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Abstract
Repetitive transcranial magnetic stimulation (rTMS) at sub-threshold intensity is a viable clinical strategy to enhance the sensory and motor functions of extremities by increasing or decreasing motor cortical excitability. Despite this, it remains unclear how sub-threshold rTMS modulates brain cortical excitability and connectivity. In this study, we applied functional near-infrared spectroscopy (fNIRS) to investigate the alterations in hemodynamic responses and cortical connectivity patterns that are induced by high-frequency rTMS at a sub-threshold intensity. Forty high-frequency (10 Hz) trains of rTMS at 90% resting motor threshold (RMT) were delivered through a TMS coil placed over 1–2 cm lateral from the vertex. fNIRS signals were acquired from the frontal and bilateral motor areas in healthy volunteers (n = 20) during rTMS administration and at rest. A significant reduction in oxygenated hemoglobin (HbO) concentration was observed in most defined regions of interest (ROIs) during the stimulation period (p < 0.05). Decreased functional connectivity within prefrontal areas as well as between symmetrical ROI-pairs was also observed in most participants during the stimulation (p < 0.05). Results suggest that fNIRS imaging is able to provide a reliable measure of regional cortical brain activation that advances our understanding of the manner in which sub-threshold rTMS affects cortical excitability and brain connectivity.
Highlights
Transcranial magnetic stimulation (TMS) is a therapeutic technique that applies a rapid alternating current to generate magnetic fields, which penetrate scalp and skull to reach the brain and induce secondary electrical fields that activate the cortical neurons (Wassermann and Lisanby, 2001)
We aimed to investigate the effects of high-frequency (10 Hz) sub-threshold Repetitive transcranial magnetic stimulation (rTMS) on brain cortical excitability and connectivity across multiple brain regions
While the present study provides important details regarding the effects of TMS application on regional blood flow and connectivity, one limitation is that a sham group was not recruited in the experiment, making it difficult to validate if the alterations of hemodynamic response and the connectivity were associated with the rTMS
Summary
Transcranial magnetic stimulation (TMS) is a therapeutic technique that applies a rapid alternating current to generate magnetic fields, which penetrate scalp and skull to reach the brain and induce secondary electrical fields that activate the cortical neurons (Wassermann and Lisanby, 2001). Cortical Hemodynamic and Connectivity Induced by rTMS inhibits cortical excitability at lower pulse frequencies (less than the 1 Hz) and enhances the cortical excitability at higher pulse frequencies (5 Hz or higher; Ridding and Ziemann, 2010; ValeroCabré et al, 2017). Since its introduction in 1985 (Barker et al, 1985), rTMS has demonstrated the ability to induce long-term changes in cortical activity (Fitzgerald et al, 2006b), leading to its adoption as a potential treatment for a variety of neurological and mental disorders, including depression and motor impairment (Takeuchi et al, 2005; Fitzgerald et al, 2006a; O’Reardon et al, 2007; Schwerin et al, 2011). In order to maximize the therapeutic potential of rTMS, it is necessary to investigate the effects that stimulation has on both brain activity and cortical connectivity
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