Living in the city or growing up in one can affect brain function during a stressful situation, a brain imaging study has found.

The world is becoming more urbanized, with almost 70 per cent of people expected to live in urban areas by 2050, according to projections by the United Nations.

Studies suggest that living in a city increases the risk of depression and anxiety, and that schizophrenia rates are higher in people born and brought up in cities. But until now, there hasn't been research into how human brain structures might be affected by urban living.

To that end, researchers at McGill's Douglas Mental Health University Institute in Montreal and the University of Heidelberg in Germany used MRIs to study brain responses of healthy German students who were taking a math test under stressful conditions.

Study participants faced time pressure and in some cases they had investigators scolding them through headphones.

When exposed to those stressful conditions, two areas of the students' brains that are known to be involved in processing emotions became more active, the researchers reported in Wednesday's online issue of the journal Nature.

Brain more active

Specifically, the brain's amygdala was more active in those who lived in cities, and the perigenual anterior cingulate cortex, or pACC, was more active in those who had been brought up in cities, the researchers found.

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"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," Jens Pruessner of McGill and his co-authors concluded.

"The findings contribute to our understanding of urban environmental risk for mental disorders and health in general."

In the study, a city was defined as more than 100,000 inhabitants, a town as more than 10,000.

Since the researchers couldn't find any difference by type of city — between high-density cities and those with a lot of green space, for instance — they believe the overall social stresses associated with living in urban areas are the key, Pruessner said.

He said the findings might point to the importance of taking time away from the hustle and bustle. 

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Areas of the brain involved in fear, stress and anxiety were more active in the brains of city people. (CBC)

"I think what's important is to realize that you might be more exposed to a higher amount of social stress if you live in the city," Pruessner said. "Therefore you should account for that by giving yourself more chances to take a vacation and pause from the stress, to find times for recreation." 

The final analysis included 32 healthy participants with rural as well as urban upbringing.

The study cannot prove a cause and effect relationship, and since the subjects grew up in the relative prosperity of Germany, the findings may not apply elsewhere, the researchers said in acknowledging limitations of the study.

'Mysterious mechanism'

The participants were also not randomized and they were mainly college students, which also limits how the findings could be applied to other populations.

An enormous number of factors could influence the results. The authors took age, education, marital and family status and aspects of health, mood, personality and social support into account.

"None of these factors significantly influenced the effects … suggesting that living in a city environment changes brain response during a social stressor by a distinct, although mysterious, mechanism," Daniel Kennedy and Ralph Adolphs of the California Institute of Technology in Pasadena said in a journal commentary.

"Given the world's increasing population, the fact that we will be living mostly in cities is inescapable. This highlights the importance of understanding the effects that such living conditions will have on human mental health."

It is too early to draw conclusions on what the results could mean for mental illness, said Elizabeth Phelps of New York University, an expert in emotion and the brain who wasn't involved in the study.

"This will raise a lot of interest in this idea. Whether or not it pans out in future research, who knows, but I think it's worth investigating," she said.

In the case of schizophrenia, scientists suspect that both genetic and environmental factors are involved. The new findings point to the social stresses of city living.

"We've all known that there has to be something else involved that triggers the onset of schizophrenia, and it's called stress," said Chris Summerville, the chief executive officer of the Schizophrenia Society of Canada in Winnipeg. "So if have higher levels of stress in urban areas, then it stands to reason you would probably have higher levels of schizophrenia."

The findings did not come as a surprise to a man rushing for a commuter train in Toronto.

"Seriously, if you lived in the country and you didn't have to deal with the traffic and trains and all the noise, I think you would be a calmer person," said David Smith.

In contrast, David Hine, who now lives in the small community of New Ross, N.S., after living in cities and towns, said there is a greater sense of connectedness in rural life.

"In the ups and downs of life, it's nice to have the security and support of a community," Hine said.

The study was funded by the European Community’s Seventh Framework Program, the German Research Foundation, and the Federal Ministry of Education and Research in Germany. 

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The research is featured on the cover of this week's issue of the journal Nature. (Courtesy of Nature)

With files from CBC's Melanie Glanz, Leslie McLaren and The Associated Press