June 23, 2011 -- The brains of people who live in cities react more strongly to stress than those who live in small towns and rural areas, a new study shows.
The study is published in the journal Nature. It may help explain why mood disorders like depression and mental illnesses like schizophrenia are more common in city dwellers than in those living in less densely populated areas.
Researchers in Germany and Canada recruited healthy adults who lived in large cities, moderately sized towns, or smaller, rural communities. Scientists recorded their brain activity as they tried to solve difficult math problems while being criticized for their poor skills. It's a test that creates social stress as people struggle, but fail, to prove their mental abilities.
As they were stressed, people who were currently living in cities had more activity in an almond-shaped area of the brain called the amygdala than those who lived in towns or rural areas.
The amygdala plays important roles in fear, emotional processing, and self-protection. It has been linked to scores of mental illnesses including posttraumatic stress disorder, depression, anxiety, autism, and phobias.
People who grew up in cities also had an interesting response to the stress. Even if they were no longer living in an urban area, their brains showed higher activity in a region called the anterior cingulate cortex, which helps to regulate the amygdala, suggesting that the early-life environment helps to shape the brain's stress response in important ways.
"It's a stronger response of those areas that typically regulate fear and emotion," says study researcher Jens C. Pruessner, PhD, director of the Douglas Mental Health Institute at McGill University in Montreal. And he says it suggests "that living in big cities with many, many people surrounding you sensitizes you to respond more strongly to stress."
How Cities Tax the Brain
Both the researchers and independent experts point out that the study can't prove that city living is causing these brain regions to light up under stress.
But the association remained after researchers tried to account for the influences of other things that could be related to living in a rural or urban area, like socioeconomic status, the size of study participants' social networks, or how anxious they were to begin with.
"I think there's a lot to the story that our environment is important to how we function and also what our mental health is like," says study researcher Andreas Meyer-Lindenberg, MD, PhD, director of the Central Institute of Mental Health in Mannheim and professor of psychiatry at Heidelberg University, in Germany.
Meyer-Lindenberg says that to tease out what parts of city life might be responsible for the stress response, he is now comparing the brains of migrants and non-migrants who live in the same city. "They have a different social environment, but same city environment," he tells WebMD.
Experts who were not involved in the research praised its use of neuroscience to try to pinpoint how complex environmental influences impact the brain.
"I hope more scientists try to do this where they combine basic kinds of neuroscience with these kinds of bigger, broader problems, that's very commendable," says Marc Berman, PhD, a research fellow at the University of Michigan, Ann Arbor. "But it's one study, and it's correlational, so we need a lot more work in this area."
But it's not the first study to question how urban environments might affect mental functioning.
In a study published in Psychological Science in 2008, Berman and his colleagues asked healthy adults to walk through an urban environment or a natural setting.
After the walks, the researchers called out sequences of numbers and had study participants repeat the digits back to them in reverse order, a test that measures working memory.
After a walk in nature, people showed about a 20% improvement in their working memory compared to after they walked down city sidewalks.
Though researchers can't explain exactly what it is about the urban environment that may tax the brain, they speculate that cities, with their competing noises, smells, and sights, drain the brain's ability to direct attention.
Natural settings, they believe, require a different kind of attention from the brain, one that doesn't appear to be as fatiguing.
"I wouldn't draw the conclusion from these studies that city living is bad or urban living is bad and we should all move into the country," says Berman.
"We need to figure out what elements about the city are harmful to us, what things we can change, what things we can add to the city to make it more restorative and better for cognitive functioning," he says.