Automatic and effortful processes in auditory memory reflected by event-related potentials.

Mismatch negativity (MMN) and N2b were elicited during a selective dichotic-listening task in 16 young (Y), 16 middle-aged (M) and 19 elderly (E) subjects to evaluate automatic and effortful memory comparison of auditory stimuli. Sequences of standard (80%) and deviant (20%) tones were dichotically presented to subjects in two runs. In each run, subjects were instructed to give a button-press response to the deviant (target) tones in the ear designated as attended and to ignore the input to the other ear. Peak latencies, peak amplitudes and mean amplitudes were calculated for MMN and N2b components in each subject. MMN latency and amplitude were quite stable regardless of age, while N2b latency was significantly longer in M and E subjects than in Y subjects. These results are interpreted as reflecting that automatic processes of comparison in auditory memory of stimuli presented at short interstimulus intervals remain quite stable from 23 to 77 years of age; however, those requiring attentional effort decline with age.