We may think about our closest friends and family using different parts of the brain than we do for the rest of our social circle, brain scans suggest.
Previous research by at the University of Oxford suggests that we have several tiers of friendship and that we spend most of our time and effort on those in our most intimate circle, which consists of around five people.
To see if these various layers of friendship are processed differently in the brain, at the Hebrew University of Jerusalem and his colleagues asked 26 people, with an average age of 26, to group their friends and family into five categories based on how close they felt to each person.
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On average, the participants included seven people in their most intimate group. 鈥淭hese are the people that are most important to us,鈥� says study co-author , also at the Hebrew University of Jerusalem.
The researchers then put the volunteers in a functional magnetic resonance imaging (fMRI) scanner. Inside, they were given the names of two friends and asked to judge which one they were closer to. The questions only ever involved people who the participants had listed in the same closeness category. Each person made 400 comparisons.
When they contemplated people in their most intimate group, the temporoparietal junction, middle temporal gyrus and dorsomedial prefrontal cortex were preferentially activated in the participants鈥� brains. These regions have been implicated in processing one鈥檚 own mental state and the mental state of others, says Dunbar, who is also a co-author of the study.
鈥淚 suspect that within the most intimate layer, friends become so emotionally close that they are more like family members,鈥� says Dunbar.
Questions about other friends led to more activity in the medial temporal lobe, retrosplenial cortex and ventromedial prefrontal cortex, which are all involved in memory processing.
This may reflect the fact that people said they based their judgements of their wider pool of friends more explicitly on memories of when they last interacted with that person, whereas judgements for each participant鈥檚 core group were more based on 鈥済ut feelings鈥�, says Arzy.
One weakness of the study is that the sample size was small. Dunbar says brain-imaging studies will always have this problem as the experiments are expensive to run. 鈥淭hat said, these findings fit very nicely into the pattern we already know exists,鈥� he says. 鈥淭he areas highlighted here are the ones that keep coming up in all studies of human social network sizes.鈥�
鈥淭here have been previous studies showing that different brain areas, especially the medial prefrontal cortex, are concerned with people who are similar to us versus others,鈥� says at University College London.
鈥淭hese researchers seem to have identified the 鈥榯heory of mind鈥� area of the brain in relation to friends,鈥� he says, regarding the areas that are active when we think of our close friends. Theory of mind refers to the ability to imagine what other people think and feel. 鈥淭hese are people whose mental states we are likely to want to think about,鈥� says Frith.
鈥淥ne interpretation is that we use our own mental models of feelings or thoughts for people similar to us, as we assume they will be thinking, behaving and feeling in similar ways to us, while we use different models 鈥� and different brain regions 鈥� to think about people dissimilar to us,鈥� says at Birkbeck, University of London.
Biorxiv
