In one test, participants were repeatedly given the choice of performing either an easy task for a low reward or a hard task for a higher reward. Those with PI-ME/CFS were less likely to choose the hard task than the healthy volunteers, suggesting less ability to exert effort. The healthy volunteers had reduced muscle function and brain motor cortex activity after repeated tasks. But people with PI-ME/CFS did not. Brain scans during one task found that those with PI-ME/CFS had lower activity in a brain region called the temporoparietal junction. These findings suggest that the fatigue of those with PI-ME/CFS might be caused by dysfunction in the way the brain decides how to exert effort.
When the team analyzed cerebrospinal fluid, they found that participants with PI-ME/CFS had reduced levels of chemicals called catechols. Catechols help regulate the nervous system. Catechol levels correlated with effort preference and motor function in people with PI-ME/CFS, but not in healthy volunteers. This suggests that altered catechol signaling in the brains of people with PI-ME/CFS may give rise to their altered effort.