It has long been assumed that stimulant drugs, such as Ritalin and Adderall, work by helping people focus.
Now a new study, published in the journal Science, shows that these medications, typically prescribed for those with attention-deficit/ hyperactivity disorder (ADHD), actually work by directing the brain to fix its attention on the benefits, rather than the costs, of completing difficult tasks.
The study marks the first time that researchers have looked at how stimulants such as Ritalin alter cognitive function. The findings could open opportunities for further studies to help medical professionals better understand how to identify and treat ADHD, depression, anxiety and other mental disorders.
“People tend to think, ‘Ritalin and Adderall help me focus,’” said Dr. Michael Frank, the study’s co-senior author and a professor of cognitive, linguistic and psychological sciences at Brown University.
“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.”
According to Frank, stimulants increase the amount of dopamine released in the striatum, a key region in the brain related to motivation, action and cognition.
Previous studies have shown that dopamine, a “chemical messenger” that carries information between neurons, has a significant influence on cognitive and physical behavior. Several past studies have shown, for example, that both rodents and humans are more motivated to perform physically demanding tasks with higher dopamine.
What has remained unclear, however, is whether dopamine can have similar motivational effects on cognition — and that’s what a new collaborative project between Frank, Brown postdoctoral researcher Dr. Andrew Westbrook and Dutch neuropsychiatry scholar Dr. Roshan Cools set out to understand.
“We’ve known for a long time that when you give people these types of stimulants, you get enhanced performance,” said Westbrook, the study’s lead author. “But is that due to an increased ability, or is it due to increased motivation? We didn’t know which of these two factors were contributing and to what degree.”
Previously, the research team had developed mathematical models suggesting that dopamine alters the degree to which the striatum emphasizes the benefits, rather than the costs, of completing physical and mental actions.
Drawing on these models, the team developed an experiment that analyzed how dopamine-raising stimulants affected people’s cost vs. benefit decisions.
The research involved 50 healthy women and men ages 18 to 43 in a lab at Radboud University in the Netherlands. First, the team measured the natural dopamine levels in each participant’s striatum using brain imaging technology. Then, the participants were asked if they wanted to take part in a series of cognitively demanding tests, some easier and others more difficult, in exchange for certain amounts of money. Subjects who agreed to take the hardest tests could make the most money.
Each participant completed the experiment three times — once after taking a placebo; once after taking methylphenidate (the generic version of Ritalin); and once after taking sulpiride, an antipsychotic that elevates dopamine levels when taken in low doses and is often used to treat symptoms of schizophrenia and major depressive disorder at much higher doses.
The study used a double-blind experiment design, where neither the researchers nor the participants knew which pill had been taken.
The results were similar to Westbrook’s computer-modeled predictions: Participants with lower dopamine levels made decisions that indicated they were more focused on avoiding difficult cognitive work — in other words, they were more sensitive to the potential costs of finishing the task.
In contrast, participants with higher dopamine levels made decisions suggesting they were more sensitive to differences in the amount of money they could earn by choosing the harder test — in other words, they focused more on the potential benefits. Westbrook said the latter held true whether the subjects’ dopamine levels were naturally higher or whether they had been artificially elevated by medications.
Westbrook said the findings support the idea that, medication or no medication, dopamine typically acts as a motivation regulator for human brains.
“The thoughts that pop into our head, and the amount of time we spend thinking about them, are regulated by this underlying cost-benefit decision-making system,” Westbrook said. “Our brains have been honed to orient us toward the tasks that will have the greatest payoff and the least cost over time.”
All of us have slightly different base levels of dopamine, said Frank. People with lower levels tend to be more risk-averse, because they spend more time focusing on the potential costs of completing a difficult task, while those with higher levels tend to be more impulsive and active, because they focus more on the benefits.
No single dopamine level is inherently better than another, Frank said. For example, an active high-dopamine person may take fulfilling, happiness-boosting risks but may also be more prone to injury. On the other hand, a risk-averse, low-dopamine person may avoid injuries and disappointments but may also miss out on adventures.
And dopamine levels don’t necessarily stay the same from one day to the next. They may decrease in response to danger or lack of sleep, and they may increase when people feel safe and supported. In other words, most people can trust their natural dopamine levels to guide them toward the right decisions, said Westbrook.
Of course, previous studies have shown that people with particularly low dopamine levels, such as those with depression or ADHD, can benefit from dopamine-boosting stimulant medications. But he said those medications are never certain to improve the lives of those who are healthy and who choose to use them recreationally. Doing so could, in fact, lead some to make poorer decisions.
“When you raise dopamine in someone who already has a high dopamine level, every decision seems like it has benefit, which could distract from the real beneficial tasks,” Westbrook said. “People might behave in ways that aren’t consistent with their goals, like taking part in impulsive gambling or risky sex behaviors.”
The researchers hope the new findings help medical professionals better understand cognitive mechanisms, allowing them to identify connections between dopamine levels and disorders such as anxiety, depression, ADHD and schizophrenia.
“We want to know, what are the drivers of what changes cognitive ability and function?” Frank said. “Our research is focused on carving nature at its joints, so to speak — disentangling neural and cognitive functions to understand people’s different thought processes and evaluate what’s best for their needs, whether it’s therapy or medication.”
Source: Brown University