Manipulating neuronal communication by using low-intensity repetitive transcranial magnetic stimulation combined with electroencephalogram

Repetitive transcranial magnetic stimulation (rTMS) modulates ongoing brain rhythms by activating neuronal structures and evolving different neuronal mechanisms. In the current work, the role of stimulation strength and frequency for brain rhythms was studied. We hypothesized that a weak oscillating electric field induced by low-intensity rTMS could induce entrainment effects in the brain. To test the hypothesis, we conducted three separate experiments, in which we stimulated healthy human participants with rTMS. We individualized stimulation parameters using computational modeling of induced electric fields in the targets and individual frequency estimated by electroencephalography (EEG). We demonstrated the immediately induced entrainment of occipito-parietal and sensorimotor mu-alpha rhythm by low-intensity rTMS that resulted in phase and amplitude changes measured by EEG. Additionally, we found long-lasting corticospinal excitability changes in the motor cortex measured by motor evoked potentials from the corresponding musle.2021-11-2

Short-lived Alpha Power Suppression Induced by Low-intensity Arrhythmic rTMS

This study was conducted to provide a better understanding of the role of electric field strength in the production of aftereffects in resting state scalp electroencephalography by repetitive transcranial magnetic stimulation (rTMS) in humans. We conducted two separate experiments in which we applied rTMS over the left parietal-occipital region. Prospective electric field simulation guided the choice of the individual stimulation intensities. In the main experiment, 16 participants received rhythmic and arrhythmic rTMS bursts at between ca. 20 and 50 mv/mm peak absolute electric field intensities. In the control experiment, another group of 16 participants received sham rTMS. To characterize the aftereffects, we estimated the alpha power (8–14 Hz) changes recorded in the inter-burst intervals, i.e., from 0.2 to 10 s after rTMS. We found aftereffects lasting up to two seconds after stimulation with ca. 35 mV/mm. Relative to baseline, alpha power was significantly reduced by the arrhythmic protocol, while there was no significant change with the rhythmic protocol. We found no significant long-term, i.e., up to 10-second, differences between the rhythmic and arrhythmic stimulation, or between the rhythmic and sham protocols. Weak arrhythmic rTMS induced short-lived alpha suppression during the inter-burst intervals

Weak rTMS-induced electric fields produce neural entrainment in humans

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