Predictability effect on N400 reflects the severity of reading comprehension deficits in aphasia

Predictability effect on N400, in which low predictability words elicited a larger N400 than high predictability words did over central to posterior electrodes, has been used to index difficulty of lexical retrieval and semantic integration of words in sentence comprehension. This study examined predictability effect on N400 in aphasic patients to determine if the properties of N400 are suited to indexing the severity of reading comprehension deficits. Patients with aphasia were divided into high and low ability groups based on scores on the reading comprehension subtest in the Chinese Concise Aphasia Test (CCAT). The two aphasia groups, a group of healthy elders who were age-matched to the aphasic participants, and a group of young adults, were requested to read sentences that either ended with highly predictable words or unexpected but plausible words, while undergoing electroencephalography (EEG). The young adult and healthy elderly groups exhibited the typical centro-parietal distributed effect of predictability on N400; however, healthy elders exhibited a reduced N400 effect in a delayed time window compared to the young adults. Compared with the elderly control, the high ability aphasia group exhibited a comparable N400 effect in a more restricted time window; by contrast, the low ability aphasia group exhibited a frontal distributed N400 in a much later time window (400-700 ms). These data suggest that the severity of reading comprehension deficits affects predictability effect on a set of N400 characteristics (i.e., amplitude, time window, and topographic distribution), which may be effective as ERP signatures in the evaluation of language recovery in aphasia.