The effect of middle age on the late positive component of the auditory event-related potential

We recently described a research study in which age-related changes in interaural asymmetry were elicited using the N400 of the auditory event-related potentials (AERP) (Davis et al, 2013). The N400 was the primary focus due to its sensitivity to various aspects of semantic processing (Kutas and Hillyard, 1984), which we measured using a quasi-dichotic semantic category judgment task in competing speech. In this article, we describe age-related changes that occurred in the late positive component (LPC) of the AERP in the same study. The LPC peak occurs subsequent to the N400 peak on the AERP waveform and has been associated with context updating and further evaluation and processing of stimulus meaning (Juottonen et al, 1996). Neither age group showed significant interaural asymmetry in the LPC. However, a robust age-related difference in LPC scalp topography was observed.

The LPC of the auditory event-related potentials was utilized to evaluate age-related differences in language processing in a quasi-dichotic competing speech task.

Electrophysiological responses were obtained on a word-pair semantic categorization task presented through a front loudspeaker while ignoring competing speech that was presented through either left (competition left [CL]) or right (competition right [CR]) loudspeakers. The LPC was compared between young and middle-aged groups in three conditions: side of competition, semantic judgment, and electrode position.

Twenty young (18-24 yr) and twenty middle-aged (44-57 yr) females with normal hearing sensitivity participated in this study.

Individual, as well as grand-averaged, AERP waveforms and scalp topographies were analyzed in response to the word pairs. The LPC component was subjected to a mixed design analysis of variance (ANOVA) for peak latency and amplitude measures in the latency range of 700-800 msec. Since statistical analyses showed little difference in the LPC component as a function of side of competition, the AERP data were collapsed for the CL and CR conditions. The LPC was analyzed in two ways: first at mid-parieto-central electrode locations, second across midline electrodes from PZ to FZ.

Analysis of the mid-parieto-central electrodes showed no amplitude or latency differences for either group or side of competition. The second analysis (across midline electrodes), however, showed a significant amplitude interaction between electrode position and group, indicating that the two age groups were equivalent in the posterior region of the scalp but divergent as electrode site moved frontally. Significant age-related scalp topography differences were found in both semantic judgment conditions. No significant latency differences were found in any condition.

The middle-aged group showed substantially greater LPC peak amplitude in the frontal regions of the scalp than young adults. These results were in concert with N400 results, which suggested that the middle-aged group required more attentional/cognitive resources than young adults in order to maintain a high performance level on a linguistic task in the presence of competing linguistic stimuli (Davis et al, 2013).