearing loss is less prevalent. To determine whether an increase in the left-ear disadvantage (LED) in dichotic listening could be demonstrated in a group of middle-aged persons. The N400 component of the auditory event-related potential (AERP) was utilized to evaluate interaural asymmetry in a quasi-dichotic competing speech task. Electrophysiological responses were obtained on a word-pair semantic categorization task presented through a front loudspeaker while the listener ignored competing speech presented through either left (competition left [CL]) or right (competition right [CR]) loudspeakers. Study Samples: Twenty young (18-24 yr) and 20 middle-aged (44-57 yr) females with normal hearing sensitivity. Individual, as well as grand-averaged, AERP waveforms and scalp topographies were analyzed for the word pairs. Peak amplitude and latency measures of the N400 component were subjected to a mixed design analysis of variance (ANOVA). No significant interaural asymmetry was found in the AERP waveform for the reference word condition in either age group. In response to the second word of the pair, however, middle-aged females showed significantly greater N400 negativity in the CR condition than in the CL condition. No significant laterality effect was found in the young females. The study of young versus middle-aged participants may be an effective way of avoiding the confound of high-frequency hearing loss in elderly persons when studying age effects on auditory processing.