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Pessiglione and colleagues, including first author of the study Antonius Wiehler, wanted to understand what mental fatigue really is. While machines can compute continuously, the brain cannot. They wanted to discover why. They suspected the reason had to do with the need to recycle potentially toxic substances that originate from neural activity.
To look for evidence to support this theory, they used magnetic resonance spectroscopy (MRS) to monitor brain chemistry over the course of a workday. They studied two groups of people: those who needed to think hard and those who had relatively simple cognitive tasks.
They saw signs of fatigue, including reduced pupil dilation, only in the group doing hard mental work. Those in that group also exhibited in their choices a change toward options proposing rewards at short delay with minimal effort. Critically, they also had higher levels of glutamate in synapses of the brain’s prefrontal cortex.
Together with earlier evidence, the scientists say it supports the hypothesis that glutamate accumulation makes further activation of the prefrontal cortex more costly, such that cognitive control is more difficult after a mentally tough workday.
There are now a plethora of supplements and changes we can make to our diet and lifestyle to help combat brain fatigue. Read about them in our latest article here