No support for the effect of tDCS on frontal alpha asymmetry and behavioral and brain activity indices of inhibitory control

Prior research links self-regulation failure to psychiatric disorders. While the left and right dorsolateral prefrontal cortices are associated with distinct components of self-regulation—approach and inhibitory systems, respectively—existing measures are considered indirect. Our preregistered study explored frontal alpha asymmetry (FAA) as a potential biomarker for self-regulation. We explored whether the assumed effects of transcranial direct current stimulation (tDCS) on behavioral and brain activity indices of inhibitory control are mediated through changes in FAA. We used a randomized controlled sham-feedback design with 65 healthy humans (46 females). Before and after 2 mA anodal tDCS on the right frontal site, we collected resting-state EEG data to assess FAA scores, and participants also completed a stop signal task with neutral and intrinsic reward (food) conditions. The tDCS had no impact on FAA or any behavioral or neural indices of inhibitory control. However, event-related potential analyses revealed a correlation between inhibitory brain activity in the reward condition and trait FAA. Higher right relative to left frontal brain activity was linked to lower early-onset inhibitory activity, possibly originating from the inferior Frontal Gyrus, but correlated with higher late-onset inhibitory control, presumably originating from the superior Frontal Gyrus. While tDCS yielded unexpected outcomes in FAA and inhibitory control metrics, we found an interesting dissociation regarding the lateralization of frontal brain activity and early and late onset inhibitory brain activity.