Monkeys Have a Specialized Brain Network for Sizing Up Others’ Actions

For many hours a day they pluck dirt, debris and bugs from each other’s fur. Between grooming sessions they travel in troops to search for food. When ignored by mom, they throw tantrums; when not ignored by zoo-goers, they throw feces.

Through these behaviors, monkeys demonstrate they understand the meaning of social interactions with other monkeys. They recognize when their peers are grooming one another and infer social rank from seeing such actions within their group.

But it has long been unclear how the brains of our close evolutionary relatives actually process what they observe of these social situations. New findings published Thursday in Science offer a clue. A team of researchers from The Rockefeller University have identified a network in the monkey brain dedicated exclusively to analyzing social interactions. And they believe this network could be akin to human brains’ social circuitry. 

In the new work—led by Winrich Freiwald, an associate professor of neurosciences and behavior—four rhesus macaques viewed videos of various social and physical interactions while undergoing functional magnetic resonance imaging. (Monkeys love watching TV, so they paid attention.) They were shown clips of monkeys interacting, as well as performing tasks on their own. They also watched videos of various physical interactions among inanimate objects.

Videos of monkeys doing anything at all activated brain regions involved in face and body recognition. Videos of objects stimulated brain regions that help with object identification. Varying forms of interaction—whether between monkeys or objects—triggered activity in two other brain regions, neither of which were stimulated by videos that did not depict such interactions.

The most intriguing finding was the discovery of a large brain network in monkeys that was only activated by viewing social interactions. Watching videos of other monkeys interacting lit up what the authors call the “social-interaction network,” which includes parts of the medial prefrontal cortex—a region behind the forehead—and its slightly more deeply placed neighbor, the anterior cingulate cortex. The prefrontal cortex is generally involved in higher complex cognition; in humans it is linked to personality traits.

If the new finding leads to the eventual pinpointing of similar neurocircuitry in humans, it may provide insight into so-called “theory of mind.” Developed in 1978 by psychologists David Premack and Guy Woodruff, the concept refers to our species’s ability to understand that other people have their own thoughts, intentions and desires—in other words, we are able to speculate about the mental motivations behind another’s behavior.

Humans have many degrees of this awareness. In his book, Human: The Science behind What Makes You Unique, University of California, Santa Barbara, psychology professor Michael Gazzaniga offers an interesting example of four degrees of awareness: “I know that you know that I know that you want me to go to Paris….” The savvier humans among us can keep going: “…and you know I can’t and I know that you know I can’t…” Although it is still a matter of debate, a number of researchers believe chimps have some degree of theory of mind. Studies have shown the grinning primates are aware what other chimps do and do not see.

These new reports reinforce the evidence for a rudimentary theory of mind in monkeys. too.  “Previously, monkeys were considered to have only a limited theory of mind—or even none at all,” explains Julia Sliwa, a Rockefeller postdoctoral researcher and lead author on the paper. “By showing that areas of the social-interaction network are closely homologous to the theory of mind network in humans, we suggest that our human theory of mind skill might have evolved from the ability of our monkey kin to read social interactions.”

What’s more, the degree of specificity uncovered in Sliwa and Freiwald’s work—in which a network of brain regions is so clearly linked to a particular mental function—was not seen with any other video stimulus. Brain circuitry is typically full of redundancy, and brain regions are often engaged in countless neural processes. Yet understanding the interactions of others, it seems, operates through more dedicated machinery.

Nancy Kanwisher, a Massachusetts Institute of Technology neuroscience professor whose own research has found that social observations occur in a different brain region, the temporal lobe, commented: “Sliwa and Freiwald demonstrate an extreme degree of functional specificity [related to social observation] in a set of regions in the frontal lobe.” But the study left her with a set of unanswered questions: “Are these areas activated by just the presence of a social interaction or does the nature of that interaction matter (for example, friendly or hostile)? Would the same regions be engaged if the monkeys simply hear a social interaction in a series of monkey calls? Is the same region engaged when monkeys are themselves socially interacting with other animals?”

Hunting, foraging, cooking and tool use increased social exchanges among our primate and hominid ancestors, and these practices are thought to have contributed to our big human brains. Many scientists have speculated that our ability to understand social situations and communicate is in part due to our so-called “mirror neuron system.” Mirror neurons are brain cells that fire when someone performs a particular activity as well as when that person observes another performing the same activity. The hypothesis holds that this system is essential to understanding the intentions of others—and to imitating them to learn new skills such as talking or using one stone to shape another into a spear tip.

The authors of the new research witnessed activity in the mirror neuron system in response to watching social interactions. They saw, however, a similar occurrence in response to videos of two monkey toys interacting, suggesting the circuitry might not be exclusive to social engagement, and may be involved in processing interactions of any kind. The importance of mirror neurons—once touted by neuroscientist Vilayanur Ramachandran as “the neurons that shaped civilization”—is at the moment unclear. As psychologist Christian Jarrett suggested a few years back, they are one of the most hyped concepts in neuroscience and we have a long way to go to understand their function in humans.

In future research Sliwa and Freiwald hope to explore how the various centers comprising the social-interaction network operate together, and what each of these brain regions contributes to understanding social encounters. For now, what does seem undeniable is that having a sense of the intentions behind the behavior of others—a sense of what they are thinking and of their motivations, and perhaps even empathizing with them—was probably critical to the evolutionary march from monkey to mankind. 

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