Steady-state visually evoked potential topography during the continuous performance task in normal controls and schizophrenia.
OBJECTIVES: To examine the latency topography of the steady-state visually evoked potential (SSVEP) in patients diagnosed with schizophrenia and normal controls while undertaking a visual vigilance task. METHODS: Twenty patients diagnosed with schizophrenia and 18 normal controls performed the A-X version of the continuous performance task (CPT A-X) where subjects are required to press a micro-switch on the unpredictable appearance of an 'X' that had been preceded by an 'A.' Brain electrical activity was recorded from 64 scalp sites and a 13 Hz spatially uniform visual flicker presented with the task was used to elicit a steady-state visually evoked potential (SSVEP). RESULTS: Following the appearance of the 'A' and 'X,' the control group demonstrated a transient SSVEP latency reduction at parietal and prefrontal sites. By contrast, the patients group showed no such SSVEP latency reduction. The prefrontal SSVEP latency changes in the 500 ms interval following the appearance of the 'X' were correlated with mean individual reaction time in both populations. CONCLUSIONS: We suggest that the SSVEP latency reduction may index excitatory processes and that the absence of prefrontal SSVEP latency reduction in schizophrenic patients may be a manifestation of reduced prefrontal activity or 'hypofrontality' observed with other neuroimaging modalities.