Effect of repetitive transcranial magnetic stimulation on fronto-posterior and hemispheric asymmetry in depression

Aim: The objective of this work was to study the impact of Transcranial Magnetic Stimulation (TMS) on brain asymmetry in patients with depression using different stimulation protocols, i.e. 1 Hz repetitive TMS over the right dorsolateral prefrontal cortex (DLPFC) (protocol G1), 10 Hz repetitive TMS over the left DLPFC (G2), and intermittent theta burst stimulation (iTBS) consisting of three 50 Hz burst bundle repeated at 5 Hz frequency (G3). Methods: EEG analysis was performed using Directed Transfer Function (DTF) and four brain asymmetry measures. The Spearman correlations between the asymmetry measures and the scores of four psychiatric tests were calculated. Results: The left hemisphere activity decreased in lower frequency bands (i.e., theta and alpha) for G1 and also in higher frequency bands (i.e., beta and gamma) for G3. Moreover, frontal lobe activity decreased in theta, alpha, beta and gamma bands for G1, but increased in delta, theta, and alpha bands for G2. The highest correlations were found between the frontal-posterior asymmetry measure and the HAM-D test independently on the frequency band when the iTBS protocol was applied, whereas the application of 1 Hz rTMS protocol caused the changes in the hemispheric asymmetry in theta and alpha bands. Conclusions: Four brain asymmetry measures derived from DTF allowed for differentiation between conditions (i. e., before and after TMS) as well as between TMS protocols. Significance: The asymmetry measures allow for evaluation of the therapeutic effect of TMS and for the choice of the most appropriate stimulation protocol.