Mentally grueling work is exhausting because it creates a build-up of toxic byproducts in the brain, new research suggests.
Chess players have to deal with it and you probably have to deal with it after a long day at work: mental fatigue. You prefer to plop on the couch and not think about anything. It now appears that this phenomenon can also be explained physiologically. Scientists from the Paris Brain Institute and the Sorbonne Universités, led by Mathias Pessiglione, discovered that thinking long and hard will poison your brain a little.
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Sort letters
For a long time, neuroscientists thought that mental fatigue is an illusion your brain creates because it simply wants to do more enjoyable things. But the current researchers wanted to know whether there is not a physiological cause. They suspected that the fatigue comes from processing toxic byproducts of brain activity.
The hypothesis was tested in 40 volunteers. These people were divided into two groups. One group had to do hard mental work for over 6 hours. That work consisted of sorting letters that appeared on the screen into vowels/consonants or uppercase/lowercase, depending on the color. The other group had to do the same but was given more time and the color change was slower.
Toxic buildup
During the ‘working day’ the participants were subjected to a brain scanning technique at regular intervals magnetic resonance spectroscopy (MRS) is called. With MRS, certain substances in the brain can be mapped. For example, Pessiglione and his colleagues saw that the concentration of the chemical glutamate in the brain rose faster in hard thinkers in less hard thinkers.
glutamate is a neurotransmitter necessary for proper communication between brain cells. But too much of it acts like a poison, slowing your brain down. So it must be cleaned up quickly before it can accumulate.
Clearing Glutamate
The research team now thinks that mental fatigue is a kind of signal from your brain: “I’m going to cut it off, because I have to clear the toxic glutamate first”. That the hard thinkers were actually more tired at the end of the ‘working day’ was evident from the higher fatigue scores they reported. The researchers also saw more fatigue signals in this group, such as reduced pupil dilation.
After the working day, the participants were also allowed to choose from various rewards. This showed that the hard thinkers opted for a quick and easy reward (such as transferring money to them) than a prize that they had to wait longer for and caused more uncertainty (such as a lottery).
According to Pessiglione and his team, this suggests that hard thinking not only has a physiological cause (the recycling of a potentially toxic substance) but also influences decision-making. The latter actually sounds quite logical; you have to sit down for a while to get rid of that accumulation of glutamate.
Refreshing experiment
“It is refreshing to see these researchers attempting to determine their association with measurable data from the brain: brain activation with fMRI and the amount of glutamate with MRS,” says cognitive neuroscientist Fren Smulders (University of Maastricht). “Furthermore, their theoretical model seems to be well put together. The results of the pupil measurements also support their arguments well.”
He says there is a ‘but’. “As the authors of the publication themselves indicate, it is difficult to draw strong conclusions about the causal relationship. So: is the accumulation of glutamate really the cause of the fatigue, or is it a consequence of it? Is it perhaps an attempt by the brain to compensate for the fatigue? That would be a completely different interpretation, with the same data.”
feet up
More research is therefore needed to find definitive evidence that too much of the neurotransmitter makes you fatigued. Pessiglione hopes that in the future it will be possible to actually stimulate the production of glutamate and other neurotransmitters. “The next step is indeed the direct manipulation of glutamine,” says Smulders.
The knowledge could then also offer new insights into the treatment of burnout, which hard workers sometimes struggle with. But for now the ancient recipe after a heavy mental task is: put those legs up and rest.
Sources: Current Biology, Cell Press via EurekAlert!
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