¹H MR Spectroscopy of the Motor Cortex Immediately following Transcranial Direct Current Stimulation at 7 Tesla

Abstract

23Transcranial direct current stimulation (tDCS) is a form of non-invasive brain stimulation that 24 may modulate cortical excitability, metabolite concentration, and human behaviour. The 25 supplementary motor area (SMA) has been largely ignored as a potential target for tDCS 26 neurorehabilitation but is an important region in motor compensation after brain injury with 27 strong efferent connections to the primary motor cortex (M1). The objective of this work was to 28 measure tissue metabolite changes in the human motor cortex immediately following tDCS. We 29 hypothesized that bihemispheric tDCS would change levels of metabolites involved in 30 neuromodulation including N-acetylaspartate, glutamate, and creatine. In this single-blind, 31 randomized, cross-over study, fifteen healthy adults aged 21-60 participated in two 7T MRI 32 sessions, to identify changes in metabolite concentrations by magnetic resonance spectroscopy. 33Immediately after 20 minutes of tDCS, there were no significant changes in metabolite levels or 34 metabolite ratios comparing tDCS to sham. However there was a trend toward increased 35 NAA/tCr concentration (p=0.08) in M1 under the stimulating cathode. There was a strong, 36 positive correlation between the change in the absolute concentration of NAA and the change in 37 the absolute concentration of tCr (p<0.001) suggesting an effect of tDCS. Both NAA and 38 creatine are important markers of neurometabolism. Our findings provide novel insight into the 39 modulation of neural metabolites in the motor cortex immediately following application of 40 bihemispheric tDCS. 2 42 3 65 concentration of phosphocreatine in the left temporo-frontal region following anodal tDCS to the 66 left dorsolateral prefrontal cortex (17). In another study, 2mA of anodal tDCS to the right 67 parietal cortex caused an increase in both Glx and total N-aceytl-aspartate (NAA + NAAG) 68 relative to sham, measured from the parietal cortex (10), while a study by Stagg and colleagues 69 found that 1 mA of cathodal stimulation to left M1 decreased Glx under the electrode(7). Other 70 studies have found no effect. For example, Kim et. al. found no changes after 1.5 mA of 71 cathodal tDCS to left M1 in any metabolite measured under the stimulating electrode (18). 72 Similarly, using 1mA of current in an M1-M1 bihemispheric montage, Tremblay et. al. found no 73 significant changes in any metabolite in left M1 (19). These conflicting results are difficult to 74 interpret, and leads to uncertainty with regards to the implementation of an optimum stimulation 75 paradigm. 76 The application of tDCS to improve motor performance and recovery in neurological 77 disorders requires optimization of stimulation parameters. Bihemispheric tDCS can enhance both 78 behaviour and physiological responses in healthy and neurologically injured individuals (20-22). 79The supplementary motor area (SMA) has proven to be an important area of the brain during the 80 execution of bimanual hand movements (23), and plays a compensatory role durin...

Effects of Transcranial Direct Current Stimulation on GABA and Glx in Children: A pilot study

“…While this cannot be confirmed in our investigation, there is literature suggesting changes in GABA concentration are time sensitive with fluctuation in GABA concentration occurring in the 90 minute window following stimulation [4,55,65]. The time sensitivity of metabolite measurements is further supported by seemingly discrepant findings in the literature in which GABA and Glx changes are not seen during tDCS [66][67][68].…”

“…They propose MRS measures of Glx lack sensitivity to consistently detect Glx changes following tDCS [4,28]. Several other studies report an absence of significant changes in Glx in response to a-tDCS at the site of stimulation with little speculation as to why [4,29,66,67,69]. It may be that tDCS influences Glx in a complex way that is network dependent.…”

Effects of Transcranial Direct Current Stimulation on GABA and Glutamate in Children: A Pilot Study

“…While this cannot be confirmed in our investigation, there is literature suggesting changes in GABA concentration are time sensitive with fluctuation in GABA concentration occurring in the 90 minute window following stimulation (4,46,57). The time sensitivity of metabolite measurements is further supported by seemingly discrepant findings in the literature in which GABA and Glx changes are not seen during tDCS (58)(59)(60).…”

“…Several other studies report an absence of significant changes in Glx in response to a-tDCS with little speculation as to why (4,26,58,59,61).…”

Patient-Specific RF Safety Assessment in MRI: Progress in Creating Surface-Based Human Head and Shoulder Models

“…An increasing number of MRI investigations that study the human brain employ multimodal setups, where additional devices are used to record complementary information or manipulate brain states [8][9][10], examples include electroencephalography (EEG), transcranial magnetic stimulation (TMS), and transcranial direct current stimulation (tDCS). This requires a dedicated setup of wires and electrodes that are in contact with human skin.…”