Abstract
Repetitive transcranial magnetic stimulation (rTMS) is a potent tool for modulating endogenous oscillations in humans. The current standard method for rTMS defines the stimulation intensity based on the evoked liminal response in the visual or motor system (e.g., resting motor threshold). The key limitation of the current approach is that the magnitude of the resulting electric field remains elusive. A better characterization of the electric field strength induced by a given rTMS protocol is necessary in order to improve the understanding of the neural mechanisms of rTMS. In this study we used a novel approach, in which individualized prospective computational modeling of the induced electric field guided the choice of stimulation intensity. We consistently found that rhythmic rTMS protocols increased neural synchronization in the posterior alpha frequency band when measured simultaneously with scalp electroencephalography. We observed this effect already at electric field strengths of roughly half the lowest conventional field strength, which is 80% of the resting motor threshold. We conclude that rTMS can induce immediate electrophysiological effects at much weaker electric field strengths than previously thought.
Highlights
Repetitive transcranial magnetic stimulation is a potent tool for modulating endogenous oscillations in humans
We obtained peak magnitudes of the absolute electric field extracted from the gray matter compartment
Using prospectively individualized intensities for Repetitive transcranial magnetic stimulation (rTMS) we showed that electric fields half the magnitude of conventionally applied fields already induced immediate electrophysiological effects in humans
Summary
Repetitive transcranial magnetic stimulation (rTMS) is a potent tool for modulating endogenous oscillations in humans. We consistently found that rhythmic rTMS protocols increased neural synchronization in the posterior alpha frequency band when measured simultaneously with scalp electroencephalography We observed this effect already at electric field strengths of roughly half the lowest conventional field strength, which is 80% of the resting motor threshold. The near threshold approach utilizes individualized stimulation intensities, the properties of the rTMS-induced electric field, including its strength, can differ substantially within and across individuals. This approach cannot account for differences in the cortical folding pattern and the cortex-scalp distance between motor and non-motor areas[8]. It is crucial to account for these known anatomical effects because the induced electric field strength plays an important role in inducing electrophysiological e ffects[9]
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