• The study found cognitive stress leads to glutamate buildup in the brain
  • Glutamate accumulation makes further activation of the prefrontal cortex difficult
  • There is no way to bypass the brain's cognitive threshold

While it's obvious that humans are not mentally sharp when their brains are exhausted, it has not been long since we started noticing that mental exhaustion can make our physical performance suffer, too.

However, this was often attributed to an illusion of the mind and an inflated symptom of mental exertion. Now a study has busted this myth, assuring all mentally tired souls that it's not all in their heads. Yes, mental fatigue is real.

The study, published in the journal Current Biology on Aug. 11, found a build-up of potentially toxic material in the region of the brain called the prefrontal cortex after intense cognitive engagement for long hours.

"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," said co-author of the study, 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," he said.

The researchers had an inkling that mental fatigue was related to the need to recycle toxic byproducts buildup in the brain from neural activity. To confirm this hypothesis, Pessiglione and colleagues, including the first author of the study Antonius Wiehler, set up an experiment to monitor brain metabolites throughout an approximate workday using magnetic resonance spectroscopy (MRS).

The participants were divided into two groups -- those who had high-demand cognitive work and those who had relatively simple cognitive work.

Participants in the former group showed signs of fatigue, including reduced pupil dilation and a preference shift toward rewards with short-delay and little-effort options.

More importantly, they also exhibited higher levels of glutamate in nerve synapses in the prefrontal cortex. This supports the hypothesis that glutamate accumulation makes further activation of the prefrontal cortex a more costly affair for the brain, making cognitive control more difficult after a mentally demanding workday.

Is there a way to bypass this limitation of our brain's cognitive threshold?

"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."

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