A brain region sensitive to social rank.

Our social environment is hierarchical and we can all guess roughly where we and others lie in this hierarchy. It rarely needs to be stated explicitly – a boss does not need to remind his employee that he’s the boss every day. Yet hierarchy acts in the background, like an invisible hand, modifying almost each of our interactions. It makes us more or less polite, familiar, or audacious with those people for whom each attitude is more or less appropriate.

A fight between two macaques. This is one of the rare events that has a chance to change the dominance relation between macaques. Other behaviors such as threat displays, starring and grunts are also used in a different fashion by subordinate and dominant monkeys.Photo by einalem released under the Creative Commons license.

 

Somewhere in our brain, there might be a neural network gathering information about the social hierarchy in order to adjust our behaviors. For now, we ignore how and where this would happen. Research has shown that humans as well as other animals, like monkeys, have detailed knowledge of the social hierarchy. Humans use that knowledge every day to interact with others. Similarly in monkeys and apes, different types of grunts are used by dominant and sub-ordinate to signal their status and resolve conflicts1,2. This does not mean that the same brain mechanisms are at play in the two species; but it remains a plausible hypothesis.

Tackling the question of how the brain represents social rank confronts us with a big issue: how would we tell that some region of the brain deals specifically with social hierarchy? One popular approach would be to put subjects in a fMRI scanner and see what parts of the brain are activated when they view pictures of people with different status. The problem with this is that some parts of the brain might deal with hierarchies in general – including the non-social ones. The branches of a tree, the letters of the alphabet and the temporal sequence of a TV schedule are all examples of hierarchies that are not particularly social. Thus running this experiment might lead us to find all the brain areas that participate in classifying anything, not just the brain areas that participate in classifying people. Yet some scientists think that social interactions were important enough in the evolutionary history of humans that we might have evolved specialized mechanisms and specialized brain regions to deal with people. The hypothesis is plausible and needs to be considered – the question is how do we identify those areas?

Localization of the human amygdala.Photo by Amber Rieder, Jenna Traynor, Geoffrey B Hall, released under the Creative Commons license.

 

A team of scientists led by Emrah Duzel has recently addressed part of this problem by presenting human subjects with two kinds of hierarchies: social and non-social3. Participants entered a brain scanner and were presented with pairs of photos that represented galaxies or people. They were told which galaxy, or which person, was higher ranking than the other. For galaxies, they were told that some galaxies were better than the others because they contained more minerals. For people, they were told that some individuals were ranked higher in a company. The scientists then looked at the brain activities of participants to see if some areas were more activated by higher ranking people and galaxies. A bunch of brain areas were activated, either by the galaxies, people, or both. However, the amygdala, a region located deeply in the brain, came out as the only region where the brain activity was correlated for the rank of people but not for galaxies.

This study makes an important step forward in separating and identifying the brain areas that are activated specifically by social hierarchy compared to other types of hierarchies. The amygdala might and likely does participate in non-social behaviors as well, but this study shows that the effect of rank on neural activity seems specific to the social domain when compared with another non-social domain.

References

1. Byrne RW, Bates LA. (2012) Primate social cognition: uniquely primate, uniquely social, or just unique? Neuron.

2. Cheney DL, Seyfarth RM, Silk JB (1995) The role of grunts in reconciling opponents and facilitating interactions among adult female baboons. Animal Behaviour.

3. Duzel E, Melo HL, Kumaran Dharshan (2012) The Emergence and Representation of Knowledge about Social and Nonsocial Hierarchies. Neuron.