The use of non-invasive brain stimulation (NIBS) techniques in conjunction with magnetic resonance imaging (MRI) is a powerful combination that can provide us with deeper insights into our understanding of the brain than one technique alone (Woods et al., 2019). Functional MRI (fMRI) techniques in particular, for instance, provide a very high spatial resolution, allowing changes in blood flow within the brain to be measured to the order of just 2-3mm.
Transcranial electrical stimulation (tES) is a very popular method of non-invasive brain stimulation to apply in conjunction with MRI, due to the relative ease of use that tES hardware provides when compared to transcranial magnetic stimulation (TMS) or transcranial focused ultrasound (TUS/tFUS) stimulation.
Combining these two techniques allows researchers not only to discern and identify causal evidence for the involvement of a specific brain region in a particular process but also enables the direct effects of NIBS to be measured with excellent spatial resolution. Such combined approaches enable researchers to accurately identify the influence of one brain region over another, revealing causal evidence for one brain site altering a process in another brain site: effective connectivity.
The use of arterial spin labelling (ASL) has been useful in tES-fMRI studies because ASL measured perfusion, which is easier to interpret than the BOLD signal (Woods et al., 2019). Polarity-specific effects of anodal and cathodal tES have been found on left DLPFC, where an increase (Zheng et al., 2011) or decrease (Stagg et al., 2013), respectively, has been found. In addition to this, perfusion in areas anatomically connected to the DLPFC has been revealed (Stagg et al., 2013). This evidence shows that tDCS can be combined with fMRI to show how effects arise at the site of stimulation and at sites anatomically connected to the site of stimulation (Woods et al., 2019).
MRI can also accomplish magnetic resonance spectroscopy (MRS), which enables changes in neurotransmitter concentration to be assessed (Woods et al., 2019). The use of tES in conjunction with MRS is also a very useful combination. Effects of tES are usually determined through a change in motor evoked potential amplitude if M1 is stimulated (e.g. Nitsche & Paulus, 2001). However, a physiological readout like the MEP is not always available - even when electroencephalography is used, there is the problem of tES-induced artefact and the inverse problem. If an effect arises, it is impossible to determine where the effect is with certainty. The combination of tES with MRI on the other hand enables an understanding how tES effects translate into macroscopic, mesoscopic and microscopic changes (Bestmann et al., 2015). For example, the application of tDCS with the anode over M1 led to a decrease in GABA levels in the stimulated area as measured by MRS (Stagg et al., 2009a, 2011a; Kim et al., 2014; Bachtiar et al., 2015). This evidence shows how tES and fMRI can be used to understand how tES affects basic physiology, and in turn, how these effects change cognition and behaviour.
In the video below, taken from the Brainbox Initiative Webinar Series, Professor Charlotte Stagg (University of Oxford) details how and why neuroscientists may combine tES and MRI techniques.
- Transcranial Direct Current Stimulation Integration with Magnetic Resonance Imaging, Magnetic Resonance Spectroscopy, Near Infrared Spectroscopy Imaging, and Electroencephalography. Adam J. Woods, Marom Bikson, Kenneth Chelette, Jacek Dmochowski, Anirban Dutta, Zeinab Esmaeilpour, Nigel Gebodh, Michael A. Nitsche, Charlotte Stagg. Practical Guide to Transcranial Direct Current Stimulation. January 2019
- Modulation of GABA and resting state functional connectivity by transcranial direct current stimulation. Velicia Bachtiar, Jamie Near, Heidi Johansen-Berg, Charlotte J Stagg. ELife. September 2015
- Understanding the behavioural consequences of noninvasive brain stimulation. SvenBestmann, Archy O.de Berker, JamesBonaiuto. Trends in Cognitive Sciences. January 2015
- tDCS-induced alterations in GABA concentration within primary motor cortex predict motor learning and motor memory: a 7 T magnetic resonance spectroscopy study. Soyoung Kim, Mary C Stephenson, Peter G Morris, Stephen R Jackson. Neuroimage. October 2014
- Widespread modulation of cerebral perfusion induced during and after transcranial direct current stimulation applied to the left dorsolateral prefrontal cortex. Charlotte J Stagg, Richard L Lin, Melvin Mezue, Andrew Segerdahl, Yazhuo Kong, Jingyi Xie, Irene Tracey. The Journal of Neuroscience. July 2013
- The Role of GABA in Human Motor Learning. Charlotte J. Stagg, Velicia Bachtiar, and Heidi Johansen-Berg. Current Biology. March 2011
- Polarity-Sensitive Modulation of Cortical Neurotransmitters by Transcranial Stimulation. Charlotte J. Stagg, Jonathan G. Best, Mary C. Stephenson, Jacinta O'Shea, Marzena Wylezinska, Z. Tamas Kincses, Peter G. Morris, Paul M. Matthews and Heidi Johansen-Berg. The Journal of Neuroscience. April 2009
- Sustained excitability elevations induced by transcranial DC motor cortex stimulation in humans. M A Nitsche, W Paulus. Neurology. November 2001
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