Testing a Model for Cognitive Fatigue in Individuals with Multiple Sclerosis

Up to 95% of individuals with multiple sclerosis (MS) experience fatigue. The cognitive component of fatigue is generally reported as the most distressing aspect of patients' fatigue, limiting their ability to sustain concentration and endure mental tasks. Investigators have recently begun to show that effort and reward are related to cognitive fatigue in MS, but the processes and dynamics linking effort and reward to cognitive fatigue need to be more precisely defined. To better characterize the processes behind cognitive fatigue, this study draws from cognitive neuroscience that proposes an effort-reward imbalance is central to cognitive fatigue. This behavioral study tests the effort-reward imbalance on subjective and objective cognitive fatigue in 40 individuals with MS and 40 healthy controls. Subjective fatigue was measured by self-report measures and objective fatigue was measured as response time, accuracy, and changepoint on a switching task that consisted of high and low effort and reward conditions. Results demonstrate a relationship between subjective and objective cognitive fatigue. Findings also suggest that effort has an effect on both subjective and objective fatigue, while reward has an effect on objective fatigue alone. Contrary to the effort-reward imbalance, this initial study did not demonstrate that effort and reward depend on each other to elicit cognitive fatigue. Results showed, however, that the MS group reported higher subjective fatigue and exhibited more objective fatigue in effortful conditions compared to the control group. In conclusion, high demand and low rewards each have an effect on objective fatigue. Findings warrant continued investigation of the additive role of effort and reward on cognitive fatigue.