ARTICLE IN BRIEF
An MRI study among college students correlated the density of grey matter in brain areas previously implicated in social perception and associative memory with the size of both online and real-world social networks.
You know what they say about Facebook friends, right? The more you have, the bigger the size of your amygdala, right superior temporal sulcus, left middle temporal gyrus, and entorhinal cortex.
At least, that is the finding of an MRI study among college students published by British researchers, who correlated the density of grey matter in brain areas previously implicated in social perception and associative memory with the size of both online and real-world social networks. The study was published online in advance of print on Oct. 19 by the Proceedings of the Royal Society B: Biological Sciences.
While questioning some aspects of the study's design and interpretation, two neurologists familiar with the paper told Neurology Today that its findings highlight new insights into understanding the neural underpinning of social behaviors.
“Social behavior is one of the last frontiers that neurologists and neuroscientists have turned their attention to,” said Bradford C. Dickerson, MD, associate professor of neurology at Harvard Medical School. “We're just getting a handle on the brain structures related to social cognition.”
Dr. Dickerson, who will be among the presenters in a course on the subject at the AAN annual meeting in April, co-authored a paper published in February in Nature Neuroscience showing that the volume of the amygdala as measured on MRI correlates with the size and complexity of real-world social networks in adult humans. That study left open the question of whether similar findings would hold for online relationships.
“What's exciting is that this new study finds a somewhat similar relationship with online networks,” Dr Dickerson said. “People with relatively larger amygdalae have more friends on Facebook.”
GREY MATTER=SOCIAL NETWORK
Hypothesizing that memory capacity would also be important in the maintenance of a large number of social ties online, the investigators of the current study measured the grey-matter density of the medial temporal lobe, which has been linked to the encoding and retrieval of face–name pairs.
Ryota Kanai, PhD, first author of the study and research associate at University College London's Institute of Cognitive Neuroscience, and colleagues, first studied 125 healthy volunteers recruited from University College London. They found a significant positive correlation between the volunteer's number of Facebook friends and the grey matter density for the left middle temporal gyrus (R=0.354, p <0.05), right posterior temporal sulcus (R=0.348, p<0.05), and the right entorhinal cortex (R=0.348, p<0.05).
The researchers asked these same participants about the size of their real-world social network, as measured by nine variables, including how many people they would invite to a party, how many they would ask for a favor and expect to have it granted, and how many friends are listed in their phonebook. Using an index of these nine variables, they found it correlated with the grey matter volume of the amygdala in a sample of 58 individuals.
A second experiment replicated the findings of the first experiment in a new sample of 40 individuals. Two additional experiments described subgroup analyses.
Although the grey matter density of the amygdala correlated with both online and real-world social network size, the investigators reported that the density of the superior temporal sulcus, left middle temporal gyrus, and entorhinal cortex were associated only with online social network size.
“The reason for this dissociation is unclear,” said Dr. Kanai. “We speculated that building and maintenance of a large-scale social network on Facebook impose different kinds of social and cognitive demands compared to real-world social networks.”
The right superior temporal sulcus and left middle temporal gyrus are both known to be activated when a person observes social signals, such as the gaze of other people, Dr. Kanai pointed out. “The entorhinal cortex is known to mediate associative memory, and we speculate that this area may contribute to learning associations between faces and names.”
A Canadian neurologist who studies the effects of frontal lobe damage on social interactions said it is unlikely that only the regions reported in the study regulate social behaviors and perceptions.
“I'm not saying the areas they studied don't have any plausible link to social networks,” said Lesley Fellows, MD, PhD, associate professor and interim chair of the department of neurology and neurosurgery at McGill University School of Medicine in Montreal. “It's just that a lot of areas could have a social link. They've identified regions that have been studied in other contexts and have been associated with social cognition. But it's a bit of a leap to understand how an area that may be important in emotion is underlying social network size. It's a big jump.”
Still, Dr. Fellows said, the study highlights how focal injuries, whether due to trauma or neurodegenerative diseases, can cause the kinds of social disabilities that clinical neurologists see in their practice on a routine basis.
“As neurologists, our observations of social behavior have been rather general,” she said. “We might soon be in a place where we can start to analyze those problems from a more theoretical point of view. Do they have an ability to perceive social cues? Maybe the person isn't understanding the emotional tone of those around her.”
The study left unanswered the question of which way the causal arrow flows —whether larger social networks cause changes in brain structure, or the other way around — but a Nov. 4 study in Science pointed to an answer, at least in macaques. A team of researchers from the University of Oxford and Mount Sinai School of Medicine neuroimaged 23 monkeys that had been placed in social groups of different sizes. Those placed in larger groups were found to have increases in grey matter in the mid-superior temporal sulcus, and rostral prefrontal cortex. Not only structure, but the function of the monkeys' brains changed as well, with increased coupling of activity in the frontal and temporal cortex.
The macaques study “provides more of a clue about causality, that is, that having larger networks may cause the brain changes, rather than vice versa.”
As knowledge of how specific neural defects can affect social interactions, neurologists might be better able to explain particular behaviors to patients and their families, and eventually to treat them, Dr. Fellows said.
“A neurologist would never start to propose treatments based only on the finding that a patient can't walk,” she said. “They would want to know more and test more to find out why. Similarly, we might be able to move beyond a general statement like ‘there's some impulsivity,’ to learn what exactly is underlying it to find what is remediable or can be worked around.”
MORE EVIDENCE OF SOCIAL SIGNALS AND BRAIN SIZE
Ryota Kanai, PhD, first author of the study and research associate at University College London's Institute of Cognitive Neuroscience, cited a 2000 paper in Trends in Cognitive Sciences as evidence of a “large body of evidence that indicates that processing of basic social signals such as gaze and body movements of others is mediated by the posterior superior temporal sulcus.”
Studies by Ralph Adolphs, PhD, of the California Institute of Technology have also demonstrated that the amygdala is another central structure in social cognition, as damage to it impairs the ability to recognize emotional facial expressions.
The paper cited a variety of research showing that high-level social cognition is associated with activity in brain regions including the temporoparietal junction, medial prefrontal cortex and precuneus.
“While the precise contribution of each of those regions to social cognition is yet to be established,” the study stated, “they are consistently activated in a broad range of tasks embedded in social contexts. Since the ability to understand other people's intention is a key to successful social interactions, the size of online social networks might be reflected in the structure of these specific social brain regions.”