Magnetic resonance spectroscopy with transcranial direct current stimulation to explore the underlying biochemical and physiological mechanism of the human brain: A systematic review

Abstract

A large body of molecular and neurophysiological evidence connects synaptic plasticity to specific functions and energy metabolism in particular areas of the brain. Furthermore, altered plasticity and energy regulation has been associated with a number of neuropsychiatric disorders. A favourable approach enabling the modulation of neuronal excitability and energy in humans is to stimulate the brain using transcranial direct current stimulation (tDCS) and then to observe the effect on neurometabolites using magnetic resonance spectroscopy (MRS). In this way, a well-defined modulation of brain energy and excitability can be achieved using a dedicated tDCS protocol to a predetermined brain region. This systematic review was guided by the preferred reporting items for systematic reviews and meta-analysis and summarises recent literature studying the effect of tDCS on neurometabolites in the human brain as measured by proton or phosphorus MRS. Limitations and recommendations are discussed for future research. The findings of this review provide clear evidence for the potential of using tDCS and MRS to examine and understand the effect of neurometabolites in the in vivo human brain.

Keywords: MRS; brain; neurometabolites; neuroscience; transcranial direct current stimulation.

FIGURE 1

Flow diagram of study selection

FIGURE 2

Risk of bias analysis traffic light plot (a) and summary plot (b) for included studies

FIGURE 3

Brain regions where tDCS was applied with their corresponding published articles and literature included in this review. Colour coding indicates one study per colour, which used the bi‐hemispheric stimulation. DLPFC, dorsolateral prefrontal cortex; IFG, inferior frontal gyrus; pSTG, posterior superior temporal gyrus. The brain images were generated using CONN software (Whitfield‐Gabrieli & Nieto‐Castanon, 2012)

FIGURE 4

All neurometabolites of interest measured by MRS in the investigated studies in different brain regions used for the current review. ACC, anterior cingulate cortex; DLPFC, dorsolateral prefrontal cortex; IFG, inferior frontal gyrus; IPS, intraparietal sulcus; pSTG, posterior superior temporal gyrus. The brain images were generated using CONN software (Whitfield‐Gabrieli & Nieto‐Castanon, 2012)