Large-Scale EEG Neural Network Changes in Response to Therapeutic TMS

Abstract

Abstract Transcranial magnetic stimulation (TMS) likely induces functional connectivity changes in aberrant circuits implicated in depression. Electroencephalography (EEG) “microstates” are voltage topographies hypothesized to represent large-scale resting networks. Canonical microstates have recently been proposed as markers for major depressive disorder (MDD), but it is not known if or how they change following TMS. Resting EEG was obtained from 49 patients with MDD before and after six weeks of daily TMS. Polarity-insensitive modified k-means clustering was used to segment EEGs into constituent microstates. Microstates were localized via sLORETA. Microstate changes over time, and between TMS responder and non-responder groups, were compared with t-tests. Six microstates (MS1-MS6) were identified. Response to TMS was associated with an increase in occurrence and coverage of MS 2, and decreased occurrence and coverage of MS 3. Non-responders showed no significant changes in any microstate. Changes in both MS 2 and MS 3 occurrence and coverage correlated with symptom change. Localization of MS2 suggests that TMS may upregulate the positive affect “reward” network (improvement in anhedonia) in responders while MS3 localization may represent diminished inhibitory activity of the frontoparietal network, possibly related to emergence from the cognitive “fog” of depression. Keywords: EEG, TMS, Microstates, Networks