Prolonged mental fatigue can wear down brain areas crucial for the individual ability to self-control, and cause people to behave more aggressively.
In a new multidisciplinary study published in the PNAS, a group of researchers from neuroscience and economics at the IMT School of Advanced Studies Lucca links the debated concept of "ego depletion", that is to say the diminution of willpower caused by previous exploitation of it, to physical changes in the areas that govern executive functions in the brain. In particular, the fatigue appears to correspond, in the awake brain, to an increase of the EEG waves typical of sleep in the frontal cortex zone dedicated to making decisions.
In the scientific literature, theories regarding so-called ego depletion emerged in the early 2000s. At their core, there is the idea that self-control is a limited cognitive resource for everyone, and therefore, the more it is exercised, the more it is exhausted. The literature in behavioral economics has used various types of cognitive manipulations typical of economic games to show the effect of ego depletion on behavior, for example less empathy towards others, a lower tendency to act altruistically, or a greater propensity to aggression.
In more recent years, however, this theory has been criticized: subsequent studies have not always managed to replicate the effect of "consumption" of willpower for individuals engaged in strenuous cognitive tasks or, if they have succeeded, they have found a much smaller effect than initially estimated. Moreover, the brain correlates of such an effect remained obscure.
The new study addresses the classic problem by adding the neuroscientific perspective. The research on sleep has identified a phenomenon called "local sleep": it happens when some brain areas in the awake individual begin to show on the EEG typical neural activity seen during sleep, namely delta waves. It has been shown that this happens particularly in cases of mental fatigue. "Our starting hypothesis was that local sleep would be the neuronal manifestation of the phenomenon of ego depletion known to psychology," observes Erica Ordali, research fellow at the IMT School and first author of the paper.
To test this hypothesis, the researchers subjected a group of individuals to some fatigue tasks lasting one hour – instead of the classic fifteen minutes usually used in this type of study - in order to make the potential effect, if present, more evident. Next, individuals played economic games that required varying degrees of aggression and cooperation, including the so-called hawk and dove game. In this game, limited resources are to be shared in a hostile environment situation, with people having the choice between collaborating or overbearing behavior, that may result in the loss of resources for both parties. Compared to a control group not subjected to cognitive fatigue, the individuals who had undergone it proved to be significantly more uncooperative and hostile. Specifically, peaceful cooperation rate dropped from 86 percent in the "No Fatigue" to 41 percent in the "Fatigue" group (p>0.001, for a total of 447 subjects).
All study participants to the experiment (n=44) underwent electroencephalograms while playing economics games. In line with the study hypothesis, fatigued individuals showed the emergence of areas with typical sleep waves in some areas of the frontal cortex, which was completely absent in the others. "Our study shows that mental fatigue has a measurable effect on behavior and that, when a certain degree of fatigue sets in, people are more likely to behave in a hostile manner," says Ordali.
"These results provide a scientific bases to popular wisdom that suggests to 'sleep on it' before making a decision, by showing that metabolic exhaustion within certain brain areas does affect our decision-making processes" says Pietro Pietrini, coauthor of the paper and Director of the Molecular Mind Lab at the IMT School, where the study was designed. "Overall, these findings have important implications for multiple situations in everyday life, including economic transactions and legal agreements, as they demonstrate that when the brain is 'tired' we may make choices that go even opposite to our own interest. As a matter of fact, this is what people do also in most criminal acts" concludes Pietrini.
The study was conducted by the IMT School for Advanced Studies Lucca in collaboration also with researchers at the University of Florence.
