2023-06-07 10:00:13
Cach day, we carry out many tasks that cost us. We gladly agree to make this effort, because we know that we will be rewarded in return by a salary, or simply by the pleasure of having completed a project. Sometimes, unfortunately, it becomes difficult to motivate oneself or to concentrate. At this stage, we speak of real “mental fatigue”. If this occurs mainly when we have exhausted all our intellectual resources, it can also be the consequence of physical fatigue or lack of sleep.
Exhausted, we see everything as insurmountable and we feel incapable of carrying out the slightest activity. What is happening at this precise moment in our brain? Is it the effort required that seems impossible to us or the reward that suddenly does not seem so motivating to us? This question is at the center of recent research which shows that these two processes could well contribute to our demotivation.
In 2010, Julian Lim and his colleagues at the University of Pennsylvania installed participants in a scanner and asked them to react to a target by pressing a button as soon as it appeared, and this, with permanent vigilance, for twenty minutes. Over the course of the experiment, they noted not only a gradual increase in the time taken to detect the target, but also an increasingly noticeable feeling of mental fatigue.
Their results find a cerebral signature of attentional and cognitive cost in the parieto-frontal network of the right hemisphere. The activity of this network, very important at the beginning of the task, tends to decrease as the feeling of mental fatigue appears. In addition, brain activity recorded at rest, before the task, in two brain regions, the thalamus and the medial right frontal gyrus, predicted the decline in performance during the task. There would therefore also be cerebral markers of mental fatigue. But that’s not all: the more we feel tired, the more the reward may well lose its value in terms of the effort required.
The limited effect of short breaks
Tanja Müller and her colleagues from Oxford (UK) also installed participants in a scanner in 2021, asking them to perform a hand pressure effort to obtain a reward. At each trial, the subject could choose between five seconds of rest (no effort, but negligible reward) and five seconds of more or less intense work associated with a variable reward. The subjects could estimate the subjective value of the reward and the fatigue felt. Using this protocol, the authors were able to describe two distinct time scales on which fatigue and reward value impairment occur. These can be observed in the very short term during a task – and, in this case, a brief pause is beneficial –, but the authors also find, in the longer term, alterations on which a short pause does not has no significant effect.
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