Dopamine affects how brain decides whether a goal is worth the effort

Some students without ADHD take medications like methylphenidate in the hope of enhancing their focus while studying. Aldomurillo / E+ via Getty Images

Medications ike methylphenidate (sold under the Ritalin and other brand names) are prescribed to treat attention deficit hyperactivity disorders (ADHD). These stimulants are also used by students without ADHD who hope to enhance their focus while studying. It’s unclear exactly how methylphenidate and similar drugs work as a study aid to improve mental performance.

Methylphenidate works in part by blocking the reabsorption of the neurotransmitter dopamine—a chemical messenger involved in reward-seeking, movement, and motivation—by the brain’s neurons. This increases the amount of dopamine in the brain’s striatum, an area involved in motor function and reward. Previous studies have shown that increases in dopamine in the striatum can motivate people and rodents to perform harder physical tasks.

Researchers suspected that dopamine might play a role in influencing how the brain evaluates whether a mental task is worth the effort. The team, co-led by Dr. Michael Frank from Brown University, performed a series of experiments to assess dopamine’s role in motivation. The study was funded in part by NIH’s National Institute of Mental Health (NIMH). Findings appeared in Science on March 20, 2020.

Fifty people, ages 18 to 43, participated in the study. The scientists first measured the levels of natural dopamine in the participant’s striatum. Participants were asked to choose between a series of memory tasks of varying difficulties. More difficult mental tasks were rewarded with more money.

The team found that those with higher dopamine levels in a region of the striatum called the caudate nucleus were more likely to focus on the benefits (the money) and choose the difficult mental tasks. Those with lower dopamine levels were more sensitive to the perceived cost, or task difficulty.

The participants next completed experiments after taking an inactive placebo, methylphenidate, or sulpiride—an antipsychotic medication that, at low doses, increases dopamine levels. Increasing dopamine boosted how willing people with low, but not high, dopamine synthesis capacity in the caudate nucleus were to choose more difficult mental tasks. It did this by changing their cost/benefit sensitivity. The results from these experiments reflected the findings for natural varying dopamine levels.

To gain more insight into the decision-making process, the researchers tracked the participant’s eye movement as they reviewed information about task difficulty and the amount of money they would receive. Their gaze patterns suggested that dopamine didn’t alter their attention to benefits vs costs. Rather, it increased how much weight people gave to the benefits once they were looking at them.

These finding suggests that Ritalin and similar drugs may work by acting on motivation rather than directly boosting cognitive function. For those with lower dopamine levels, boosting dopamine can affect the mental cost-benefit analysis so that they focus more on reward than cost. That, in turn, increases their willingness to attempt harder tasks.

“People tend to think, ‘Ritalin and Adderall help me focus,’” Frank says. “And they do, in some sense. But what this study shows is that they do so by increasing your cognitive motivation: Your perceived benefits of performing a demanding task are elevated, while the perceived costs are reduced. This effect is separate from any changes in actual ability.”

—by Erin Bryant