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Dopamine Might Be Behind Impulsive Behavior

A monkey study suggests the brain chemical makes what's new and different more attractive.
A micrograph of dopamine crystals. (Photo: wellcomeimages/Flickr)

A micrograph of dopamine crystals. (Photo: wellcomeimages/Flickr)

A certain amount of exploration is a good thing—without it, we’d have a hard time figuring what we like and don’t like—but it has a dark side, too, in the form of impulsivity and behavioral addictions. A small new study suggests a brain chemical called dopamine helps regulate that trade off, with consequences for how we understand our own preferences and decisions.

Dopamine has a funny history in the burgeoning field of neuroeconomics. Though it’s hardly dopamine’s only job, researchers believe the chemical is a key player in human and animal decision making. In particular, dopamine neurons in the brain’s limbic system—itself thought to support emotion, motivation, and other functions—seem to keep track of how much pleasure we expect when making a given choice and what we actually get. That, theorists say, is how we learn what we like and what we don’t. Still, dopamine's function is less than clear, in particular with regard to why we seek out new experiences, sometimes in excess.

The results "do seem to imply that dopamine specifically heightens novelty seeking without affecting overall learning or general exploration."

Thus motivated, Vincent Costa, Bruno Averbeck, and colleagues at the National Institute of Mental Health’s Laboratory of Neuroscience in Bethesda, Maryland, decided to see how a little extra dopamine affected the decision making of three of their rhesus monkeys. Across six sessions, the monkeys got injections of a dopamine transport inhibitor, or DAT, a drug that increases the effect of dopamine. Then, the monkeys made a series of choices between three pictures, each of which had a different chance of rewarding a monkey with a small amount of juice. Over the course of several hundred such choices, the monkeys would have ample chance to learn about those rewards, but Costa and company set things up so that every so often one of the pictures changed to something new. Crucially, the team followed each session with another one two days later that used saline—salt water—in dopamine’s place.

The team found that their monkeys chose the novel picture around 60 percent of the time when they’d administered the dopamine transport inhibitor, compared with about 55 percent of the time with saline. That difference, they discovered with the help of a computer model of learning and decision making, corresponded to the drug roughly doubling the value of a novel picture, compared to the other pictures. Though they're hesitant to draw any strong conclusions yet, the results "do seem to imply that dopamine specifically heightens novelty seeking without affecting overall learning or general exploration," in which monkeys might sample from different options but wouldn't be unusually drawn to brand-new options, Costa says in an email.

Next up, Costa writes, the team is trying to identify what neural circuits are behind the choice to seek out what's new and different, which will involve recording the activity of individual neurons in a number of brain regions thought to be related to reward learning and decision making. They'll also be working on other ways of modeling their data, with an eye toward how rational or irrational novelty-seeking behavior really is.