Hard cognitive work leads to the accumulation of glutamate in the lateral prefrontal cortex. Which will cause mental fatigue.
Highlights
- Mental fatigue is explored with magnetic resonance spectroscopy during a workday.
- Hard cognitive work leads to the accumulation of glutamate in the lateral prefrontal cortex.
- The need for glutamate regulation reduces the control exercised over decision-making.
- Reduced control favors the choice of low-effort stocks with short-term rewards.
Summary
Behavioral activities that require control over automatic routines usually feel strained and result in cognitive fatigue. Beyond the subjective report, cognitive fatigue has been conceived as an increased cost of cognitive control, objectified by more impulsive decisions. However, the origins of such cost inflation of control with cognitive work are much debated. Here we suggest a neurometabolic explanation: the cost would be related to the need to recycle potentially toxic substances accumulated during the exercise of cognitive control.
We validated this hypothesis by using magnetic resonance spectroscopy (MRS) to monitor brain metabolites over an approximate workday, during which two groups of participants performed high- or low-demand cognitive control tasks, interspersed with economic decisions. Election-related fatigue markers were only present in the high-demand group, with reduced pupil dilation during decision-making and a shift in preference toward low-effort, short-delayed options (a low-cost bias captured using computational models).
At the end of the day, high-demand cognitive work resulted in higher glutamate concentration and increased glutamate/glutamine diffusion in a cognitive control brain region (lateral prefrontal cortex [lPFC]), relative to low-demand cognitive work and to a reference brain region (primary visual cortex [V1]).
Taken together with previous functional magnetic resonance data, these results support a neurometabolic model in which glutamate accumulation triggers a regulatory mechanism that makes lPFC activation more expensive, explaining why cognitive control is more difficult to mobilize after a strenuous workday.
Feedback
Even professional chess players begin to make mistakes, usually after 4 to 5 hours in the game, which they would not make when they rested well. A consensus explanation for why playing chess induces cognitive fatigue is that movement planning cannot be based on routines learned effortlessly (except at the beginning of the game) because the space of possibilities is too large.
It’s no surprise that hard physical labor exhausts him, but what about hard mental labor? Sitting around thinking a lot for hours also makes one feel exhausted. Now, researchers have new evidence to explain why this happens and, based on their findings, the reason you feel mentally exhausted (rather than sleepy) from intense thinking isn’t just in your head.
Their studies, published in Current Biology, show that when intense cognitive work lasts for several hours, it causes the accumulation of potentially toxic byproducts in the part of the brain known as the prefrontal cortex. This, in turn, alters their control over decisions, so it shifts toward low-cost actions that don’t require effort or waiting as cognitive fatigue develops, the researchers explain.
Theories about mental fatigue
“Influential theories suggested that fatigue is a kind of illusion invented by the brain so that we stop doing what we are doing and move on to a more rewarding activity,” says Mathias Pessiglione of Pitié-Salpêtrière University in Paris, France. “But our findings show that cognitive work results in a true functional impairment, the accumulation of harmful substances, so fatigue would be a signal that makes us stop working, but with a different purpose: to preserve the integrity of brain functioning.”
Pessiglione and his colleagues, including the study’s first author, Antonius Wiehler, wanted to understand what mental fatigue really is. While machines can calculate continuously, the brain cannot. They wanted to know why. They suspected that the reason had to do with the need to recycle potentially toxic substances that arise from neural activity.
To look for evidence of this, they used magnetic resonance spectroscopy (MRS) to monitor brain chemistry over the course of a workday. They looked at two groups of people: those who needed to think a lot and those who had relatively easier cognitive tasks.
They saw signs of fatigue, including reduced pupil dilation, only in the group that was doing hard work. Those in that group also showed in their choices a shift towards options that proposed rewards in a short time and with little effort. Critically, they also had higher levels of glutamate at synapses in the brain’s prefrontal cortex. Along with the above evidence, the authors say it supports the idea that glutamate accumulation makes additional activation of the prefrontal cortex more expensive, so that cognitive control is more difficult after a mentally hard day’s work.
Is there a way to avoid mental fatigue?
“Not really, I’m afraid,” Pessiglione said. “I would use good old recipes: rest and sleep! There is good evidence that glutamate is removed from synapses during sleep.”
There may be other practical implications. For example, the researchers say, tracking prefrontal metabolites could help detect severe mental fatigue. Such a skill can help adjust work schedules to avoid burnout. He also advises people to avoid making important decisions when they’re tired.
In future studies, they hope to learn why the prefrontal cortex seems especially susceptible to glutamate accumulation and fatigue. They are also curious to know if the same markers of fatigue in the brain can predict recovery from health problems, such as depression or cancer.
