Theta-gamma phase-amplitude coupling as a marker of cognitive deficits in schizophrenia

Cortical theta-gamma phase-amplitude coupling (TG-PAC), as is measured via EEG, has been suggested as a possible marker of visual working memory deficits in patients with schizophrenia. To test this we collected EEG data from 30 patients with psychosis and 30 healthy controls across three tasks: a visual working memory task, a visual search task, and a passive perception task. We found significant phase-amplitude coupling between theta and two distinct gamma bands: 25-35Hz and 45-55Hz. Overall TG-PAC did not differ across task, and was at similar levels between patients and healthy controls. However, the within-subjects difference between coupling at 23-35Hz and coupling at 45-55Hz shows a strong correlation with working memory capacity in both patients and controls. A similar correlation between TG-PAC frequency dynamics and cognitive symptoms is also seen in patients. No such correlation was seen when looking at positive or negative symptoms of psychosis. Taken together, this suggests that TG-PAC arises from a general process that is not specific to working memory tasks, but reflects some aspect of cognition that is tied to working memory performance.