INSTANTANEOUS EFFECTS OF PREFRONTAL TRANSCRANIAL MAGNETIC STIMULATION ON BRAIN OXYGENATION: A SYSTEMATIC REVIEW
Authors: Xia *Weili , Jin Minxia , Qin Ping , Kan Laidi , Zhang Bingbing , Lin Tim T Z , Li Ami S M , Kranz Georg S 
Published: February 2025

Abstract

Background
Prefrontal transcranial magnetic stimulation (TMS) is an efficacious treatment for psychiatric disorders such as major depression. Stimulation-induced changes in the function and structure of disease-related brain networks are presumed to be the mechanism of action underlying the therapeutic effects of TMS. The concurrent use of TMS with neuroimaging, such as functional magnetic resonance imaging (fMRI) or functional near-infrared spectroscopy (fNIRS), makes it possible to directly measure the instantaneous effects of prefrontal TMS on brain oxygenation [1, 2]. However, a thorough understanding of the local and remote brain activation changes during and immediately after prefrontal TMS stimulation remains unclear.

Aims & Objectives
Here, we conducted a comprehensive review of concurrent TMS/fMRI and TMS/fNIRS studies to investigate the engagement of local and remote brain regions during and immediately after prefrontal TMS.

Method
Key terms were searched in PubMed, Embase, PsycINFO, Web of Science, the Cochrane Library Database, and the references of screened articles to identify human studies investigating the “online” effects of prefrontal TMS on brain excitability monitored by fMRI or fNIRS.

Results
Thirty-seven studies (13 concurrent TMS/fNIRS studies, 24 concurrent TMS/fMRI studies) were included in a qualitative synthesis. Despite methodological inconsistencies, a distinct pattern of activated nodes of the frontoparietal central executive network, the cingulo-opercular salience network and the default-mode network emerged. Nodes included the prefrontal cortex (especially dorsolateral prefrontal cortex), insula cortex, striatal regions (especially caudate, putamen), anterior cingulate cortex and amygdala. Meanwhile, high-frequency repetitive TMS most consistently induced expected facilitatory effects in these brain regions.

Discussion & Conclusion
Our results demonstrate that prefrontal TMS immediately modulates the excitability of several brain networks and their constituent nodes. However, there was high variability in TMS-induced brain excitability. Various stimulation parameters (e.g., intensity, coil orientation, target sites) and the inter and intra-individual variability of brain state may contribute to the observed heterogeneity of target excitability and co-activated regions [3]. Given the considerable methodological and individual variability across the limited evidence, conclusions should be drawn with caution.

References
[1]Bergmann, TO, R Varatheeswaran, CA Hanlon, KH Madsen, A Thielscher, and HR Siebner, Concurrent TMS-fMRI for causal network perturbation and proof of target engagement. Neuroimage, 2021. 237: p. 118093.
[2]Curtin, A, S Tong, J Sun, J Wang, B Onaral, and H Ayaz, A systematic review of integrated functional near-infrared spectroscopy (fNIRS) and transcranial magnetic stimulation (TMS) studies. Frontiers in Neuroscience, 2019. 13(FEB).
[3]Huang, YZ, MK Lu, A Antal, J Classen, M Nitsche, U Ziemann, et al., Plasticity induced by non-invasive transcranial brain stimulation: A position paper. Clin Neurophysiol, 2017. 128(11): p. 2318-2329.