The neural mechanisms underlying feelings of mental fatigue are poorly understood. Though neuroimaging has been used to show differences in brain responses to movement and motor skills, less attention has been paid to understanding brain responses to demanding cognitive tasks that produce mental fatigue. The relative lack of research in this area is surprising because a primary complaint of individuals reporting ongoing fatigue is the perceived inability to adequately perform cognitive tasks.
This spurred a well-known CFS brain researcher, Gudrun Lange, PhD, and her team of researchers led by Dane Cook, PhD, to use functional magnetic resonance imaging (fMRI) to determine the association between feelings of mental fatigue and brain response in people with CFS.
Functional magnetic resonance imaging is one of the most recently developed forms of neuroimaging. When nerve cells are active they consume oxygen carried by red blood cells from local capillaries. The local response to this use of oxygen is increased blood flow to regions of the brain with increased neural activity. An fMRI scan allows researchers to track neural activity by detecting these changes in blood flow and oxygenation. So this scan looks at the way the brain works, rather than its physical anatomy, as a traditional MRI does.
In the study by Cook, Lange and colleagues, published in NeuroImage, small groups of healthy non-fatigued control subjects and subjects with CFS performed three types of tests: a fatiguing cognitive test, a non-fatiguing cognitive test and a non-fatiguing motor test. fMRI scans were conducted with each of the three tests. Each participant’s fatigue was measured prior to scanning and following each task during the fMRI data collection.
The researchers hypothesized that mental fatigue would be significantly related to brain activity during the fatiguing cognitive task, but not during the other tasks. Did the test and MRI scans bear that out?
Indeed, the results showed that mental fatigue was related to brain activity. The participants with CFS didn’t differ from control subjects for either of the non-fatiguing tasks, but exhibited significantly greater activity in several regions of the brain during the fatiguing cognitive task.
This suggests a direct association between subjectively reported feelings of mental fatigue and brain responses during fatiguing cognition in essence validating that CFS patients’ complaints of mental fatigue have a verifiable physiologic component. With replication by other research groups and against other types of control groups, this type of study could have important applications in documenting both the diagnosis of CFS and disability associated with it.