Why Is It Tiring to Think?

Summary

Behavioral activities that require control over automatic routines generally feel effortful 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 control cost inflation with cognitive work are highly 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 using magnetic resonance spectroscopy (MRS) to monitor brain metabolites over the course of an approximate workday, during which two groups of participants performed high- or low-demand cognitive control tasks, interspersed with economic decisions. Choice-related fatigue markers were only present in the high-demand group, with a reduction in pupil dilation during decision making and a shift in preference toward low-effort, short-delay options (a low-delay bias). cost captured using computational models).

At the end of the day, high-demand cognitive work resulted in higher glutamate concentration and greater glutamate/glutamine diffusion in a cognitive control brain region (lateral prefrontal cortex [lPFC]), relative to high-demand cognitive work. low demand and with a reference brain region (primary visual cortex [V1]).

Taken together with previous fMRI data, these results support a neurometabolic model in which glutamate accumulation triggers a regulatory mechanism that makes lPFC activation more costly, explaining why cognitive control is more difficult to move after a day of strenuous work.

Comments

Even professional chess players start making mistakes, usually after 4-5 hours in the game, that they wouldn’t make when well rested. A consensus explanation for why playing chess induces cognitive fatigue is that move planning cannot be based on effortlessly learned routines (except at the beginning of the game) because the possibility space is too large.

It’s no surprise that hard physical work exhausts you, but what about hard mental work? Sitting down and thinking hard for hours also makes one feel exhausted. Now, researchers have new evidence to explain why this happens, and according to 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 continues 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 your control over decisions, so you shift toward low-cost actions that require no effort or waiting as cognitive fatigue develops , the researchers explain.

“Influential theories suggested that fatigue is a kind of illusion invented by the brain to make us stop 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 alteration, the accumulation of harmful substances, so fatigue would be a signal that makes us stop working, but with a different purpose: preserving the integrity of the functioning of the brain." brain ".

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 the reason had to do with the need to recycle potentially toxic substances that arise from neuronal 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 doing hard work. Those in that group also showed in their choices a shift toward options that offered rewards in a short time and with little effort. Critically, they also had higher levels of glutamate in synapses in the brain’s prefrontal cortex. Together with the evidence above, the authors say it supports the idea that glutamate buildup makes additional activation of the prefrontal cortex more costly, so that cognitive control is more difficult after a day of mentally hard work.

So is there a way around this limitation on our brain’s ability to think hard?

" Not really, I’m afraid," Pessiglione said. “I would use good old recipes: rest and sleep! “There is good evidence that glutamate is cleared from synapses during sleep . ”

There may be other practical implications. For example, the researchers say, monitoring 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 are tired.

In future studies, they hope to learn why the prefrontal cortex seems especially susceptible to glutamate buildup and fatigue. They are also curious whether the same markers of fatigue in the brain can predict recovery from health problems, such as depression or cancer.