Decoding hemispheric differences through time-varying effective connectivity after occipital TMS: a WP3-WP4 collaboration

The human brain hemispheres have long been considered functionally comparable in the domain of visual information processing. However, understanding the connection between brain regions and the asymmetrical transmission of visual signals across hemispheres holds significant importance in many clinical applications. Our study involved eighteen healthy participants undergoing a session of transcranial magnetic stimulation (TMS) combined with EEG acquisition. We proposed a signal processing pipeline to capture the rapid dynamics changes by a metric of time-varying effective connectivity both at the scalp and source level. We applied several preprocessing steps. After the preprocessing, a Bayesian approach was used to accurately estimate the TMS-evoked potentials (TEPs), and the inverse solution was calculated relying on the estimated evoked potentials using the eLORETA algorithm. The cerebral cortex was then parcellated into 100 regions of interest (ROIs) according to the Schaefer atlas. Once the source currents were estimated, we exploited the time variance of the signal to capture temporal connectivity modulation of the ROIs. Our results showed distinct effects between hemispheres, evident both at the scalp and at the source level.