Abnormal N400 Responses But Intact Differential Hemispheric Processing of Ambiguity in Schizophrenia

Disordered thinking in schizophrenia may be a consequence of the selection of conceptual associates of dominant meanings of ambiguous words despite contextual information suggesting subordinate meanings are more appropriate. Previous work using short sentences showed a large N400 event-related potential to subordinate meaning associates and a behavioral semantic bias, but results were variable. The current experiment used word pairs to simplify the procedure and to less tax memory maintenance. Furthermore, hemispheric responses were compared, as evidence suggests the left hemisphere may select dominant meanings, while the right hemisphere may keep all possible meanings active. Subjects indicated whether two words (CUE, TARGET) were related. The CUE, presented for 1 second, could be an ambiguous or an unambiguous noun, and the TARGET, presented 1.25 seconds after the onset of the CUE, was a dominant or subordinate associate, or a related or an unrelated word, respectively. The N400-effect was calculated from difference waveforms over 400-600 msec. Groups (23 schizophrenia, 25 matched controls) showed significantly different N400-effects to the words (group x word, p =.04). Controls showed a graded response, with dominant < subordinate < unrelated. Schizophrenia patients showed the largest N400-effect to subordinate associates, with less activity to dominant meaning associates and unrelated words. Both groups showed a right hemisphere distribution to unrelated words and substantial left hemisphere activation to subordinate associates (word x hemisphere, p <.001). These data support a semantic bias in schizophrenia. They also demonstrate a special role of the right hemisphere in maintaining broad homograph meaning hierarchies. This hemispheric specialization appears to be intact in schizophrenia.