Differential alteration of automatic semantic processing in treated patients affected by bipolar mania and schizophrenia: An N400 study

Abstract

Various formal thought disorders are presented as symptoms by manic patients including pressure of speech, flight of ideas, and more complex speech with strong emotional components. N400 is the event-related potential, in which amplitude is suggested to be a general index of efforts to retrieve stored semantic context, which depends on the stored representation itself and the retrieval cue stimuli. The present study examines N400 components induced by a word-matching task in manic patients, and compare these responses to those induced by the task in schizophrenia and healthy controls. Twenty manic patients, twenty schizophrenic patients, and twenty healthy controls performed the word-matching task, in which they were presented with 120 (60 congruent and 60 incongruent) word pairs, they were instructed to discriminate whether each word pair was congruent or incongruent. During the task, we recorded the electroencephalogram. Reaction time analysis revealed a main effect for priming, in which reaction times were longer in response to incongruent words than to congruent words in all three participant groups (F=43.1, p<0.001) with no group effects (F=2.3, p=0.11). N400 analysis showed the main effect for priming (F=30.2, p<0.001), for group (F=5.0, p=0.01), and the interaction of priming×group (F=4.6, p=0.02). Post-hoc analysis of this interaction revealed larger N400 amplitudes to congruent words in manic patients (F=4.0, p=0.02) and smaller N400 to incongruent words in schizophrenic patients than in other groups (F=6.1, p=0.004). No correlations were found between N400 and symptom severity within patient groups. These findings suggest that priming effects of contextually related word pairs are decreased in patients with bipolar mania, whereas priming N400 responses of contextually unrelated word pairs are increased in schizophrenia. This may be the neurophysiological evidence of abnormal automatic semantic processing in patients with bipolar mania, and it also reflects a qualitative difference in thought and speech disorders between bipolar manic and schizophrenia.