Negative difference (Nd), an ERP-marker of stimulus relevance: different lateral asymmetries for paranoid and non-paranoid schizophrenics
We studied Nd in psychotic patients with varying degrees of paranoid symptoms. Nd is an ERP measure of the difference registered by a non-attended tone and the same tone used later as a discrimination target. Methods: In a 3-tone oddball paradigm we recorded from 19 sites in 22 young healthy subjects and 28 patients with schizophrenia in a passive tone presentation and an active discrimination. Results: 1/ Nd (200-260 ms) was fronto-central and bilaterally symmetrical in healthy subjects. 2/ Paranoid patients showed a left-frontal/right temporoparietal amplitude reduction which was reversed in non-paranoid subjects, (reduced at right-frontal/left posterior sites). 3/ These asymmetries were similar but clearer when the groups were separated by active paranoid symptoms than by diagnosis. Conclusions: Thus there are functional asymmetries mediating stimulus relevance (controlled processing) in schizophrenic patients which differ from controls and differ between patients with and without active paranoid symptoms.
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