Abstract:

Response inhibition refers to the ability to inhibit inappropriate or irrelevant actions so that one can make flexible and goal-directed behavioral responses based on environmental changes. Response inhibition is an essential component of cognitive ability. It is critical for the executive control of behavior in healthy individuals and a fundamental cognitive ability required for people to function properly. Previous studies have shown that response inhibition is mainly related to the functions of the right inferior frontal gyrus (rIFG), the dorsolateral prefrontal cortex (DLPFC), and the pre-supplementary motor area (pre-SMA). An increasing number of studies have used transcranial direct current stimulation (tDCS) to modulate the activities of these brain regions, thereby affecting response inhibition. This review examined existing studies concerning the modulatory effects of tDCS targeting relevant brain regions on response inhibition in healthy participants, summarized the parameters of tDCS used in these studies and their main behavioral findings, and pointed out the shortcomings of previous studies and the future research directions of response inhibition enhancement by tDCS.

The neural mechanism of tDCS modulating response inhibition has yet to be clarified, mainly for the following two reasons: First, efforts are still needed to clarify the neural circuit of response inhibition itself, especially the time process of each brain region in the circuit. Second, the key brain region for improving response inhibition (i.e., the brain region that most greatly affects response stimulation after being stimulated) has not been confirmed yet. Solving these two problems can provide a basis for a better understanding and application of tDCS in response inhibition enhancement. This review also provides some insights on how to solve these problems. Currently, a majority of studies focus on healthy young adults, but have neglected the needs of the elderly and the children for enhanced response inhibition. Previous studies have shown that the effects of tDCS on response inhibition are age-dependent. Considering the age-related differences in physiological characteristics (especially brain anatomy characteristics), targeted studies should be conducted in the future to investigate the age-dependent effects of tDCS on response inhibition enhancement, so as to better use tDCS in healthy people of different ages.

Altogether, tDCS is an effective method to improve the ability of response inhibition, but the outcomes of previous studies are heterogeneous. Most studies have found that anodal tDCS can improve response inhibition; however, some studies have obtained inconsistent results. This review assumed that such inconsistency can be explained by three categories of factors, that is, individual differences (genetics, personality, baseline performance, etc.), stimulation parameters (current intensity, electrode placement, electrode size, duration, etc.), and behavioral tasks (difficulty, analytical methods, etc.). We have put forward valuable suggestions on how to control the effects of these factors, so that future research can introduce more standardized designs to reduce the heterogeneity of related studies.

Finally, in order to further investigate how to use tDCS to improve response inhibition more efficiently, this review proposed several directions for future research. First, existing studies are mostly based on conventional tDCS. Future research should actively embrace high-definition tDCS (HD-tDCS), as HD-tDCS has higher spatial resolution than conventional tDCS and can produce more prominent behavioral or neurophysiological effects and obtain results with higher reliability and validity. Second, few studies have explored the combination of tDCS with cognitive training of response inhibition. It is worthwhile to further clarify the effects of this protocol on response inhibition enhancement. Third, the effects of different stimulation patterns need to be explored, such as multisession repeated stimulation, combination of tDCS with other stimulation methods, and multitarget stimulation over multiple brain regions.

Key words: response inhibition, tDCS, IFG, DLPFC, pre-SMA, stop signal task, go/nogo task