Scientists have identified two brain regions that together determine whether we are through mental exhaustion or simply giving up.
Using functional MRI scans, researchers at John Hopkins found that cognitive fatigue activates specific neural circuits, linking our perception of fatigue to decisions on persistent arduous tasks, which can help treat debilitating mental fatigue in depression and PTSD.
The study, published in the Journal of Neuroscience, performed increasingly difficult memory tasks when scanning the brain, tracking 28 healthy volunteers. Participants must remember the alphabetical sequence, face greater challenges and provide greater cash rewards.
The brain’s fatigue circuit
When people report feeling mental fatigue, both brain regions show a sharp increase in activity. The right island buried deep in the brain has long been associated with feeling of fatigue. Meanwhile, the dorsal prefrontal cortex – areas on both sides of the brain – control memory and decision-making.
“Our study aims to induce cognitive fatigue and understand how people try to choose when they feel tired and how they change when they determine the location of the brain that makes these decisions,” explained Vikram Chib, senior author of the study and associate professor at Johns Hopkins.
During cognitive fatigue, the activity level of the two brain regions exceeded twice as high. The connection between these areas seems to create a feedback loop that affects whether we continue to push ourselves mentally.
Money speaks when the mind is tired
The study reveals counterintuitives about motivation and psychological efforts:
- Overcoming cognitive fatigue requires a lot of economic incentives
- Participants are more likely to avoid more difficult tasks when mentally exhausted, even with moderate rewards
- External motivation is crucial for ongoing psychological performance
- People’s perception of capabilities does not always match their actual brain capacity
“In order to make participants put in more cognitive efforts, economic incentives need to be high, suggesting that external incentives are driving this effort,” Chib noted. This pattern reflects previous findings from his team.
The meaning of the real world
What is important for the discovery is the laboratory environment. Cognitive fatigue seriously affects people with neurological and psychiatric disorders, especially those with depression and PTSD, who often struggle with mental fatigue.
“Two areas of the brain may jointly decide to avoid more cognitive efforts unless more incentives are provided,” Chib explained. However, he warns, “there may be a difference between the perception of cognitive fatigue and the actual abilities of the human brain.”
Study participants aged 21 to 29 received $50 in participation and performance bonuses, ranging from $1 to $8. Before responding to memory challenges, all people underwent baseline brain scans.
Possibility of treatment
Understanding this neural circuit opens up new avenues of treatment. Chib suggests that targeted medications or cognitive behavioral therapies may aid excessive mental fatigue by changing the activity of these brain areas.
The research framework can also provide physicians with objective methods to measure and classify cognitive fatigue, which will go beyond subjective patient reports beyond measurable patterns of brain activity.
However, there are still important limitations. Functional MRI technology measures blood flow rather than direct neuronal activity and may lack subtle brain changes. The controlled laboratory environment is also very different from the psychological challenges of reality.
Chib admits: “This study was conducted in an MRI scanner and has very specific cognitive tasks. It is important to understand how these results generalize to other cognitive work and real-world tasks.”
The next step involves studying this fatigue circuit behavior differently in people with depression, PTSD and other diseases in which mental fatigue becomes debilitating. Such studies may eventually lead to more effective treatments for millions of fallen into cognitive fatigue.
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