Brain asymmetry in resting state and during linguistic tasks in major depression vs. dysthymia: an EEG alpha study

Introduction: Mood disorders are relatively heterogeneous and there is limited research on the psychobiological distinctive features of Dysthymia with respect to Major Depression. In line with past studies, they are expected to exhibit inverted brain asymmetries. Objectives: We aimed to investigate altered asymmetry of EEG Alpha band in MDD, Dysthymia patients and healthy controls during resting state and two linguistic tasks: phonological and semantic. We hypothesized, in patients with Dysthymia compared with MDD, a greater extent of inverted hemispheric asymmetry due to the structured lifetime nature of their disorder. Methods: We recorded EEG Alpha activity as an index of cortical inhibition, in 20 MDD patients, 20 Dysthymic patients and 20 healthy controls (all groups matched) during the three tasks. Electrodes were clustered in four main regions, two anterior (left and right) and two posterior (left and right). Statistics were carried out by means of ANOVA. Results: In frontal sites, no Alpha asymmetry was found in the groups, but Dysthymic patients had an overall greater Alpha activity across all tasks. The same effect was found in posterior regions, but, in addition, greater Alpha on the left was found during resting state for all groups. Dysthymic patients only maintained this altered asymmetry also during the linguistic tasks. Conclusions: Our findings highlight the role of linguistic tasks in assessing deficits in hemispheric integration in mood disorders. Dysthymic patients, who suffer from a lifetime disorder, exhibited an overall greater cortical inhibition especially on the left posterior sites, which was not counterbalanced by left lateralization-inducing tasks.