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."
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.