Social Rewards and Gray Matter: Why Are They Linked?

Whether it is savoring an ice cream cone or learning that you got a good deal on a recent purchase, two regions of your brain may move into gear.

Those regions, the nucleus accumbens and the orbitofrontal cortex, appear to process the rewarding information that comes from engaging in such pleasurable activities.

Now it looks as if these two brain regions may also be involved in processing the rewarding information that comes from engaging in social interactions. The reason? People who are emotionally warm, sentimental, and eager for people contact have been found to have significantly more gray matter in these two brain areas than do people who are more self-contained and aloof.

The study was headed by psychiatrist Graham Murray, M.D., a Medical Research Council clinician scientist at the University of Cambridge in England, and his colleagues. Results were published online on May 20 in the European Journal of Neuroscience.

As part of a large population study in Finland, some 2,000 young men were assessed on temperament with the Cloninger Temperament and Character Inventory, and out of these 2,000, 62 also underwent a structural MRI scan of their brains. Satisfactory MRI scan results were obtained for 41 of the 62. Murray and his team used the scan results for these 41 men, as well as the temperament information about them, for their study. Specifically, they looked to see whether there were any structural brain differences between subjects who had scored high on “reward dependence,” that is, on human attachment, human dependence, openness to warm communication, and sentimentality, and subjects who had scored low on this personality trait.

Density Differs

There were differences. Gray matter was significantly denser in the orbitofrontal cortex and the nucleus accumbens in the subjects who had scored high on reward dependence than in the subjects who had scored low on it.

These findings, the researchers noted, not only “provide strong evidence for a brain structural disposition to social interaction,” but suggest that the same two areas of the brain that are critical for processing reward information for some other types of activities are also critical for processing reward information for social interactions.

Still another interesting finding emerged from the study. Gray matter on the poles of the temporal lobes was significantly denser in the subjects who had scored high on reward dependence than in the subjects who had not. So these brain areas may also participate in the processing of reward information that stems from social interactions.

What Do Data Mean?

As tantalizing as such findings are, they raise provocative questions. For example, even though Murray and his colleagues found a link between a social personality and the density of gray matter in specific regions of the brain, can they really deduce from it that those particular brain areas process social rewards?

Only indirectly, he admitted to Psychiatric News. “It could be that social interaction itself drives brain growth in these regions rather than the other way around. Personally I think it's likely to be a two-way relationship; the more gray matter in these regions, the more pleasurable social interaction is, and the more you interact, the more growth there is in these regions. We'll need longitudinal studies of children or adolescents to confirm that.”

Since the same areas of the brain seem to be involved in processing various types of rewards, could one type of reward perhaps be substituted for another and thus satisfy people's need for rewards?

“It's a nice idea!” Murray replied. “But just because the same brain circuits process information about different rewards, it doesn't mean that your brain doesn't know the difference between sex and chocolate.”

Finally, what are the implications of these findings for psychiatrists?“ We studied individual differences in how much pleasure people take in social interaction and related it to brain structure,” Murray said.“ But of course there are psychiatric disorders where the patients may take little pleasure from social interaction—autism and schizophrenia, for example. Could it be that variance in brain structure in these conditions contributes to the lack of social pleasure experienced by the patients? I'm keen to now look at this issue in patient studies. If brain structure is important for this issue in patients, how can we address it? Biological treatments could be one solution, but actually it's possible that long-term social interventions may partly work by altering brain structure.”

The study was funded by the Academy of Finland, Sigrid Juselius Foundation, Stanley Medical Research Institute, Medical Research Council, and NARSAD.

An abstract of “The Brain Structural Disposition to Social Interaction” is posted at<www3.interscience.wiley.com/journal/122394540/abstract>.▪