Effort and motivation to characterize mental fatigue: behavior and functional connectivity

Despite its pervasive presence in the general population, mental fatigue remains difficult to tackle from a scientific point of view. A promising framework consists of conceptualizing fatigue and fatigue recovery as a dynamic process resulting from effortful cognitive activities and rest periods, which in turn affects motivated behavior. The brain mechanisms underlying fatigue recovery during rest and its prolonged effects are, however, not fully elucidated. In this study, 19 healthy young adults completed two versions of a dual-task paradigm varying with difficulty level. The presence of mental fatigue was characterized through effort and motivation perception, and task performance (d’ and response bias indices from the signal detection theory). Participants further completed resting-state functional Magnetic Resonance Imaging (fMRI) during the recovery period following the tasks to assess how the functional connectivity of the brain was affected by the mental state induced by each task version. First, behavioral analyses showed that, compared with the easy condition, participants in the difficult task condition reported greater changes in effort perception and reduced motivation, indicative of a heightened mental fatigue state. This motivational decline was further associated with a more conservative response bias. Second, neuroimaging results showed reduced connectivity between the ventromedial prefrontal cortex and the right anterior supramarginal gyrus following the difficult vs. easy condition, with lower connectivity values associated with heightened effort perception. These findings underscore the interplay between fatigue, effort, and motivation, and suggest that persistent alterations in brain connectivity at rest may underlie the extended timescales of fatigue recovery.