Stump-tailed macaques (Macaca arctoides) socialising.
Credit: Wikimedia
SYDNEY: By comparing the brains of monkeys living in large groups to those living in smaller groups, scientists have found that the brain can change shape to accommodate social network size.
The finding, published in Science today, reveals that there are still opportunities for the brain to change, even during adulthood. It also suggests that a complex social environment puts pressure on improving brain plasticity - our brain's ability to efficiently adapt to changes.
The researchers found that areas of the brain known to process social information such as facial expressions were larger in monkeys who lived in larger groups, and vice versa. There was also more activity between neurons in the monkeys who had larger social networks. The results answer the chicken-or-the-egg question: which came first, the large social network or the increase in grey matter?
"Living in larger groups caused increases in grey matter to several brain regions in the temporal and prefrontal cortexes," said lead author Jerome Sallet, an experimental psychologist at Britain's University of Oxford. "The correlation we observed between network size and grey matter volumes was particularly interesting because it was more than a correlation. Indeed, it revealed a causal effect."
Monkey business
Previous studies have shown that an individual's preferred social network size is reflected in the amount of grey matter in parts of their brain. Whether changing an animal's social network size actually changed the shape of parts of the brain, however, hasn't been seen until now.
Social comparisons between 23 macaque monkeys were made using Magnetic resonance imaging (MRI) machines. Before-and-after scans showed brain differences between the monkeys made to live with one friend and the monkeys living with a bunch of mates.
Like humans, monkeys have differing levels of sociability. Sallet and colleagues were careful to make sure that the group placings were as random as possible. "The monkeys' social environment in the research facility where the animals are housed is controlled by researchers. It did not depend on monkey's sociability," said Sallet.
Better socialisers
Monkeys living in larger social groups were found to have more complex information in the part of their brain that deals with social information.
And the increase in the amount of grey matter volumes in some brain regions with social network size may also imply a particular animal’s success in a social context – animals in more social groups may become better at interpreting social cues.
The results show us that not just new skills (such as a monkey taught to use a tool, or a human learning how to juggle) induce brain plasticity, but also the social environment we interact with, said Sallet.
"This socially driven plasticity seemingly affects only very specific areas of the cortex, rather than the brain as a whole," commented Jason Mattingley, a cognitive neuroscientist from the University of Queensland in Brisbane, who was not involved with the study. "Thus, areas thought to play a role in such social processes as the ability to recognise facial expressions and vocalisations, or to infer others' intentions, are altered with increasing social network size."
One limitation, however, was how complex social interaction could be in small groups, he added. "One could readily imagine an extremely rich and harmonious social grouping of just three monkeys, and a highly fractious and isolating grouping of eight or nine monkeys in another [grouping]."
"In the future it will be important to consider precisely what kinds of social interactions drive changes in brain structure and function."
