Neuropsychological Vulnerability to Schizophrenia

Increasing evidence indicates that certain of the abnormalities of processing information that occur in schizophrenia are not limited to the periods of active positive symptoms, but rather may be enduring indicators of vulnerability to schizophrenic episodes. This chapter will focus on deficits on tachistoscopic measures of vigilance or sustained, focused attention, one of the most prominent impairments found across offspring of schizophrenic patients, actively psychotic schizophrenic patients, and clinically remitted schizophrenic patients. The evidence for similar signal detection impairment during demanding vigilance conditions before, during, and after schizophrenic psychotic episodes will be examined. We will also review recent data on the stability of these impairments within schizophrenic patients across psychotic and remitted periods. The potential neuropsychological significance of these tachistoscopic signal detection deficits during vigilance will be discussed in the light of recent research using positron emission tomography (PET) and EEG coherence procedures. A PET study suggests that a vigilance task involving identification of blurred stimuli, the degraded-stimulus Continuous Performance Test, activates prefrontal cortex and produces differential right hemisphere activation in normal subjects. Schizophrenic patients performing this vigilance task show less prefrontal activation and less differential activation of right compared to left temporoparietal areas than normal subjects. An analysis of EEG coherence completed with R. Hoffman during the same task activation condition extends these results. EEG coherence examines similarity of EEG waveforms between spatially distributed brain modules and may thereby clarify functional interrelationships. During activation by the degraded-stimulus Continuous Performance Test, schizophrenic patients were found to have reduced EEG coherence from right prefrontal to right occipital regions and from right prefrontal to posterior 54temporal regions. The possible implications of these findings for neuropsychological vulnerability to schizophrenia are examined. The possibility that a specific right hemisphere impairment is implicated in vulnerability to schizophrenia is considered. The hypothesis of specific right-hemisphere impairment is contrasted with an alternative interpretation that emphasizes the role of a prefrontal supervisory attentional system interacting with taskspecific demands.