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Almost all primates live in groups with an observable and definable social hierarchy, and humans aren’t an exception. We may overlook it in our day to day lives, but every so often it becomes evident that we interact best when we understand the pecking order. The social brain hpyothesis argues that the cognitive demands of living in complexly bonded social groups selected for increases in executive brain. Two new papers in the current issue of the journal Neuron investigate this phenomenon by looking at the activity in specific regions of the brain, like the striatum, which reflects a common signal of reward in both the economic and social domains.

The research was conducted by researchers at the National Institute of Mental Health. fMRI was used to monitor the activity of the brain of 72 participants who were playing an interactive computer game for money. From this press release,

“They were assigned a status that they were told was based on their playing skill. In fact, the game outcomes were predetermined and the other “players” simulated by computer… Participants intermittently saw pictures and scores of an inferior and a superior “player” they thought were simultaneously playing in other rooms.

Although they knew the perceived players’ scores would not affect their own outcomes or reward –and were instructed to ignore them – participants’ brain activity and behavior were highly influenced by their position in the implied hierarchy.”

Several interesting observations where made when the researchers compiled all the fMRI data. For example, the striatum showed activity in a situation where a rise or fall in rank was a possibility as much as it did to the monetary reward. The stratium is a critical part of the brain where dopamine is regulated, and a previous study investigated the genetics of dopamine and the linkage it had to agressive social behaviors. Overall, this observation implies that social status is highly valued in our subconscious minds, even as much as money. The press is gorging itself on this sound bite, they just love it when something as complex as social hierarchy and brain functions are reduced to something as simple as gaining money.

Another interesting observation involved subjects that were presented a ‘superior competitor’ in the game. When that happened, it triggered activity in,

“an area near the front of the brain that appears to size people up – making interpersonal judgments and assessing social status. A circuit involving the mid-front part of the brain that processes the intentions and motives of others and emotion processing areas deep in the brain activated when the hierarchy became unstable, allowing for upward and downward mobility.”

Also when the player preformed better than any superior competitors, another area towards the front of the brain which controls planning was activated. In contrast, when the player did worse than an inferior competitor different activity was shown in centers of the brain associated with emotional pain, frustration, and stress. Pretty cool.

One last cool results was associated with players who were at the top of the hierarchy, not only did they say they had a more positive experience but more activity was associated in the emotional pain circuitry when they perceived an outcome that could drop them down in rank.

These results kinda thwart any Utopian anarchists out there. This data shows that our brain’s hierarchical consciousness seems to be ingrained in the human brain, so much so that there are distinct circuits activated by concerns over social rank.

Coinciding with these two studies is this short little paper in the latest Nature investigating the genetics and expression of Neuropeptide Y (NPY). Neuropeptide Y is just that a peptide that functions as a neurotransmitter, it is involved in regulation of energy balance, memory and learning. In mice and monkeys, it has been observed that stress stimulates the expression of this gene product. That’s not very surprising because Neuropeptide Y alters adrenergic receptors, the ones that bind adrenaline and noradrenaline, two stress hormones. As seen in the above results, stress is an important behavioral response in social hierarchy.

Anyways the new Nature study finds that,

“haplotype-driven NPY expression predicts brain responses to emotional and stress challenges and also inversely correlates with trait anxiety… Lower haplotype-driven NPY expression predicted higher emotion-induced activation of the amygdala, as well as diminished resiliency as assessed by pain/stress-induced activations of endogenous opioid neurotransmission in various brain regions. A single nucleotide polymorphism (SNP rs16147) located in the promoter region alters NPY expression in vitro and seems to account for more than half of the variation in expression in vivo. These convergent findings are consistent with the function of NPY as an anxiolytic peptide and help to explain inter-individual variation in resiliency to stress…”

Like I said above, I’m pretty sure we’ve all had experiences where we felt threatened by inferior individuals and we’ve all had the glee when we thought we were at the top of our game. The fMRI study has shown what areas of the brain are active in these situations, and the genetics of NPY indicate how the allelic differences of NPY affect stress responses. All in all, I’m impressed with these trio of papers. They illuminate a lot about how we subconsciously process social hierarchy, which is a very human thing.

    Zhou, Z., Zhu, G., Hariri, A.R., Enoch, M., Scott, D., Sinha, R., Virkkunen, M., Mash, D.C., Lipsky, R.H., Hu, X., Hodgkinson, C.A., Xu, K., Buzas, B., Yuan, Q., Shen, P., Ferrell, R.E., Manuck, S.B., Brown, S.M., Hauger, R.L., Stohler, C.S., Zubieta, J., Goldman, D. (2008). Genetic variation in human NPY expression affects stress response and emotion. Nature, 452(7190), 997-1001. DOI: 10.1038/nature06858
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