You’ve probably found yourself in this situation. Left on the couch, after a long and tiring day at work, in which he had to “think too much”. You don’t want to focus on anything else, you’re dating the delivery app or aimlessly browsing social media. But why?
In a study published in the scientific journal Current Biology, French researchers suggest that this is related to “the need to recycle potentially toxic substances accumulated during the exercise of cognitive control”. The substance in question is glutamate, the main excitatory neurotransmitter in the brain, which plays an important role in learning and memory.
According to research, the substance accumulates in “stressful conditions” or “with increasing demands of tasks”. “The problem with very high concentrations of extracellular glutamate (outside the cell) is not only the disruption of the excitation/inhibition balance, but also the induction of bursts of activation, which can impair the transmission of information and cause excitotoxicity (which can cause death or nerve damage) in the most severe cases,” they write.
The study’s lead author, Antonius Weihler, a psychiatrist at the GHU Paris Psychiatry and Neurosciences, told Science magazine that science is still far from being able to say that “working hard mentally causes a toxic buildup of glutamate in the brain.” To another scientific publication, Nature, he explained that he wants to use the results to learn how to recover from mental exhaustion. “Does sleep help? How long do breaks need to be to have a positive effect?”
To test this metabolic hypothesis of cognitive fatigue, the scientists analyzed 40 people divided into two groups, performing cognitive tasks for about six hours. Some performed more complex activities and others, tasks considered simpler.
After blocks of activities, participants had to make four economic choices, associated with monetary rewards. They served for fatigue analysis. This is because the highest values were associated with high effort demands – thinking or exercising physically, for example – and with a longer time to receive it – delay in receiving it.
While making these choices, the scientists analyzed glutamate levels in the lateral prefrontal cortex using MRI. The brain region is involved in decision-making and emotional regulation – it is one of the last regions of the brain to develop and has important maturation in adolescence. They also performed eye tracking to observe pupil dilation, which, they said, “has already been validated as an index of cognitive effort.”
The researchers found that the group that performed more complex tasks had a “high level” of glutamate in the lateral prefrontal cortex, and a reduction in pupil dilation when making economic choices. Unlike those who did simpler activities, who preferred immediate reward options that involved less effort, even if, in the long run, they represented smaller gain. They say the results replicate and extend research that has shown that exerting “intense cognitive control” in intellectual work or endurance sport induces a form of cognitive fatigue that “manifests itself as a greater preference for immediate options.”
But the study has limitations. “Our results are only correlational and cannot be taken as proof that what limits the effort of cognitive control is the need to prevent the accumulation of glutamate”, they warn. In addition, there are technical limitations: the scanners used are not able to quantify the presence of other substances.
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