The Effects of Transcranial Electric Stimulation on the Modulation of Electric Fields and Neuronal Firing Rates Within the Human Brain

Abstract

Testing the therapeutic claims of Transcranial Electrical Stimulation (TES) is increasingly facilitated by the simultaneous deployment of various imaging and neurophysiological methods. However, various meta-analyses evaluating the aforementioned claims have questioned the ability of weak electrical currents from non-invasive TES to effectively and predictably modulate neural activity. Many studies have recently shown that the electric currents used in contemporary TES experiments may not be large enough to directly affect neuronal circuits. This uncertainty in the effectiveness of TES exists in part due to a lack of ground truth information of the electric fields produced within the brain tissue. The objective of this research was to analyze this essential ground truth data collected from intracerebral recordings in humans during TES to determine the in situ electric field magnitude in the human brain and its effect on local neuronal activity. This analysis also helped determine whether multielectrode stimulation that is guided by computational models of current flow can achieve sufficient electric field magnitude at targeted brain regions. These models were expected to guide electrode placement and to predict electric field distribution, independent of magnitude. The results of this study showed that electric field strengths at targeted brain regions are sufficiently strong (i.e., >0.25 V/m) for some but not all subjects, and that the strength of the electric field may bear a direct relationship with changes in neuronal firing rate. Further progress in this project can help shed light on the acute effects of TES and also its plastic effects that are expected to follow as a consequence of short-term changes. Therefore, the results of this study contribute to the foundation for a mechanistic understanding of TES and can have significant implications for facilitation of clinical translation.