State-dependent tDCS modulation of the somatomotor network: A MEG study.

OBJECTIVE:Transcranial direct current stimulation (tDCS) is a non-invasive technique widely used to investigate brain excitability and activity. However, the variability in both brain and behavioral responses to tDCS limits its application for clinical purposes. This study aims to shed light on state-dependency, a phenomenon that contributes to the variability of tDCS. METHODS:To this aim, we investigated changes in spectral activity and functional connectivity in somatomotor regions after Real and Sham tDCS using generalized additive mixed models (GAMMs), which allowed us to investigate how modulation depends on the initial state of the brain. RESULTS:Results showed that changes in spectral activity, but not connectivity, in the somatomotor regions depend on the initial state of the brain, confirming state-dependent effects. Specifically, we found a non-linear interaction between stimulation conditions (Real vs Sham) and initial state: a reduction of alpha and beta power was observed only in participants that had higher alpha and beta power before Real tDCS. CONCLUSIONS:This study highlights the importance of considering state-dependency to tDCS and shows how it can be taken into account with appropriate statistical models. SIGNIFICANCE:Our findings bear insight into tDCS mechanisms, potentially leading to discriminate between tDCS responders and non-responders.