Neural dynamics of constraint relaxation and problem representation changes in single-trial insight problem solving: An fNIRS study

Insight problem solving involves overcoming an impasse when a solution seems unreachable, often experienced as an ‘Aha!’ moment. In such solving, shifting from an incorrect representation imposed by constraints to a correct representation through constraint relaxation is critical. Prior research compared brain activity when constraint relaxation and representation change occurred versus when they did not occur, but neural activity before and after such changes within trials has remained underexplored. This study examines neural correlates of problem solving progress, focusing on transitions from impasse—defined by an incorrect, constrained representation—to the state immediately preceding the ‘Aha!’ moment when constraints relax and correct representation emerges. Functional near-infrared spectroscopy (fNIRS) monitored brain activity during a slot machine task. Task performance and eye movement data identified the representations adopted by participants and whether constraint relaxation occurred. Participants were categorized into a success group, which reached the solution, and a failure group, which did not. In the success group, the state just before ‘Aha!’ showed increased oxygenated hemoglobin in the right dorsolateral prefrontal cortex (DLPFC) and superior temporal gyrus (STG) compared with the impasse state. In contrast, the failure group showed increased activity in the right angular gyrus. These results suggest that right DLPFC activation supports constraint relaxation via executive function and working memory, and right STG activation facilitates the transition to a new representation. Integrating fNIRS measures of right DLPFC/STG activation with eye tracking indices of saccade direction shifts enables detection of constraint relaxation preceding the ‘Aha!’ moment.