In two experiments, event-related brain potentials
were recorded to word pairs simultaneously presented to
both ears, with instructions to attend to one ear and detect
occasional nonwords in that ear. This attentional manipulation
yielded four patterns of word repetition on successive
trials: first and second presentations attended (AA), both
unattended (UU), and across ears (AU and UA). A prominent
attenuation of N400 due to immediate repetition of words
was observed on AA trials. However, when first presentations
were ignored on UU and UA trials, no repetition effect
was obtained. These findings indicate that the repetition
effect on N400 depends on attentional processing of first
presentations.