The development and modelling of devices and paradigms for transcranial magnetic stimulation

Abstract

Magnetic stimulation is a noninvasive neurostimulation technique that can evoke action potentials and modulate neural circuits through induced electric fields. Biophysical models of magnetic stimulation have become a major driver for technological developments and the understanding of the mechanisms of magnetic neurostimulation and neuromodulation. Major technological developments involve stimulation coils with different spatial characteristics and pulse sources to control the pulse waveform. While early technological developments were the result of manual design and invention processes, there is a trend in both stimulation coil and pulse source design to mathematically optimize parameters with the help of computational models. To date, macroscopically highly realistic spatial models of the brain as well as peripheral targets, and user-friendly software packages enable researchers and practitioners to simulate the treatment-specific and induced electric field distribution in the brains of individual subjects and patients. Neuron models further introduce the microscopic level of neural activation to understand the influence of activation dynamics in response to different pulse shapes. A number of models that were designed for online calibration to extract otherwise covert information and biomarkers from the neural system recently form a third branch of modeling.

Mechanistic link between right prefrontal cortical activity and anxious arousal revealed using transcranial magnetic stimulation in healthy subjects

“…We used electric-field modeling to define the roll, pitch, and yaw vectors of the TMS coil for each subject during stimulation (See Fig. 2b) [13]. Tissue compartments were created from the T1/T2 images for the skin, skull, cerebrospinal fluid, gray matter, and white matter using SimNIBS [21].…”

“…Because brain state at the time of stimulation influences response to the stimulation [11], we delivered the stimulation during the maintenance interval of the Sternberg working memory paradigm, a task known to activate the dlPFC [12], to facilitate the effectiveness of this stimulation. We also used this task to identify individualized functional TMS targets, representing the peak BOLD activity during the maintenance interval in the dlPFC for each subject, and used iterative electric-field modeling to optimize the coil position [13].…”

Conditions for numerically accurate TMS electric field simulation

“…For computational modeling of TMS, the induced E-field can be partitioned into a primary Efield due to the coil currents and a secondary E-field due to charge accumulation on regions where there is a change in conductivity [10]. Numerical error of the primary E-field is dependent on the method used for modeling the TMS coil windings and its currents.…”

Coupling magnetically induced electric fields to neurons: longitudinal and transverse activation

“…Computational models of neuronal activation by magnetically induced E-fields provide an approach to understand stimulation mechanisms and to optimize stimulation parameters (11)(12)(13)(14)(15). Like electrical stimulation, a two-stage approach is commonly used to simulate magnetic stimulation (16).…”