Effects of coil orientation on the electric field induced by TMS over the hand motor area

Abstract

Responses elicited by transcranial magnetic stimulation (TMS) over the hand motor area depend on the position and orientation of the stimulating coil. In this work, we computationally investigate the induced electric field for multiple coil orientations and locations in order to determine which parts of the brain are affected and how the sensitivity of motor cortical activation depends on the direction of the electric field. The finite element method is used for calculating the electric field induced by TMS in two individual anatomical models of the head and brain. The orientation of the coil affects both the strength and depth of penetration of the electric field, and the field strongly depends on the direction of the sulcus, where the target neurons are located. The coil position that gives the strongest electric field in the target cortical region may deviate from the closest scalp location by a distance on the order of 1 cm. Together with previous experimental data, the results support the hypothesis that the cortex is most sensitive to fields oriented perpendicular to the cortical layers, while it is relatively insensitive to fields parallel to them. This has important implications for targeting of TMS. To determine the most effective coil position and orientation, it is essential to consider both biological (the direction of the targeted axons) and physical factors (the strength and direction of the electric field).

An integrated framework for targeting functional networks via transcranial magnetic stimulation

“…However, this assumption is not universally held in the field. In particular, several authors have suggested that the normal component of the electric field (Fox et al, 2004; Janssen et al, 2015; Laakso et al, 2014) is the determinant of the stimulation area, not the field strength. These models differ somewhat in the prediction of stimulation areas, with absolute field strength predicting strongest stimulation at the gyral crowns (Opitz et al, 2011; Thielscher et al, 2011), where the normal component model predicting peak stimulation deeper in the sulci (Fox et al, 2004).…”

5 kHz Transcranial Alternating Current Stimulation: Lack of Cortical Excitability Changes When Grouped in a Theta Burst Pattern

“…We assume that the same motor cortex area in the sulcus of M1 (Laakso et al, 2014) is being activated here as in other studies using tACS of the motor cortex with a comparable electrode montage (Moliadze et al, 2010; Chaieb et al, 2011). When applying a posterior-to-anterior (PA)-directed current via TMS, it first activates the cortical layer 1 ending in layer 6, thus depolarizing the soma of pyramidal tract cells (Jefferys, 1981).…”

Mapping the visual brain areas susceptible to phosphene induction through brain stimulation

“…However, this is not necessarily true: Recent studies have proposed that a subcomponent of the electric field that is induced by TMS can best predict stimulation outcomes in the motor cortex (Laakso et al 2014; Janssen et al 2015). This subcomponent is perpendicular to and directed into the cortical surface and, for TMS, maximally affects neurons situated in the sulcal wall.…”