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.
In an initial study, the researchers recruited 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 during social stress than did rural denizens. Previous studies have suggested that the amygdala, among other roles, evaluates social threats and is overactive in people with anxiety disorders. People who had 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.
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 reported last week in the journal Nature.
Meyer-Lindenberg suspects that city living actually causes these differences in brain activity, though he acknowledges that the data so far can’t prove that.
John Cacioppo, a social neuroscientist at the University of Chicago who was not involved in the study, said he thinks the idea that city living increases the risk of mental illness by altering the brain’s sensitivity to social stress is worth pursuing.
This article was produced by ScienceNOW, the daily online news service of the journal Science.