Much like physical fatigue, our minds also get tired after a bout of intensive work that involves mental application. However, what causes mental fatigue wasn’t particularly well understood. As this article says, the earlier theories suggesting lack of energy were proven wrong as cognitive work consumes very few calories. New research points to accumulation of a certain type of chemical in the brain which results in us feeling mentally exhausted.

“A team of scientists led by Antonius Wiehler of Pitié-Salpêtrière University Hospital, in Paris, looked at things from what is termed a neurometabolic point of view. They hypothesise that cognitive fatigue results from an accumulation of a certain chemical in the region of the brain underpinning control. That substance, glutamate, is an excitatory neurotransmitter that abounds in the central nervous systems of mammals and plays a role in a multitude of activities, such as learning, memory and the sleep-wake cycle.

In other words, cognitive work results in chemical changes in the brain, which present behaviourally as fatigue. This, therefore, is a signal to stop working in order to restore balance to the brain. In their new paper in Current Biology, the researchers describe an experiment they undertook to explain how all this happens.

To induce cognitive fatigue, a group of participants were asked to perform just over six hours of various tasks that involve thinking. Half were assigned easy things to do and half hard ones. For example, in one task, letters were displayed on a computer screen every second or so. Those in the easy group had to remember whether the current letter matched the previous letter or, for the hard group, the one shown three letters earlier.

….During the experiment the scientists used a technique called magnetic-resonance spectroscopy to measure biochemical changes in the brain. In particular, they focused on the lateral prefrontal cortex, a region of the brain associated with cognitive control. If their hypothesis was to hold, there would be a measurable chemical difference between the brains of hard- and easy-task participants. And indeed, that is what they found. Their analysis indicated higher concentrations of glutamate in the synapses of a hard-task participant’s lateral prefrontal cortex. Thus showing cognitive fatigue is associated with increased glutamate in the prefrontal cortex. Dr Wiehler speculates that this is the result of a mechanism in the brain that is computing a sort of cost-benefit analysis, with fatigue and increased glutamate adding to the cost of mental effort.

There may well be ways to reduce the glutamate levels, and no doubt some researchers will now be looking at potions that might hack the brain in a way to artificially speed up its recovery from fatigue. Meanwhile, the best solution is the natural one: sleep.”

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