Focal Hemodynamic Responses in the Stimulated Hemisphere During High-Definition Transcranial Direct Current Stimulation

Abstract

Objective: High-definition tDCS (HD-tDCS) using a 4x1 electrode montage has been previously shown to constrain the electric field within the spatial extent of the electrodes. The aim of this study was to determine if functional near-infrared spectroscopy (fNIRS) neuroimaging can be used to determine a hemodynamic correlate of this 4x1 HD-tDCS electric field on the brain. Materials and Methods:In a 3 session cross-over study design, 13 healthy males received sham (2mA, 30s) and real (HD-tDCS-1 and HD-tDCS-2, 2mA, 10min) anodal HD-tDCS targeting the left M1 via a 4x1 electrode montage (anode C3, 4 return electrodes 3.5cm from anode). fNIRS was used to measure changes in brain hemodynamics (oxygenated hemoglobin integral-O2Hbint) during each 10min session from 2 regions of interest (ROIs) in the stimulated left hemisphere that corresponded to "within" (Lin) and "outside" (Lout) the spatial extent of the 4x1 electrode montage, and 2 corresponding ROIs (Rin and Rout) in the right hemisphere. Results:The ANOVA showed that both real anodal HD-tDCS compared to sham induced a significantly greater O2Hbint in the Lin than Lout ROIs of the stimulated left hemisphere; while there were no significant differences between the real and sham sessions for the right hemisphere ROIs. Intra-class correlation coefficients for the 2 real HD-tDCS sessions showed "fair to good" reproducibility for Lin (0.54) and Lout (0.52) ROIs. Conclusion:The greater O2Hbint "within" than "outside" the spatial extent of the 4x1 electrode montage represents a hemodynamic correlate of the electrical field distribution, and thus provides a prospective reliable method to determine the dose of stimulation that is necessary to optimize HD-tDCS parameters in various applications.

Current intensity‐ and polarity‐specific online and aftereffects of transcranial direct current stimulation: An fMRI study

“…Merzagora et al () reported a significant increase in the concentration of oxyhemoglobin following 10 min of anodal tDCS, which also led to after‐effects with a delayed peak. A more recent fNIRS study in healthy humans by Muthalib, Besson, Rothwell, and Perrey () also found elevated oxyhemoglobin levels in the ROI underneath the anodal M1 stimulation electrode, compared to an ROI that was outside the spatial extent of the target electrode. A similar finding was observed when Zheng et al () investigated CBF using ASL‐fMRI during and immediately following short durations of M1 tDCS at intensities ranging from 0.8 to 2.0 mA.…”

“…Merzagora et al (2010) reported a significant increase in the concentration of oxyhemoglobin following 10 min of anodal tDCS, which also led to after-effects with a delayed peak. A more recent fNIRS study in healthy humans by Muthalib, Besson, Rothwell, and Perrey (2018) leagues, we note that their study may not be completely comparable with ours, because in the latter study, an alternating on-off-on paradigm was used to probe the effect of different intensities, and previous work has shown that homeostatic and interference mechanisms may affect synaptic plasticity when stimulation is intermittently repeated (Fricke et al, 2011;Monte-Silva et al, 2013). Our results are also in general accordance to a previous ASL study by Stagg et al, who observed an increase of 2-3% in perfusion during and immediately after 20 min of 1.0 mA anodal tDCS over the left DLPFC (Stagg et al, 2013).…”

How to consider animal data in tDCS safety standards

“…5) Chattbar et al correctly note that 2 A/m 2 is above electrode current densities that are used in conventional sponge-pad tDCS, but 2 A/m 2 is in line with High-Definition tDCS [9–13] and so represents a conservative comparison for brain injury. We do not understand the implication by Chattbar et al that the ratio of human to rat brain volume supports a ~2000-fold safety factor; a large brain does not tolerate higher intensities per se, and the lesions from rodent studies [2,7] were observed beneath the electrodes.…”

Effects of HD‐tDCS on Resting‐State Functional Connectivity in the Prefrontal Cortex: An fNIRS Study

“…These modeling studies have been applied to determine the most favorable electrode configuration [33] and multiple small-electrode (~3 cm 2 ) high-definition (HD) montages to control the distribution of the current applied to a specific brain region [34]. In a study using anodal HD-tDCS, one anode electrode was placed at the center and four return electrodes were placed approximately 3.5 cm away from the anode in a ring configuration [35]. HD-tDCS has been used to investigate a method to control the electric field and thereby precisely stimulate a target cortical region [36,37] to potentially increase the long-term excitability aftereffects [38].…”