Non-invasive electrical stimulation of the brain (ESB) modifies the resting-state network connectivity of the primary motor cortex: A proof of concept fMRI study

An innovative method to obtain fMRI resting-state network maps during non-invasive electrical stimulation of the brain (ESB) was developed and tested. Five healthy volunteers participated in 2 fMRI sessions. In session one, a transcranial direct current stimulator (tDCS) was applied placing the positive electrode (31.5 cm²) over the right M1 of the cortex and the negative electrode (31.5 cm²) over the left supra-orbital area of the head. In session two, a monophasic pulsed current stimulator (tPCS) was applied using the identical electrode placement. Imaging was performed on a Siemens 3 T Tim Trio scanner with a 12-channel head coil. At each session, five consecutive functional scans were obtained: 1) resting-state without stimulation (Rest-1), 2) a motor scan consisting of self-paced, bilateral finger-thumb opposition task, 3) resting-state with ESB (Stim-1), 4) resting-state without stimulation (Rest-2), and 5) resting-state with ESB, replicating Stim-1 (Stim-2). Data were analyzed using AFNI and MATLAB. For motor task fMRI analysis, a general linear model (GLM) determined the voxels in the right and left M1 that were significantly correlated with the motor task paradigm. The resting-state time series from the voxels in the R-M1 were averaged and the resulting time series used as a regressor in a GLM analysis to identify M1 connectivity maps. Connectivity maps were quantified as R² values, and then combined to give overlap maps for each of the experimental conditions. Fourier analysis determined the energy in the normalized signal average time courses extracted from L-M1 and R-M1 for each of the resting-state scans. Both tDCS and tPCS lowered the R² values and energy of the averaged time course in the right and left M1 ROI. The effect of the tPCS appeared more pronounced and less variable among subjects. Applying non-invasive ESB during fMRI scanning may down regulate the motor cortex's resting-state network connectivity.