Abstract:

Response inhibition is a cognitive process required to cancel an intended movement and can protect from danger. Functional magnetic resonance imaging (FMRI) studies showed that the dorsolateral prefrontal cortex (dlPFC) is a crucial brain region for response inhibition. Repetitive transcranial magnetic stimulation (rTMS) is a technique that can determine the contribution of specific cortical regions to behavior. Previous studies have found that repetitive transcranial magnetic stimulation of the dlPFC affects response inhibition. In the current study, we used transcranial direct current stimulation (tDCS), a non-invasive, painless brain stimulation technique with no known side effects, to alter neuronal excitability. A number of tDCS studies have suggested that tDCS may affect response inhibition. However, to date, limited work has been done to explore whether tDCS over the right dlPFC could alter response inhibition. Therefore, this study aimed to investigate the anodal stimulus effect of tDCS on response inhibition. We hypothesized that exciting the neural activity of the right dlPFC with anodal tDCS would enhance the ability of response inhibition.

A total of 34 healthy subjects (15 males, 19 females) participated in this within-subjects study. Stop-signal task (SST) was established with E-prime software. Participants received both active and sham stimulation on separate days. SST was used to measure the participants’ capacity for response inhibition. In the active stimulation condition, we delivered a 1.5 mA direct current for 25 min (fade-in/fade-out time: 8 s); in the sham condition, we delivered a 1.5 mA direct current for 30 s at the beginning and 30 s at the end of the stimulation time. Anodal and cathodal stimulation electrodes were placed on F4 and FP1, respectively. Participants completed the SST, Stroop color-word matching task, Verbal Fluency test, and Digit Span test before and after the stimulation.

We first calculated the mean reaction time (RT)in the go trials and stop-signal delay for each participant using ANALYZE-IT software. To calculate the individual stop-signal reaction time (SSRT), ANALYZE-IT first computes the mean RTs for all trials without a stop signal and then subtracts the mean stop-signal delay from this value. We performed a series of paired samples t-tests to compare the SSRT of each experimental condition with the SSRT of the sham and active conditions. There were no significant differences in SSRT pre-stimulation in sham or active stimulations, indicating equal response inhibition capacity among the participants. Interestingly, the difference in SSRT before and after stimulation was statistically significant in the active condition, t(33) = -2.25, p < 0.05, Cohens d = 0.38. This demonstrates that participants who received anodal stimulation over the right dlPFC had significantly reduced SSRT change scores on the SST compared to participants in the sham condition.

This study demonstrated that transcranial electrical stimulation of the right dlPFC can regulate response inhibition, in that anodal stimulation improved participants’ response inhibition. We confirm previous work suggesting that the right dlPFC is an important brain region of response inhibition.

Key words: response inhibition, the dorsolateral prefrontal cortex (dlPFC), transcranial direct current stimulation (tDCS)