Mentally strenuous work is exhausting because it creates a build-up of toxic byproducts in the brain, new research shows.
Chess players have to deal with it and so can you 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 at the Paris Brain Institute and the Sorbonne Universités, led by Mathias Pessiglione, found that thinking long and hard can poison your brain a bit.
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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 called magnetic resonance spectroscopy (MRS) at regular intervals. 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 different 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 indicated 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.
feet up
The researchers themselves are still critical about the link found between glutamate and fatigue. For example, they indicate that the equipment was limited in measuring the substance. In addition, other neurotransmitters may also play a role, as well as other feelings, such as hunger and stress. Pessiglione hopes that in the future it will be possible to actually stimulate the production of glutamate and other neurotransmitters. If mental fatigue also occurs, that would be the ultimate proof.
This knowledge could perhaps 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!