Living in a city, or growing up in one, is associated with heightened brain sensitivity to social stress

Without fanfare or formal announcement, human civilisation has passed a momentous milestone. For the first time, more of us now live in cities than in rural communities.

The benefits are numerous: more jobs, better access to educational and health services, more potential friends, and on the list goes. Yet city living has its dark side. Crime, deprivation and inequality are usually higher and so are rates of mental illness, including more anxiety, depression and schizophrenia. A new paper has made one of the first attempts to understand the neural effects mediating this link between urban life and mental strife.

Across several studies, Florian Lederbogen and his team (at the University of Heidelberg and Douglas Mental Health Institute) placed volunteers in a brain scanner and engaged them in a task designed to create social stress. Participants had to answer tricky arithmetic problems as fast as possible, whilst receiving negative, critical feedback from the researchers and others, via headphones or a video display.

The crucial question was whether the effect of this task on the brain would vary as a consequence of whether each participant currently lived in a city, a town or the countryside, and also where they grew up. Some participants were recruited via local newspaper advertisements, but unfortunately the majority were university undergrads.

There were two striking results. The stressful task triggered more amygdala activity in city-dwellers than townies, and more amygdala activity in townies compared with rural folk. Second, the task aroused more activity in the anterior cingulate cortex (ACC) of those participants raised in a more urban environment, regardless of where they currently lived.

These associations were highly specific – no other brain areas were differentially activated according to urban/rural status. A raft of demographic variables including household size, income, personality, and self-reported health, played no part in the results. Also, demanding tasks (memory and face recognition), with the social stress element removed, did not lead to differential activity in the amygdala or ACC according to participants' current urban/rural status or upbringing.

The amygdala, often likened to an almond, is part of the brain's limbic system and is involved in emotional processing. That this region was apparently sensitised to social stress in the city dwellers "can plausibly be related to epidemiological observations," the researchers said, such as the higher rates of anxiety disorder in cities. The ACC, meanwhile, is involved in stress regulation, among other things. It's also been called the "oh shit" centre, for its function in looking out for unexpected outcomes.

The researchers pointed out that schizophrenia, which is more common in cities, is associated with reduced ACC volume and connectivity abnormalities with the amygdala. Schizophrenia usually emerges in adolescence so it's notable that the city-link with ACC activity was based on participants' upbringing location rather than their current dwelling location. A follow-up study by Lederbogen's team further established that an urban upbringing was associated with reduced connectivity between the ACC and amygdala.

There's no question these are interesting results but they are crude. For example, we don't know what aspects of city living led to sensitised amygdala activation, or what aspect of an urban upbringing is associated with ACC function. Moreover, cities vary hugely and these results are based specifically on German urban and rural environments – perhaps the results would be different if the study were replicated on a different continent (although the researchers predict their results would be even larger in countries where the rural/urban discrepancies are greater).

We also don't know what it means to have an amygdala that's more aroused by social stress, or whether that sensitivity is permanent or not. For some broader context, consider that people with larger, more complex social networks have been shown to have bigger amygdalae.

Perhaps – and this is pure speculation – city living is associated with having a more complex social life, and therefore an enlarged, more sensitive amygdala. By this account, the amygdala finding in the current study has provided evidence of adaptive neural plasticity, just as much as it may have uncovered a pathological vulnerability.

Consistent with this interpretation, it's notable that the study participants were all psychologically healthy (potential volunteers were excluded if they had past or present mental health problems). The cross-sectional nature of the current research also means we don't know if city living causes the observed brain differences, or if people with certain kinds of brain are drawn to urban versus rural environments.

A final short-coming is that the brain differences associated with urban life did not correlate with cortisol levels triggered by the stressful tasks. Cortisol is a biological marker of stress, so if heightened amygdala and ACC activity were indicative of sensitivity to stress you'd expect participants with extra activity in these brain regions to have shown corresponding increases in cortisol.

Lederbogen and his colleagues said their study had shown "neural effects of urban upbringing and habituation on social stress processing in humans" and was a first step in what they hope will be "a new empirical approach for integrating social sciences, neurosciences and public policy to respond to the major health challenges of urbanisation."

References

F Lederbogen, P Kirsch, L Haddad, F Streit, H Tost, and six others (2011). City living and urban upbringing affect neural social stress processing in humans. Nature : 10.1038/nature10190