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 two potential mechanisms: first, poor attentional engagement should be associated with elevated levels of pre-stimulus alpha-band activity within the EEG, whereas impaired working memory encoding should be associated with reduced post-stimulus alpha suppression.

Methods: We collected EEG data from 68 people with schizophrenia, 43 people with bipolar disorder with a history of psychosis, and 53 people with major depressive disorder, as well as 90 healthy comparison subjects (HCS), 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 both 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 pre-stimulus alpha power estimated using traditional wavelet analysis predicted behavioral errors; (2) post-stimulus alpha suppression was reduced in the patient groups; and (3) reduced suppression was associated with lower likelihood of memory storage. However, we also observed that pre-stimulus alpha was larger among HCS compared to patients, and single-trial analyses showed that it was the aperiodic elements of the pre-stimulus 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 post-stimulus encoding processes rather than reduced attentional engagement prior to stimulus onset.