Remission of depression is associated with asymmetric hemispheric variation in EEG complexity before antidepressant treatment

Background: Intense increased alpha activity in the anterior brain has been recognized as one of the biomarkers of depression. This study aimed to explore whether complexity measures of electroencephalogram (EEG) signals vary across the frontal region between patients with or without remission in depression. 

Methods: Ninety-four patients with moderate-to-severe major depression underwent the experiment. Hilbert-Huang transform was applied to extract individual alpha EEG instantaneous amplitude. Averaged regional complexity (SampEn_Mean) on frontal and frontal-central areas as well as bilateral frontal areas were assessed according to multiscale entropy between the remitters (n = 26/94, 27.66%) and poor responders (n = 29/94, 30.85%). Standard deviation of regional complexity (SampEn_SD) was used as a parameter to test the homogeneity of frontal alpha complexity. 

Results: No significant differences in SampEn_Mean for the frontal or frontal-central area were observed between the remitters and poor responders. However, the SampEn_SD (i.e., regional homogeneity of frontal alpha complexity) significantly increased in the frontal-central area after treatment among the poor responders in comparison to their baseline, whereas the remitters showed an opposite trend. The remitters further showed significantly larger SampEn_SD on the left frontal area at baseline, and this lateralization disappeared after antidepressant treatment except for the poor responders. 

Limitations: This study was limited by a small sample size and without healthy controls. 

Conclusions: Homogeneity in frontal-central alpha complexity and interhemispheric asymmetry normalization of anterior EEG complexity are associated with antidepressant efficacy, suggesting that homogeneity of dynamical brain activity is a key linked to the successful treatment of major depression.