City dwellers worldwide enjoy several advantages over their rural compatriots, including, on average, better job prospects and better access to food and health care (not to mention nightlife). At the same time, city living can be stressful, and studies have found that mental health problems, such as schizophrenia, depression, and anxiety disorders, are more common in urbanites. Now, researchers have taken a crack at understanding this connection by looking for differences in how the brains of people from urban and rural environments react to certain kinds of stress.
Psychiatrist Andreas Meyer-Lindenberg and collaborators at the Central Institute of Mental Health and the University of Heidelberg Medical Faculty in Mannheim, Germany, have previously used brain-imaging methods to search for abnormalities in the brains of people with genetic risk factors for mental illness. In the new study, Meyer-Lindenberg says, the group wanted to apply the same approach to environmental risk factors, which can be even more powerful than genetic factors. "Urbanicity ... has a much higher associated risk than any gene," he says. "The idea was to take people with that risk factor and see if there's anything different in their brains."
In an initial study, the researchers placed ads in local newspapers to recruit 32 healthy German adults from cities (with more than 100,000 inhabitants), towns (with more than 10,000 inhabitants), or rural areas. Inside a functional magnetic resonance imaging (fMRI) scanner, which monitors brain activity, a subject worked on difficult arithmetic problems while a fake "performance monitor" indicated a dismal success rate compared with other subjects. Then the researchers ramped up the stress. Meyer-Lindenberg explains: "We would call them in between runs and say, 'We notice this seems to be very hard for you, but please understand these experiments are very expensive, so if you could just try to at least be above the bottom quarter, we'd really appreciate it.' " Measurements of the subjects' heart rates, blood pressure, and stress hormone levels indicated that the stress was indeed getting to them.
The fMRI scans showed that volunteers who currently lived in a city exhibited greater activation in the amygdala than did rural denizens during social stress. Previous studies have suggested that the amygdala, among other roles, evaluates social threats and is overactive in people with anxiety disorders. People who'd been raised in a city, regardless of their current home, showed a different pattern: more activation in the perigenual anterior cingulate cortex (pACC), another region thought to be involved in emotion and social processing, and implicated in some studies on schizophrenia. To Meyer-Lindenberg, the findings suggest that the pACC may be susceptible to lasting effects from the environment early in life, whereas the amygdala is more sensitive to one's current situation.
Two follow-up experiments with new groups of volunteers and different tasks inside the scanner reinforced these findings, suggesting that the bigger the city someone currently lives in, the more amygdala activity he or she exhibits during social stress. And the more time spent in a city as a child, the more the pACC revs up, the team reports online today in Nature.
Meyer-Lindenberg suspects that city living actually causes these differences in brain activity. He acknowledges that the data so far can't prove that, but he notes that his team found no correlations between brain activity and several measures of subjects' mood and personality, or with demographic factors, such as education and income.
"I like the idea of going from population studies into the lab to test mechanisms," says John Cacioppo, a social neuroscientist at the University of Chicago in Illinois. He's not yet convinced that the German team has ruled out alternative explanations for their findings, but he thinks the idea that city living increases the risk of mental illness by altering the brain's sensitivity to social stress is worth following up on. Cacioppo says he'd like to see future studies delve into specifically what kinds of social stress in urban environments are potentially harmful. Being mocked by a scientist isn't a daily occurrence for many city dwellers, he notes, but feelings of loneliness and exclusion (or on the other hand, overcrowding), or perceptions of discrimination, powerlessness, or low status, might be.