Objective:To explore any effect of excitatory and inhibitory multi-focal transcranial direct current stimulation (m-tDCS) on cortical functioning in healthy adults.Methods:Fifteen healthy participants received 20-minute excitatory and inhibitory episodes of m-tDCS targeting C1-C2-FC1-FC2. Finite element analysis and functional near-infrared spectroscopy were then used to model the normal component of the electric field (En) applied and to monitor the oxygenated hemoglobin (HbO), deoxygenated hemoglobin (HbR), and total hemoglobin (THb) concentrations in the targeted regions during the stimulation.Results:The excitatory protocol induced En values of 0.057V/m and 0.058V/m in the left and right hemispheres, respectively, with an average of 0.058V/m, while the inhibitory one evoked corresponding En values of -0.057V/m and -0.058V/m with an average of -0.058V/m. During excitatory m-tDCS, HbO and THb concentrations in the target cortices were higher than in the inhibitory protocol, with significant differences at FC1, FC2, and C2 for HbO, and at FC2 and C2 for THb. However, no significant inter-group differences in HbR concentrations were observed. Moreover, either protocol induced severe adverse reactions including pain, though the pain decreased with prolonged stimulation.Conclusions:Both excitatory and inhibitory m-tDCS modulate cortical function among healthy individuals. Excitatory m-tDCS is the more effective in enhancing cortical excitability.

Objective:To explore any effect of excitatory and inhibitory multi-focal transcranial direct current stimulation (m-tDCS) on cortical functioning in healthy adults.Methods:Fifteen healthy participants received 20-minute excitatory and inhibitory episodes of m-tDCS targeting C1-C2-FC1-FC2. Finite element analysis and functional near-infrared spectroscopy were then used to model the normal component of the electric field (En) applied and to monitor the oxygenated hemoglobin (HbO), deoxygenated hemoglobin (HbR), and total hemoglobin (THb) concentrations in the targeted regions during the stimulation.Results:The excitatory protocol induced En values of 0.057V/m and 0.058V/m in the left and right hemispheres, respectively, with an average of 0.058V/m, while the inhibitory one evoked corresponding En values of -0.057V/m and -0.058V/m with an average of -0.058V/m. During excitatory m-tDCS, HbO and THb concentrations in the target cortices were higher than in the inhibitory protocol, with significant differences at FC1, FC2, and C2 for HbO, and at FC2 and C2 for THb. However, no significant inter-group differences in HbR concentrations were observed. Moreover, either protocol induced severe adverse reactions including pain, though the pain decreased with prolonged stimulation.Conclusions:Both excitatory and inhibitory m-tDCS modulate cortical function among healthy individuals. Excitatory m-tDCS is the more effective in enhancing cortical excitability.