After a training period, the subjects were asked to perform the task while inside an fMRI machine, which measures blood flow in the braina proxy for brain activity, since wherever a brain is active, it needs extra oxygen, and thus a larger volume of blood. By monitoring blood flow, the researchers can pinpoint areas of the brain that turn on when a particular task is performed.
The task began with the researchers offering the participants a randomized range of rewardsfrom $0 to $100if they could successfully place the object into the square within the time limit. At the end of hundreds of trialseach with varying reward amountsthe participant was given their reward, based on the result of just one of the trials, picked at random.
As expected, the team found that performance improved as the incentives increasedbut only when the cash reward amounts were at the low end of the spectrum. Once the rewards passed a certain threshold, which depended on the individual, performance began to fall off.
Incentives are known to activate a part of your brain called the ventral striatum, Chib says; the researchers thus expected to see the ventral striatum become increasingly active as they bumped up the prizes. And if the conventional thought were correctthat the reason for the observed performance decline was over-motivationthey would expect the striatum to continue showing a lot of activation when the incentives became high enough for performance to suffer.
What they found, instead, was that when the participants were shown their potential rewards, activity in the striatum did indeed increase with rising incentives. But once the volunteers started doing the task, striatal activity decreased with rising incentives. They also noticed that the less activity they saw in a participant's striatum, the worse that person performed on the task.
Other studies have shown that decreasing striatal acti
|Contact: Deborah Williams-Hedges|
California Institute of Technology