A new study has found that mental fatigue may be caused by toxic byproducts that build up in the brain.
The study, which was published in Current Biology earlier this week, looked at the brain activity of two groups of people: those who needed to think hard and those who had relatively easier cognitive tasks. The study found that the group who had to think harder showed signs of mental fatigue.
This included reduced pupil dilation, a shift toward tasks that offered quick rewards with little effort and higher levels of glutamate in synapses of the brain's prefrontal cortex. The potentially toxic byproducts build up to force people to take a break or focus on something else, the researchers theorised.
Mathias Pessiglione of Pitié-Salpêtrière University in Paris, France, says: "Influential theories suggested that fatigue is a sort of illusion cooked up by the brain to make us stop whatever we are doing and turn to a more gratifying activity.
"But our findings show that cognitive work results in a true functional alteration -- accumulation of noxious substances -- so fatigue would indeed be a signal that makes us stop working but for a different purpose: to preserve the integrity of brain functioning."
The build up in the prefrontal cortex Isn't thought to be harmful, but is instead thought to be a prompt to switch up your activity while the byproducts are recycled. It alters your control over decisions, so you shift toward low-cost actions requiring no effort or waiting as cognitive fatigue sets in, the researchers explain.
Researchers suggest that the recycling of these potentially toxic substances that arise from neural activity is what causes what we perceive as mental fatigue. This may be why our brains have limitations when it comes to computing information, unlike machines.
Is there some way around this limitation of our brain's ability to think hard?
"Not really, I'm afraid," Pessiglione said. "I would employ good old recipes: rest and sleep! There is good evidence that glutamate is eliminated from synapses during sleep."
The researchers say that monitoring of prefrontal metabolites could help to detect severe mental fatigue which could help people adjust work agendas to avoid burnout. Pessiglione also advises people to avoid making important decisions when they're tired, as the glutamate build-up could impair judgement.
In future studies, they hope to learn why the prefrontal cortex seems especially susceptible to glutamate accumulation and fatigue.
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