The N400 event-related potential in aphasia

Although the N400 component of event-related potentials (ERPs) is suggested to reflect language processing, exactly which language processing functions N400 is sensitive to is not clear. We investigated this component in aphasic patients with some impairments of language processing. Meaningful and meaningless words in Kana (Japanese characters) were used as stimuli under a visual oddball paradigm. Increases in N400 latency and amplitude in the aphasic group were significant in comparison with the control group. In the aphasic group, N400 latency correlated significantly with the performance intelligence quotient employed besides language quotients. Moreover, the N400 effects were seen more clearly in the left hemisphere than in the right hemisphere for both groups. We propose that the abnormal variations in amplitude or latency of N400 in the aphasic group reflect language processing functions (controlled processing and automatic processing) that are different between slight and severe cases of aphasia. Moreover, N400 effects are sensitive to intellectual abilities besides language ability. We also suggest that N400 effects in the left hemisphere for the aphasic group are a reflection of active language processing as the substitution function.