The use of non-invasive brain stimulation (NIBS) with magnetic resonance imaging (fMRI) is a powerful combination (Woods et al., 2019). Functional MRI has very good spatial resolution, meaning that changes in blood flow can be measured on the order of 2-3 mm (Woods et al., 2019). It is easier to apply transcranial electric stimulation (tES) in a magnetic resonance environment compared to transcranial magnetic stimulation, making it a strong candidate for NIBS during fMRI. This would enable tES to be used to only provide causal evidence for the involvement of a brain region in a particular process but it would also enable the consequences of NIBS to be measured with excellent spatial resolution. Such an approach would enable the causal influence one brain region over another, revealing causal evidence for one 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., 2009). 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 understand 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 to how tES and fMRI can be used to understand how tES affects basic physiology, and in turn, how these effects change cognition and behaviour.
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