Friday, July 15, 2011

Stress and the City

The number of the world's people living in cities has increased from 30% to 50% since 1950, and by 2050 is projected to be ~70%. Many experiments, done on insects, rodents, primates, and humans, have shown that extremes of either social isolation or crowding can have harmful effects. Lederbogen et al. have now used functional magnetic resonance imaging to examine specific human brain structures that are affected by urban living, comparing people living in rural areas, towns with more than 10,000 residents, and cities with more than 100,000 residents. They replicated their findings in several separate samples, used two different stress-inducing tasks, and demonstrated that there were no effects of urbanicity on brain activation when participants performed a non-stressful cognitive task. Stress increased participants' heart rate, blood pressure, saliva cortisol, and activity in the amygdala, with city dwellers showing the largest increases. They found that participants' age, education, income, marital and family status, as well as aspects of their health, mood, personality and the amount of social support they had did not significantly influence the effects of urbanicity. Thus they suggest that living in a city environment changes brain response during a social stressor by a distinct, but mysterious, mechanism. Here is the abstract:
More than half of the world’s population now lives in cities, making the creation of a healthy urban environment a major policy priority. Cities have both health risks and benefits1, but mental health is negatively affected: mood and anxiety disorders are more prevalent in city dwellers and the incidence of schizophrenia is strongly increased in people born and raised in cities. Although these findings have been widely attributed to the urban social environment, the neural processes that could mediate such associations are unknown. Here we show, using functional magnetic resonance imaging in three independent experiments, that urban upbringing and city living have dissociable impacts on social evaluative stress processing in humans. Current city living was associated with increased amygdala activity, whereas urban upbringing affected the perigenual anterior cingulate cortex, a key region for regulation of amygdala activity, negative affect and stress. These findings were regionally and behaviourally specific, as no other brain structures were affected and no urbanicity effect was seen during control experiments invoking cognitive processing without stress. Our results identify distinct neural mechanisms for an established environmental risk factor, link the urban environment for the first time to social stress processing, suggest that brain regions differ in vulnerability to this risk factor across the lifespan, and indicate that experimental interrogation of epidemiological associations is a promising strategy in social neuroscience.

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