Chess grandmasters are known to lose a lot of weight during major tournaments. The brain, like the muscle of thought, guzzles energy. And while not everyone will lose the pounds after strenuous mental exertion, most will recognize that thinking can be exhausting. New French research shows: this fatigue is not an illusion – but it is indeed between the ears.
To make decisions, people use their prefrontal cortex, a part of the brain located below the forehead. This is done by sending signals around through nerve cells, which use the substance glutamate for this purpose. The harder and the longer the nerve cells have to work, the more glutamate accumulates in the prefrontal cortex.
But glutamate is toxic: if it builds up too much, it can damage nerve cells. To prevent this, the brain must actively intervene to regulate the amount of glutamate, which inhibits the prefrontal cortex. This is noticeable when making decisions after a long period of worrying becomes increasingly difficult. As a result, after prolonged mental effort, we become more impulsive.
The French researchers concluded this on the basis of an experiment in which two groups of test subjects performed mental tasks for hours under a scanner. One group was given easy versions of the assignments, the other group received more difficult ones. After these long sessions, both groups were presented with small economic puzzles, in which they had to choose between a large amount that they would “get” only after a long period of time, or an immediate but small amount.
Different kind of scanning technique
The result: The subjects who completed the difficult tasks showed higher concentrations of glutamate in the prefrontal cortex, and they increasingly opted for the small, fast amount as the day went on. According to the researchers, this is proof that the brain becomes more impulsive and therefore more fatigued.
‘It is an innovative project with interesting, beautiful results’, says Floris de Lange, a cognitive neuroscientist at Radboud University Nijmegen and not involved in the research. Normally, neuroscientists measure the brain activity of their subjects on the basis of the blood supply in different brain regions. The French study used a different kind of scanning technique that allows you to track specific substances such as glutamate. ‘As a result, they really map out a mechanism.’
By contrast, this study did not examine all possible steps in the mechanism. ‘That is not possible in one go,’ says De Lange. ‘It remains to be seen exactly what role glutamate and other neurotransmitters play in brain fatigue.’
In addition, the study was conducted with a relatively small number of test subjects, forty in total. But De Lange also sympathizes with this: ‘This is quite a heroic study, for the test leaders and the test subjects, who performed tiring tasks under a scanner for more than six hours.’