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New brain network identified for social interactions

While showing monkeys videos of social interactions, scientists scanned their brains and tracked their gaze (red dot). C.J. Machado and D. Amaral

Understanding social interactions and what others are thinking is essential for relationships. Monkeys, like humans, can recognize and interpret social interactions. Primates use behaviors like playing, grooming, and fighting to determine social rank and gain allies in their group. But how the brain processes social interactions is unknown.

Drs. Julia Sliwa and Winrich A. Freiwald at Rockefeller University investigated the neural circuitry underling how social interactions are interpreted. They examined the brains of rhesus macaque monkeys using whole-brain functional magnetic resonance imaging (fMRI). The study was funded in part by NIH’s National Institute of Mental Health (NIMH) and National Eye Institute (NEI). Results were published in Science on May 19, 2017.

Monkeys were placed in an fMRI scanner while being shown one of eleven different videos. The videos were designed to help researchers distinguish which brain areas respond to social and non-social cues. The footage of social cues showed monkeys interacting with each other. Footage of non-social cues showed physical objects, such as monkey toys. One set of videos showed the monkeys and/or objects in a state of inactivity. Another showed objects interacting physically with one another, such as colliding into one another. Yet another set showed monkeys playing with toys or monkeys playing or grooming other monkeys.

Researchers tracked where the monkeys gazed while the videos played and then mapped the brain areas that were activated by the different objects, monkeys, actions, and interactions. The team found that two brain regions were activated when the monkeys watched either objects or animals interacting: the posterior portion of anterior intraparietal area and the premotor area 5. The activity in these two areas overlapped with the mirror neuron system, a brain circuit that responds the same way to others performing an action as though you’re performing the action yourself. These findings suggest a role for mirror neurons beyond understanding social interactions.

The researchers also found a network of brain regions that was activated by watching the social, but not the physical, interactions. This social interaction network included regions of the prefrontal and temporal cortices, the mirror neuron system, and subcortical systems known to be involved in reward and emotional processing. Large parts of this network responded only when the monkeys viewed social interactions. This exclusive social interaction network appears very similar to the brain regions humans use to understand the plans and actions of others.

“We uncovered a new high-level social cognition network in monkeys that may be a precursor to how humans evolved the ability to understand what one another may be thinking,” Freiwald explains.

—by Tianna Hicklin, Ph.D.