Why are some people able to stay focused for hours while others lose focus after a few minutes? Why do some tasks seem exhausting to us while others accomplish them without apparent effort? A new study, published in Nature Communicationssheds new light on the role of dopamine, this molecule often associated with pleasure and motivation.
Researchers show that it doesn’t just serve to reinforce our habits. It would also play an essential role in our working memory and in the way our brain “evaluates” the cost of mental effort.
The same molecule for two different ways of learning
The study was led by Andrew Westbrook, a psychiatrist at Rutgers University. For several years, the researcher has been interested in a seemingly simple question: why does thinking sometimes require so much energy?
As he explains to PsyPost : “I’ve always been interested in cognitive effort: why does thinking feel like work, why are some tasks much more costly than others, why are the same tasks effortless for some people, and why do I have trouble staying focused even when it’s important for me to do so?
To answer this, his team looked at dopamine produced in the striatum, a region of the brain involved in learning, motivation and decision-making.
How researchers studied the role of dopamine
The scientists recruited 100 healthy adults, aged 18 to 43, in the Netherlands.
Each participant had to carry out a trial-and-error learning exercise consisting of associating different images with one of three buttons. To make the task more or less difficult, the number of images to memorize varied from 2 to 5, placing greater demands on working memory.
In parallel, the brain’s capacity to produce dopamine was measured using PET brain imaging ((18F)-FDOPA). During three separate sessions, participants then received either a placebo, 20 mg of methylphenidate, or 400 mg of sulpiride, two drugs known to modify the functioning of dopamine.
More dopamine, more efficient working memory
The results show that people naturally producing more dopamine relied more easily on their working memory, especially when the amount of information to remember remained limited.
Conversely, when participants received sulpiride, which blocks certain dopamine receptors (D2 receptors), their memory became less reliable: information was forgotten more quickly.
Methylphenidate produced a different effect. It further enhanced long-term reinforcement learning, with faster performance progression, particularly in people who already had high dopamine levels.
For Andrew Westbrook, these results extend what researchers already suspected: “It has long been known that striatal dopamine signaling helps animals mobilize when they need to exert physical effort to obtain a reward. We wanted to know if it also played a role in the decision to provide, or not, a cognitive effort.”
Dopamine also modifies our perception of effort
One of the most surprising results appeared during a final test.
Participants had to choose, between two images, the one that earned the most points. Overall, they answered correctly.
But the researchers observed a discreet phenomenon: the rewards obtained during the most difficult exercises seemed to have less value in the eyes of the participants, as if the effort made made them less rewarding.
On the other hand, under methylphenidate, this “devaluation” decreased.
In other words, dopamine not only facilitates learning: it also seems to influence the way our brain perceives the cost of mental effort.
As Andrew Westbrook summarizes: “Striatal dopamine causes us to rely more on costly working memory to solve difficult problems and also influences how we learn the cognitive effort required to complete these tasks.”
What these results could change
Using computer models, the researchers also show that, when the role of working memory is properly taken into account, reinforcement learning ultimately plays a much more limited role in the early phases of acquisition.
These results therefore invite us to review certain models used until now to understand learning and motivation.
The team now wishes to apply this work to diseases in which dopamine is already involved, such as attention deficit disorder with or without hyperactivity (ADHD) or schizophrenia.
For Andrew Westbrook, the objective is clear: “We need to better understand how abnormal dopamine functioning influences decisions related to cognitive effort.”
Ultimately, these discoveries could also be of interest to the world of education, by helping to better understand how to adapt the level of difficulty of learning without unnecessarily overloading the brain.