Clinical symptoms of schizophrenia affect reference-independent measures of task-induced EEG alpha asymmetry

Differential patterns of EEG alpha asymmetry during verbal and spatial cognitive activity are commonly described and are thought to reflect predominance of left- vs. right-sided cortical activation. Although these patterns have been difficult to elicit reliably in schizophrenics, the authors have previously suggested that clinical status may have confounded results. Therefore, EEG data from 17 additional schizophrenic patients, 16 mood disorder patient controls, and 17 normal controls were used to examine the relationship between severity of schizophrenic symptoms and task induced alpha asymmetry. Subjects performed verbal and spatial tasks during recording of 16-channel EEG. After transformation to the average reference, log alpha power from central and temporal leads was analyzed by MANOVA and MANCOVA. The expected task-side interaction (P < 0.02) was present for the total sample and for each control group when analyzed alone. However, it was only present in schizophrenics when the BPRS score was entered as a covariable. Patterns of correlations between BPRS scores, left temporal alpha power, right/left temporal alpha ratios, and task differences in ratios suggested that higher symptom levels were associated with excessive left-sided activation during spatial activity. This is consistent with other evidence of left hemisphere overactivity in schizophrenics.