To find clues, they used magnetic resonance spectroscopy (MRS) to monitor the chemical processes in the brain over the course of a working day. MRS is a specialized technique associated with the better known MRI, magnetic resonance imaging.
They used this technique to look at two groups of people: one who had to think hard and another who had to perform relatively easier cognitive tasks.
Only in the group that had been given the difficult tasks did they see indications of fatigue, including a reduced dilation of the pupils. The people in that group also showed a shift in their choices toward options that would yield rewards in the short term and with little effort.
Essentially, they also had higher levels of glutamate in the synapses of the prefrontal cortex. Glutamate is a so-called neurotransmitter, a signal substance that transfers nerve impulses between nerve cells and gland or muscle cells, and the synapses are the contact points where that transfer takes place.
Together with previous evidence, this supports the theory that the accumulation of glutamate makes further activation of the prefrontal cortex more difficult. And that in turn makes cognitive control more difficult after a mentally tough day at work.
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