The Brain’s Social Network

Keeping track of an extended network of Facebook friends, Twitter followers, and LinkedIn connections—not to mention those old-fashioned face-to-face relationships with neighbors and offi ce mates—can certainly feel like a mental challenge. But does having a larger, more complex social network actually change the brain? A new study with macaques suggests it can.

On page 697, a team led by neuroscientists Jérôme Sallet and Matthew Rushworth of the University of Oxford in the United Kingdom reports that housing monkeys in larger groups increased the amount of gray matter in several parts of the brain involved in social cognition. Previous research has implicated these regions in a variety of tasks, from interpreting facial expressions and gestures to predicting what other individuals intend to do. The researchers also found correlations between gray matter volume and a monkey’s dominance rank within its group, suggesting that beefi ng up neural circuitry in certain areas somehow promotes or enables social success.

The work may help to resolve a quandary raised by a handful of recent studies that have correlated variations in human brain anatomy with social network size, some experts say. Those studies couldn’t resolve which causes which. That is, do certain neuroanatomical features predispose people to building a large social network, or does having a large network cause certain brain regions to expand? “In humans, it’s difficult to show anything but a correlation,” says Lisa Barrett, a neuroscientist at Northeastern University in Boston. Although she has some reservations about the design of the experiment, Barrett says “this clearly shows changes in gray matter in response to experience.”

The new monkey study and the recent human research point to many of the same regions of the temporal and frontal lobes as potential contributors to social success. As researchers continue to refine this picture of the brain’s social circuitry, there may be implications for hypotheses about brain evolution, such as the social brain hypothesis, which posits that the increasing complexity of social networks in primates in general and humans in particular drove the expansion of the cerebral cortex, or at least certain parts of it.

In the Science study, Sallet and colleagues used magnetic resonance imaging (MRI) scans to look for variations in brain anatomy in 23 adult monkeys housed in groups of two to seven, plus one monkey housed alone. The monkeys were originally purchased in groups of different sizes and used in experiments by other researchers at Oxford, and their housing arrangements were determined by the requirements of these studies, Sallet says. Each monkey was scanned after it had been in a group of a constant size for 15 months on average. Before-and-after scans were not feasible due to the constraints of the various experiments, Sallet says.

Another important caveat, Barrett and others say, is that the monkeys were not assigned to groups at random. To keep the peace, caretakers may have tended to put sociable animals together in larger groups and kept the mean ones alone or in pairs, says Michael Tomasello, a psychologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. “Preexisting characteristics of the individuals in the groups might plausibly be what is being measured, not any effects of social experience,” Tomasello says. Sallet and colleagues insist that the housing assignments were made independently of any social characteristics of the animals and were as random as possible given their circumstances (although they note that one especially large group of 13 was split up to prevent fights).

Monkeys housed in larger groups had more gray matter in several regions of the temporal cortex, including the superior temporal sulcus (STS). Previous monkey studies have found that neurons in this region fire in response to faces and body movements and seem to track where other individuals are directing their attention. Gray matter may increase in the STS in response to a growing need to decode the expressions, gestures, and movements of more individuals as social network size increases, the researchers suggest.

Human MRI studies have also linked the STS to social network size. In a study published online 19 October in the Proceedings of the Royal Society B, Ryota Kanai and Geraint Rees of University College London and colleagues report that the right STS is one of several temporal lobe regions in which gray matter density correlates with the size of people’s Facebook networks.

Sallet and colleagues also found increased gray matter in the amygdala of monkeys housed in larger groups. This structure deep in the temporal lobes is best known as a hub of emotional regulation. The recent study by Kanai and Rees implicated the amygdala as well, as did a study published online by Barrett and colleagues in December in Nature Neuroscience. They found greater amygdala volume in people with larger and more complex social networks, as measured by questionnaires. Barrett suggests that the amygdala’s role in individuals and events that are worth paying attention to and remembering. For example, she says, your amygdala might help you scan a crowded party for friends and foes.

The Oxford researchers also found  more gray matter in the rostral prefrontal cortex in monkeys housed in larger groups. In humans, regions of prefrontal cortex have been linked to “theory of mind,” or the recognition that other individuals have beliefs and intentions that may differ from one’s own. Monkeys don’t appear to possess this talent, but more gray matter in this region did seem to confer a social advantage. In a subset of 11 male monkeys, the researchers found that those with more gray matter in the rostral prefrontal cortex tended to be more dominant.

The prefrontal cortex may also play a role in social success in humans. A study published in the 15 August issue of Neuro Image found that people with more gray matter in the ventromedial prefrontal cortex (and a few other regions of temporal and frontal cortex) performed better on tests requiring multiple layers of mind reading, such as determining from a short story whether Sam thought Harry intentionally gave him misleading directions. That study, led by Oxford evolutionary anthropologist Robin Dunbar, also found that people with more gray matter in these regions tend to have larger social networks, which the researchers assessed by asking participants to write down the initials of everyone with whom they’d had a social interaction in the past month, using their cell phones to jog their memories.

“With these several studies, you’re seeing a lot of the same sorts of brain areas implicated in correlations with social network size,” says Robert Barton, who studies brain evolution at Durham University in the United Kingdom. But the evolutionary implications aren’t yet clear, Barton says. “We don’t know what the relationship is between these studies that look at individual differences and the comparative studies that look at differences between species.” It remains to be seen, for example, whether the same regions that are expanded in individuals with large social networks are the same ones that are expanded in large-brained social primates compared with less social species, Barton says.

Dunbar sees it differently: “Variations among individuals provide the platform for selection to act on.” He views the recent findings as strong support for the social brain hypothesis, of which he is an ardent advocate. “The ability to form cooperative networks of individuals is what’s driving the evolution of big brain sizes,” Dunbar says. “The brain is a social tool.”

SOURCE : SCIENCE MAGAZINE VOL 334