A photo taken in the ski resort of Courchevel in the French Alps shows a Rodin ‘Thinker Statue’ on display atop La Vizelle.
©JEAN-PIERRE CLATOT / AFP
Mentally intensive activities can be exhausting. Why does thinking tire us so much? Neuroscientists have just published a study in the journal Cell Biology about how mental performance declines over time and what we can do to take care of our brains.
Atlantico: We’ve all noticed that mentally intensive activities can be exhausting. A long day of thinking, even with little physical activity, can make you want to collapse on the couch. How to explain this phenomenon ? Why does thinking tire us so much?
Mathias Pessiglione: It’s not so much thinking that tires as what we call cognitive control. Control processes are set in motion whenever the brain cannot rely on pre-learned routines. This is the case when we think about a problem (by definition the solution is not known so there is no automatic response in store but also in situations that do not require reflection, for example a social situation where we would like to yawn, scratch or leave (automatic responses) but we do not do it out of respect for propriety. Or when we make an effort of endurance, like a marathon, that the pain commands us stop (it’s the automatic answer) but that we continue because we want to achieve a certain performance.
A day when it is necessary to exercise a high level of cognitive control makes regions of the prefrontal cortex work continuously. However, neurons to communicate use a neurotransmitter, glutamate, which poses no problem at low doses but which can prove to be toxic at high doses. If the neurons work continuously, they end up accumulating glutamate in the synaptic cleft. According to our results, this is the origin of the phenomenon of fatigue: our brain can no longer cope with the increase in glutamate in the prefrontal cortex and generates a signal that leads us to stop working and rest. When the prefrontal cortex is put to rest (a fortiori when we sleep), glutamate is progressively eliminated from the synapses (either reintegrated into the neurons, or converted into glutamine by other cells, the astrocytes).
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How did you identify this phenomenon?
By the technique of magnetic resonance spectroscopy, which makes it possible to measure the diffusion of molecules such as glutamate. Diffusion is faster in open spaces, like synapses, than in small behaviors, like vesicles inside cells. The increase in diffusion therefore indicates the release of glutamate from the vesicles into the synaptic cleft. This is what we observed during the day in the prefrontal cortex of participants who performed demanding tasks (requiring a high degree of cognitive control), but not in those who performed the same tasks for the same duration but in a easier version. Moreover, fatigue in these participants was evident in tests where they were made to choose between several possible options. At the end of the day, they favored options that required neither waiting nor effort, which reflects the fatigue of cognitive control.
The “pomodoro” technique could help us keep a steady pace throughout the day without getting exhausted. What is it and how do I set it up?
I don’t know, I don’t really know this technique. A priori it consists in planning breaks. It’s a little trivial but yes, taking regular breaks avoids the accumulation of glutamate and therefore working longer without exhausting yourself. But I don’t believe that exhaustion is the problem that this method aims at, rather made to fight against procrastination.
Are there other similar techniques that can allow us to be more efficient at work? What are the rules to know to reduce our cognitive fatigue?
Rest and sleep! There are no good alternatives. One could imagine biological interventions, but they are likely to lead to side effects, and to create rebound effects when they are stopped. We could also, eventually, try to monitor the state of his prefrontal, perhaps with EEG electrodes placed on the scalp, but this is pure speculation, there is no demonstration that such a system works.
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