Transcranial magnetic stimulation (TMS) is a noninvasive brain stimulation technique that employs a high-intensity pulsed magnetic field sent through the scalp by a stimulating coil. According to Faraday's law, a time-varying magnetic field induces inside the brain tissue an electric field, which may elicit a neuronal response. Due to the lack of physical contact, TMS results in almost a painless stimulation compared to electric noninvasive techniques, and for this reason, it has been deeply investigated over the past decades. Many studies have demonstrated the effectiveness of TMS as a therapeutic solution for the treatment of different neuropsychiatric conditions, among which are major depression [1], chronic pain [2-5], epilepsy [6], and obsessive-compulsive disorder (OCD) [7]. Furthermore, TMS is extensively adopted in neuroscience research to investigate intracortical, cortico-cortical, and corticosubcortical interactions [6, 8, 9] and to assess causal relations between brain activity and behavior, as during speech [10-12] and motor mapping [13-19]. Despite the success, there are critical barriers to employing TMS in neuroscience research. One
This study aims at quantifying the effect that using different skin conductivity values has on the estimation of the electric (E)-field distribution induced by transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) in the brain of two anatomical models. The induced E-field was calculated with numerical simulations inside MIDA and Duke models, assigning to the skin a conductivity value estimated from a multi-layered skin model and three values taken from literature. The effect of skin conductivity variations on the local E-field induced by tDCS in the brain was up to 70%. In TMS, minor local differences, in the order of 20%, were obtained in regions of interest for the onset of possible side effects. Results suggested that an accurate model of the skin is necessary in all numerical studies that aim at precisely estimating the E-field induced during TMS and tDCS applications. This also highlights the importance of further experimental studies on human skin characterization, especially at low frequencies.
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