The Emory scientists used functional magnetic resonance imaging (fMRI) to measure brain activity in the striatum, which is a part of the brain previously associated with reward processing and pleasure. Although other experiments have studied and noted brain activity associated with rewards, until now these studies have not distinguished between the pleasurable effects of receiving a reward and the "saliency" or importance of the reward.
Study volunteers in the Emory experiment were asked to play a simple target-detection computer game. During the game, a money bill appeared occasionally and automatically dropped into a bag of money on the screen. The participant was given the amount of money that dropped in the bag at the end of the game, but because receiving the money had nothing to do with their performance on the computer game, it was not particularly arousing or salient to them. In another version of the game, a money bill occasionally appeared on the screen and the participant had to momentarily interrupt the target detection game and push a button to make the bill drop into the bag. In this case, whether or not the participant received the money did depend on their performance, which made the appearance of the money bill more salient to them. In yet another version, participants played the same computer game except the bag on the screen did not appear to have money in it and a blank "blob" drop! ped into the bag instead of money.
The investigators performed fMRI on the subjects while they were playing the game, particularly focusing on the reward centers. They found that some reward centers of the brain were activated whenever the money was received, but that other parts, particularly the striatum, were activated only when the participants were actively involved in receiving the reward.
"Scientists have conducted tests with monetary rewards in the past and noted that the striatum was activated, but it has been unclear whether it was because of the pleasure surrounding the money or the fact that the money was presented to participants in a salient or behaviorally important manner," said Zink. "We differentiated the saliency aspect by having the participants receive money that had nothing to do with their actions and having them receive money through active participation."
The investigators confirmed that the appearance of money that required a response was more salient to participants than money received passively by measuring skin conductance responses during the game -- a measurement of general arousal used as part of lie detector tests. The active participation in receiving the reward was the only condition that elicited a higher skin conductance measure, indicating greater arousal.
"Being actively engaged in the pursuit of rewards is a highly important function for the brain, much more so than receiving the same rewards passively," Dr. Berns explains. "It is like the difference between winning the lottery and earning the same amount of money. From the brain's perspective, earning it is more meaningful, and probably more satisfying."