Negative difference (Nd), an ERP marker of stimulus relevance: different lateral asymmetries for paranoid and nonparanoid schizophrenics.

We studied Nd in psychotic patients with varying degrees of paranoid symptoms. Nd is an ERP measurement of the difference in the way a nonattended tone is registered and the way the same tone is registered when used later as a discrimination target. In a three-tone oddball paradigm with passive-tone presentation and active discrimination, we recorded from 19 sites in 22 young healthy subjects and 28 schizophrenics. Nd (200-260 ms) was bilaterally symmetrical in healthy subjects. Paranoid patients showed a left frontal/right temporoparietal amplitude reduction which was reversed in nonparanoid subjects (reduced at right frontal/left posterior sites). These asymmetries were clearer when the groups were separated according to active paranoid symptoms rather than by diagnosis. There would therefore seem to be functional asymmetries mediating stimulus relevance in schizophrenics which differ from controls and differ between patients with and without active paranoid symptoms.