Dysfunctional Alpha Modulation as a Mechanism of Working Memory Impairment in Serious Mental Illness

Background: People with psychosis and mood disorders experience disruptions in working memory; however, the underlying mechanism remains unknown. We focused on 2 potential mechanisms: poor attentional engagement should be associated with elevated levels of prestimulus alpha-band activity within the electroencephalogram (EEG), whereas impaired working memory encoding should be associated with reduced poststimulus alpha suppression. Methods: We collected EEG data from 68 people with schizophrenia, 43 people with bipolar disorder with a history of psychosis, 53 people with major depressive disorder, and 90 healthy comparison subjects while they completed a spatial working memory task. We quantified attention lapsing, memory precision, and memory capacity from the behavioral responses, and we quantified alpha using traditional wavelet analysis as well as a novel approach for isolating oscillatory alpha power from aperiodic elements of the EEG signal. Results: We found that 1) greater prestimulus alpha power estimated using traditional wavelet analysis predicted behavioral errors; 2) poststimulus alpha suppression was reduced in the patient groups; and 3) reduced suppression was associated with a lower likelihood of memory storage. However, we also observed that the prestimulus alpha was larger among healthy control participants than patients, and single-trial analyses showed that it was the aperiodic elements of the prestimulus EEG—not oscillatory alpha—that predicted behavioral errors. Discussion: These results suggest that working memory impairments in serious mental illness primarily reflect an impairment in the poststimulus encoding processes rather than reduced attentional engagement prior to stimulus onset.