A long session of thinking could temporarily change our willingness to make an effort, even when the reward is worth it.
After a day spent solving problems or studying, a task that is actually simple can become discouraging. This is not necessarily because the brain can no longer work. A new experiment shows that intense mental activity can be accompanied by a chemical change in a region of the brain involved in our decisions.
To examine this phenomenon, researchers brought together 31 healthy adults. Each volunteer completed a demanding memory exercise for 20 minutes, twice, on different days. The scientists compared the participants' responses before and after this effort.
One feature made the experiment particularly interesting: during one of the sessions, the researchers applied mild electrical stimulation to the head. This increased the activity of a region located toward the front of the brain. During the other session, sham stimulation served as a comparison.
Why stimulate this region? It is involved in particular when information must be kept in memory and manipulated. By increasing its activity, the researchers first observed improved performance. But this immediate gain seemed to come with a trade-off.
After the exercises, the volunteers had to choose between new memory tasks. The easier ones paid less money, while the more difficult ones paid more. After the genuine stimulation, participants were less willing to choose exercises requiring a great deal of effort. Their ability to succeed and their desire to try again therefore did not necessarily follow the same pattern.
The researchers also measured the concentrations of certain brain molecules before and after the sessions, using a non-invasive imaging technique called magnetic resonance spectroscopy.
They observed a higher amount of glutamate in the stimulated region. This substance is used by nerve cells to transmit messages. It is normally present in the brain and plays a role in its functioning.
In other words, the results link increased brain activity, a measurable chemical change, and reduced motivation. However, they do not prove that glutamate alone causes all mental fatigue.
It remains to be determined whether this mechanism also plays a role in people suffering from persistent fatigue. The scientists are considering studying whether these chemical changes could help better identify certain forms of pathological fatigue, and then test suitable treatments.