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Researchers now have new evidence to explain why this is, and according to their findings, the reason you feel mentally exhausted (as opposed to drowsy) from intense thought isn't all in your head.
Their studies, which were published in Current Biology on August 11th, indicate that when intense cognitive work is prolonged for several hours, potentially toxic byproducts accumulate in the prefrontal cortex. As cognitive fatigue sets in, this in turn alters your control over decisions, so you shift toward low-cost actions requiring no effort or waiting, as explained by the researchers.
"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," says Mathias Pessiglione of Pitié-Salpêtrière University in Paris, France. "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."
Pessiglione and colleagues, including the study's lead author Antonius Wiehler, desired to comprehend the nature of mental fatigue. While machines can continuously compute, the human brain cannot. They desired to learn the reason. They hypothesized that this was due to the need to recycle potentially toxic substances generated by neural activity.
In order to find evidence of this, they utilized magnetic resonance spectroscopy (MRS) to monitor brain chemistry throughout a workday. They examined two groups of individuals: those who were required to think intensively and those whose cognitive tasks were relatively simple.
They observed signs of fatigue, including constricted pupils, only in the hard-working group. Those in this group also exhibited a preference shift toward options promising rewards with minimal effort and short delays. Importantly, they also had higher levels of glutamate in the prefrontal cortex's synapses. In conjunction with previous evidence, the authors conclude that their findings support the notion that glutamate accumulation makes further activation of the prefrontal cortex more expensive, thereby making cognitive control more challenging after a mentally taxing workday.
Is there a way to get around this restriction on how hard our brains can think? "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."
There might be additional useful implications. For instance, according to the researchers, monitoring prefrontal metabolites could aid in the detection of severe mental fatigue. This capability may aid in adjusting work schedules to prevent burnout. He also recommends that individuals avoid making significant decisions when they are exhausted.
In future research, they hope to determine why the prefrontal cortex appears to be particularly susceptible to glutamate accumulation and fatigue. They are also interested in determining whether the same markers of fatigue in the brain can predict recovery from diseases such as depression and cancer.
Journal Reference: Antonius Wiehler, Francesca Branzoli, Isaac Adanyeguh, Fanny Mochel, Mathias Pessiglione. A neuro-metabolic account of why daylong cognitive work alters the control of economic decisions. Current Biology, 2022; DOI: 10.1016/j.cub.2022.07.010
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