Where You Fit In: Two new studies shed light on the neural mechanisms used in perceiving “social rewards”

Two new studies shed light on the neural mechanisms used in perceiving “social rewards”

by Jim Schnabel

July 16, 2008

Separate groups of researchers report using functional magnetic resonance imaging (fMRI) to map the parts of the brain involved in processing social rewards such as increases in reputation or status. Their results suggest that the human brain is highly sensitive to cues about social hierarchies and treats social rewards in a manner similar to monetary rewards.

In the April 24 issue of Neuron, Caroline Zink and colleagues at the National Institute of Mental Health described a series of studies in which three sets of 24 healthy volunteers underwent fMRI scans while participating in a simple reaction-time task for money.

In the first two sets of experiments, researchers simulated a simple, three-person social hierarchy. Images shown to each volunteer suggested that one of two other players—roughly matched in age and appearance to the volunteer—was playing the same game at the same time. The three players were assigned simple ranks, purportedly on the basis of some prior achievement. The volunteer was given two stars, the other players one or three.

Unknown to each volunteer, these “inferior” and “superior” players’ actions were merely simulated, their game-playing responses predetermined. Nevertheless, the fMRI data indicated that at some level each volunteer was keenly aware of the hierarchy and his or her place within it.

Even when the hierarchy was stable, so that the game outcomes could not affect the volunteers’ positions vis-à-vis the other “players,” key areas of the volunteers’ brains showed greater activation, on average, when viewing images of the “superior” player than when viewing images of the “inferior” player.

“It suggested that our participants were paying more attention to the superior person than they were to the inferior person,” says Zink. “So the superior person is just more meaningful to them, and they are processing the superior player more. Which is exactly what they say in the primate literature: Behaviorally, monkeys value and pay much more attention to more dominant monkeys.”

In the third set of games, Zink and her colleagues ran a control experiment in which the other “players” were described as computers, not humans. Especially when the hierarchy was unstable, and the game was therefore in a sense competitive, the researchers observed little or no activation in a number of brain areas including the amygdala and medial prefrontal cortex—areas known to be involved in processing emotions and judgments of other people.

Another result suggested that “no pain, no gain” applies also to status competitions. Those subjects who seemed from questionnaires to be the most competitive and motivated to reach the top showed greater activation in a brain region called the insula when they learned that they had been moved down a notch in the social hierarchy.

“The insula is known to process emotional pain and frustration,” Zink says. “And along the same lines in these more status-conscious individuals, their amygdalas, which respond to negative emotions, were more active when they were simply viewing superior players.”

Whenever the subjects reacted correctly in their game, they were told that they had received a reward of one dollar; an incorrect response brought no money. Zink and her colleagues looked at the fMRI activity patterns after the dollar rewards in a brain region called the ventral striatum, which encodes information relating to the valuation of rewards or punishments. The researchers then compared this activation to what they observed when subjects moved up or down in status.

“It was indistinguishable,” says Zink. “And if this is where valuation is being processed, then it suggests that changes in status are as valuable to the brain as money-related changes.”

Similar results were reported in the same issue of Neuron by a group led by Keise Izuma of Japan’s National Institute for Physiological Sciences. In this study, 19 subjects were given monetary and reputational rewards while undergoing fRMI scans. The scans indicated that perceived boosts in reputation lit up virtually the same parts of the striatum, at similar intensity, as did monetary rewards.

“It’s an important finding—that social rewards and punishments, such as doing better or worse than rivals, engage the same brain regions as other rewards such as money or food,” says Chris Frith, a social-neuroscience expert at the Wellcome Trust Center for Neuroimaging at University College London.

Zink and her colleagues now plan to investigate how status-related circuits in the brain affect behavior and how they might be altered in disorders involving social deficits, such as depression and schizophrenia.