Enhancing cognitive control components of insight problems solving by anodal tDCS of the left dorsolateral prefrontal cortex

Background: Executive functions play a vital role in semantic processing. Recently, transcranial direct current stimulation (tDCS) over frontal regions associated with cognitive control has been found to enhance verbal insight problem solving. The aim of the current study was to extend our understanding of the cognitive control processes modulating verbal insight problem solving. Hypotheses: Anodal stimulation over the left Dorsolateral Prefrontal Cortex (DLPFC) was predicted to improve performance in solving insight problems. Particularly, it was expected that this effect would be focused on solution recognition, which is hypothesized to be directly related to control processes, and would be more pronounced for difficult problems, hypothesized to require more cognitive demand. Moreover, we predicted that this effect would be modulated by approach motivation tendencies, due to cognition and motivation interactions in the DLPFC. Methods: 21 participants performed a verbal insight problem task twice, once under active anodal tDCS over the left DLPFC and once under sham stimulation, with a one week interval between sessions. Participants had 7 s to generate a solution for each problem, and then were requested to indicate whether a target word was the correct solution or not. Results: Stimulation significantly enhanced solution recognition for difficult problems. This effect was modulated by trait motivation, i.e. was larger for participants with lower approach motivation. No effects were found for easy problems, or solution generation. Conclusions: Left DLPFC executive control modulates semantic processing of verbal insight problems. The observed synergy between language, cognition and motivation carries theoretical implication as well as practical consideration for future stimulation research.