Age-Related Effects of Dichotic Attentional Mode on Interaural Asymmetry: An AERP Study with Independent Component Analysis.

Background: The degree of interaural asymmetry (IA) obtained on a dichotic listening task is affected by attentional demands attributable to the mode of test administration. Previous research has shown that IA in the elderly is more influenced by increased attentional demands than young adults (YAs), but the effect of attentional mode on IA in middle-aged adults (MAs) has not been established. Auditory event-related potentials (AERPs), such as the N400, allow the evaluation of subtle differences in linguistic and cognitive processing between YAs and MAs that are imperceptible by behavioral means. Purpose: To determine the effect of dichotic attentional mode on IA in the N400 in YA and MA listeners. Research Design: Participants listened to groups of words that consisted of a reference word followed by dichotic probe words. Participants judged whether probe words were semantically related or unrelated to the reference word. This semantic judgment task was elicited in both divided-attention (DIV) and directed-attention (DIR) modes. Study Sample: Twenty-three YA (19-25 yr) and twenty-three MA (47-59 yr) females participated in the study. Data Collection and Analysis: Individual, as well as grand-averaged, AERP waveforms, scalp topographies, and event-related potential-image plots were analyzed. A mixed design analysis of variance was used to compare the N400 amplitude and latency response between ears in both attentional modes. Independent component analysis was used to isolate temporally overlapping neural sources that contributed to the negativity in the latency range of the N400 component. Results: N400 amplitude was significantly more negative in the DIV mode than DIR in both age groups. IA differences between age groups were evident only in DIV, as indicated by an age-related shift in the direction of IA in the N400 from greater asymmetry on the right in YAs to greater asymmetry on the left in MAs. ICA revealed that the age-related difference in IA in the AERP waveform reflected differences between YAs and MAs primarily in an electroencephalographic source process consistent with attentional processing. Conclusions: IA differences between YAs and MAs were revealed in the N400 only in DIV, which was the result of an increased information-processing load. ICA successfully separated temporally overlapping EEG sources that contributed to the N400 component, allowing a refined interpretation of differences in the AERP waveform among groups.