Oct 18 2004
You walk into a room and spy a plate of gooey doughnuts dripping with chocolate frosting. But wait: You were saving your sweets allotment for a party later today. If it feels like one part of your brain is battling another, it probably is, according to a newly published study.
Researchers at the University of Pittsburgh, Carnegie Mellon University, Harvard University and Princeton University found two areas of the brain that appear to compete for control over behavior when a person attempts to balance near-term rewards with long-term goals. The research involved imaging people's brains as they made choices between small but immediate rewards or larger rewards that they would receive later. The study grew out of the emerging discipline of neuroeconomics, which investigates the mental and neural processes that drive economic decision-making.
The study was a collaboration between Jonathan Cohen and Samuel McClure at Princeton's Center for the Study of Brain Mind and Behavior (Cohen also is a professor of psychiatry at Pitt); David Laibson, professor of economics at Harvard University; and George Loewenstein, professor of economics and psychology at Carnegie Mellon. Their study appears in the Oct. 15 issue of Science.
"This is part of a series of studies we've done that illustrate that we are rarely of one mind," Cohen said. "We have different neural systems that evolved to solve different types of problems, and our behavior is dictated by the competition or cooperation between them."
The researchers examined a much-studied economic dilemma in which consumers behave impatiently today but prefer to act patiently in the future. For example, people who are offered the choice of $10 today or $11 tomorrow will likely choose to receive the lesser amount immediately. But if given a choice between $10 in one year or $11 in a year and a day, people often choose the higher, delayed amount.
In classic economic theory, this choice is irrational because people are inconsistent in their treatment of the daylong time delay. Until now, the cause of this pattern was unclear, with some arguing that the brain has a single decision-making process with a built-in inconsistency, and others, including the authors of the Science paper, arguing that the pattern results from the competing influence of two brain systems.
The researchers studied 14 Princeton students who were asked to consider delayed reward problems while undergoing functional magnetic resonance imaging (fMRI), a procedure that shows what parts of the brain are active at all times. The students were offered choices between Amazon.com gift certificates ranging from $5 to $40 in value and larger amounts that could be obtained only by waiting some period, from two weeks to six weeks.
The study showed that decisions involving the possibility of immediate reward activated parts of the brain influenced heavily by brain systems that are associated with emotion. In contrast, all the decisions the students made — whether short- or long-term — activated brain systems that are associated with abstract reasoning.
Most important, when students had the choice of an immediate reward but chose the delayed option, the calculating regions of their brains were more strongly activated than their emotion systems, whereas when they chose the immediate reward, the activity of the two areas was comparable, with a slight trend toward more activity in the emotion system.
The researchers concluded that impulsive choices or preferences for short-term rewards result from the emotion-related parts of the brain winning out over the abstract-reasoning parts. "There are two different brain systems and one of them kicks in as you get really proximate to the reward," McClure said.
The finding supports the growing view among economists that psychological factors other than pure reasoning often drive people's decisions.
"Our emotional brain has a hard time imagining the future, even though our logical brain clearly sees the future consequences of our current actions," Laibson said. "Our emotional brain wants to max out the credit card, order dessert and smoke a cigarette. Our logical brain knows we should save for retirement, go for a jog and quit smoking. To understand why we feel internally conflicted, it will help to know how myopic and forward-looking brain systems value rewards and how these systems talk to one another."
The findings also may cast light on other forms of impulsive behavior and drug addiction.
"Our results help explain how and why a wide range of situations that produce emotional reactions, such as the sight, touch or smell of a desirable object, often cause people to take impulsive actions that they later regret," Loewenstein said. Such psychological cues are known to trigger dopamine-related circuits in the brain similar to the ones that responded to immediate rewards in the current study.
Concerning addiction, said Loewenstein, the findings help explain some aspects of the problem, such as why addicts become so focused on immediate gratification when they are craving a drug. The dopamine-related brain areas that dominated short-term choices among the study subjects also are known to be activated when addicts are craving drugs.
The researchers are now trying to pin down what kinds of rewards and how short a delay are needed to trigger the dopamine-related reaction. Their ultimate goal is to better understand how the emotion-related and calculating systems interact and to understand how the brain governs which system comes out victorious.