By Matt Wood
Each day people make decisions about how much effort they’re willing to put into various tasks. The decision about how much effort to invest in an activity is influenced by the reward for doing something and the probability of actually getting it. You might be willing to work hard at your job because the reward—a paycheck—is both valuable and fairly certain. But you might not be willing to try a complicated new recipe for dinner, even though it sounds delicious, because of the chance that it won’t turn out well.
Animal studies suggest that the neurotransmitter dopamine plays an important role in this type of decision-making, especially the decision to expend effort. In rats, dopamine levels influence tolerance for effort and probability costs. Rats with higher levels of dopamine are more willing to press levers and climb over barriers to reach better food, whereas those with lower levels will settle for food that’s less tasty but within easy reach.
Little is known about how dopamine affects effort-based decision-making processes in humans. But a new study published in The Journal of Neuroscience by Margaret Wardle, a post doctoral researcher in the Department of Psychiatry and Behavioral Neuroscience at the University of Chicago, her mentor Harriet de Wit, Professor and Director of the Human Behavioral Pharmacology Laboratory, and colleagues at Vanderbilt University shows for the first time that people behave in much the same way. Their research, sponsored by the National Institute on Drug Abuse, not only sheds light on how dopamine influences decision-making in humans, but also points toward possible treatment for psychiatric disorders like depression.
In rats, researchers manipulate dopamine by injecting it into the nucleus accumbens, a part of the brain that plays a key role in reward and pleasure. This precise manipulation is possible in animal studies but not very practical as a treatment for people. Instead, Wardle and her team gave their human test subjects amphetamine to increase levels of dopamine. Amphetamine, which can be abused, has a variety of medical applications, such as treating Attention Deficit Hyperactivity Disorder and other conditions.
The test subjects were healthy adult volunteers who also had experience with some kind of mind-altering drug, whether alcohol, marijuana or a stimulant like amphetamine, so they would be comfortable with that experience in the lab. They were given doses of either 10 or 20 milligrams, which is in the therapeutic range but still enough to produce euphoric effects.
They were then given a series of tasks on a computer to test their willingness to exert effort for varying rewards. In each trial subjects were allowed to choose between performing an easy task that required lower effort to earn a smaller reward, such as tapping a key 30 times with the dominant index finger within seven seconds to win $1, or a hard task with a higher reward, such as tapping 100 times with the pinky on the non-dominant hand in 21 seconds to earn anywhere from $1.25 to more than $4.
As expected, people who had taken amphetamine instead of a placebo were more willing to exert the extra effort for the higher reward. They chose the high-cost, high-reward option more often, especially when the probability of completing the task successfully was lower. While overall effort and tolerance for probability costs increased for people on the amphetamine, the effect of incremental increases in the size of the reward didn’t change. Amphetamine lowered the absolute dollar amount at which people were willing to work for a reward, but it didn’t alter the effect of those small changes on decisions. This suggests that the amphetamine didn’t alter the valuation of benefits. The gross motor effects of the amphetamine, that is, simply being able to tap a keyboard faster because they had taken a stimulant, didn’t appear to affect decision-making either.
This experiment sheds light on cost-benefit decision-making in humans, but the findings about how dopamine influences these processes also could lead to treatments for mental disorders, especially depression. Depression is commonly treated by addressing levels of serotonin, another neurotransmitter that contributes to feelings of happiness and well-being, but Wardle pointed out that depression is a constellation of symptoms beyond just general happiness.
“There are subsets of depressed people who have really strong amotivational symptoms. This is like the ‘don’t want to get out of bed, don’t want to leave the house’ feeling,” she said. “It’s not just that they don’t enjoy things; it’s more that they seem unmotivated to pursue them.” If dopamine has such a strong influence on motivation, as shown in this study, antidepressants could be designed to treat both serotonin and dopamine dysfunction.
“We know that those are some of the symptoms that don’t really remit when people get antidepressants, and they’re also some of the most debilitating,” Wardle said. “They’re the things that are associated with the most lost-work productivity, all kinds of other things that really impact your life, so if we can get a better understanding of how this brain system is related to effort-based decision-making and this get up and go, maybe we can develop better treatments for people with those symptoms.”
Wardle MC, Treadway MT, Mayo LM, Zald DH, & de Wit H (2011). Amping Up Effort: Effects of d-Amphetamine on Human Effort-Based Decision-Making. The Journal of neuroscience : the official journal of the Society for Neuroscience, 31 (46), 16597-602 PMID: 22090487