N400 and P300 modulation as functions of processing level in schizophrenia patients exhibiting formal thought disorder

In a semantic priming paradigm, the effects of different levels of processing on N400 were assessed by changing task demands in 10 schizophrenics and 10 matched controls. In the lexical decision task subjects had to discriminate between words and nonwords, and in the physical task subjects had to discriminate between cyan and blue ink letters. A lexicality test of reaction times demonstrated that the physical task was performed non-lexically in both groups. Moreover, a semantic priming reaction time effect was obtained only in the lexical decision task for the control group. The level of processing clearly affected event-related potentials. An N400 priming effect was only observed for the control group in the lexical decision task. By contrast, in the physical task a P300 effect was observed for either related or unrelated targets in both groups. Taken together, these results indicate that FTD schizophrenics are impaired specifically when task performance induces the semantic aspects of words as indexed by reduction of the N400 priming effect. (C) 2009 Elsevier B.V. All rights reserved.