Searching for the social brain

Humans exhibit complex social behavior. Understanding the brain function underlying this behavior would help to explain how it goes wrong in social disorders. Evidence shows that the neurotransmitter serotonin is involved in regulating emotional and social behaviors in humans and other primates, particularly negative traits such as anxiety and aggression. It has also been implicated in the formation of social behavioral traits such as social affiliation, dominance and openness.

Despite this, no direct link has been made between such behaviors and the action of serotonin in specific brain regions. Most work has focused on the primitive regions of the brain, such as the amygdala, which is involved in negative emotions. But there is also some evidence for involvement of the cerebral cortex, the outer layer of the brain responsible for higher cognitive function.

A team led by Hirotaka Onoe at the RIKEN Center for Molecular Imaging Science, Japan has studied the brain and behaviors of common marmosets to show that specific regions of the midline cortex, close to the center-line of the brain, are indeed crucial in the formation and regulation of social behavior traits¹. Onoe says it is the first study of non-invasive brain functional imaging in conscious marmosets that has been analyzed in combination with measurements of the animals’ social behavior.

The researchers started by observing and classifying behavioral traits displayed by the male marmosets when they encountered an unfamiliar male. They identified three traits: aggressiveness, anxiety and sociability (Fig. 1). The combination of these traits expressed in an individual can be considered equivalent to ‘personality’ in humans. The team then moved on to see how serotonin function in the brain related to these traits.

To do this, they measured levels of the serotonin transporter protein (SERT) in the brains of the same marmosets. This protein transports serotonin back into cells after it has been released from neurons. High levels of SERT correspond to high levels of serotonergic nerve terminals in the same region. Measurements were made by injecting a radioactive compound that binds to SERT into the marmosets and detecting the radioactivity in their brains by positron emission tomography (PET). Higher levels of SERT in specific regions were revealed by higher levels of radioactivity.

The researchers found that SERT levels in specific regions correlated with the social traits seen in the marmosets (Fig. 2). Low levels of SERT correlated with lower unfriendliness and higher aggression in the posterior cingulate cortex (PCC). In the anterior cingulate cortex (ACC) and the lateral prefrontal cortex (LPFC), low levels of SERT correlated with higher anxiety. These results indicate that the cognitive basis of these traits is localized to these specific subregions of the midline cortex.

The researchers also measured activity in these regions in different social situations. To do this, they again used PET, but this time they used a radioactive compound to measure the level of metabolism across the brain, corresponding to the amount of neural activity. They found that activity in the PCC, but not the ACC, varied according to whether a marmoset remained alone or was put with an unfamiliar male. Using the same approach, they also showed that functional connections of the PCC and ACC with other regions of the brain were altered by different social situations. The differences in activity show that these regions are central to the cognitive basis of social traits.