Between the crowds and the noise and the pressure, city life often seems to set one's brain on edge. Turns out that could literally be true.

A study of German college students suggests that urbanite brains are more susceptible to stress, particularly social stress, than those of country dwellers. The findings don't indicate which aspects of city life had changed the students' brains, but provide a framework for future investigations.

"Whether people are exposed to noise, live near a park, have a big group of friends or not – you can do those experiments, and tease apart which parts of urban living are associated with these changes," said Andreas Meyer-Lindenberg, a psychiatrist at German's Central Institute of Mental Health.

Meyer-Lindenberg's findings, published June 23 in Nature, are a neurological investigation into the underpinnings of a disturbing social trend: As a rule, city life seems to generate mental illness.

Compared to their rural counterparts, city dwellers have higher levels of anxiety and mood disorders. The schizophrenia risk of people raised in cities is almost double. Literature on the effect is so thorough that researchers say it's not just correlation, as might be expected if anxious people preferred to live in cities. Neither is it a result of heredity. It's a cause-and-effect relationship between environment and mind.

What those causes are is unknown, but many researchers have speculated that urban social environments are partly responsible. After all, cities are hyper-social places, in which residents must be constantly on guard, and have mathematically more opportunity to experience stressful interaction. Too much stress may ultimately alter the brain, leaving it ill-equipped to handle further stress and prone to mental illness.

"Most people speculated that it had something to do with social environments, but there was never any direct data," said Meyer-Lindenberg. "We provide the first mechanism that links cities to mental illness via social stress."

Meyer-Lindenberg and colleagues initially tested 16 male and 16 female college students. Before the test, the students' heart rate, blood pressure and stress-hormone levels were measured. There were no significant differences between country and city kids. Neither were there appreciable differences in mood or personality.

Duing the test itself, the students were put inside a brain-scanning fMRI machine, then asked to take a computerized math test designed to be socially stressful: Each correct answer was followed by more difficult questions, false feedback told each student that his or her score was exceptionally low, instructors glared disapprovingly and bemoaned the waste of money.

The city kids displayed heightened levels of activity in two brain regions: the amygdala, which is central to processing emotion and stress, and the perigenual anterior cingulate cortex, which regulates the amygdala. In short, city brains had disproportionately amplified responses to social stress. They'd become sensitized.

Meyer-Lindenberg's team repeated the study twice more with a total of 70 more students. Each time the same pattern emerged. The researchers then looked for links to age, education, income, marital and family status, mood and personality. But when those were taken into account, the pattern still remained.

The larger the city in which a student lived, the more active their amygdala. The longer they'd lived in a city as a child, the more active their cingulate cortex. In other studies, the cingulate cortex has been described as especially sensitive to early-life stress, with alterations linked to adult psychological problems.

Communication between the cingulate cortex and amygdala also seemed to be less efficient in city dwellers. In a commentary accompanying the study, California Institute of Technology neurobiologists Daniel Kennedy and Ralph Adolphs noted that similar patterns are seen in people with genetic predispositions to psychiatric disorders.

"Taken together, the findings suggest that the cingulate–amygdala circuit is one on which genetic and environmental risks for mental illness may converge," they wrote.

Meyer-Lindenberg and colleagues noted that, while they consider social stress to be the most likely trigger, other factors – pollution, crowding, as-yet-unanalyzed demographic and socioeconomic factors – could be involved. German college students also represent a very limited test group.

According to Meyer-Lindenberg, residents of cities in the developing world may be subject to more intense stresses than his students. "The divide between urban and rural is not that strong in Germany. You can go between city and country very quickly," he said. "We would expect the differences in stress to be bigger in places with a larger divide between urban and rural."

In future studies, researchers could run similar studies on people with varying urban experiences and backgrounds, possibly filling in by identifying exactly which aspects of city life make brains more vulnerable to stress. Urban planners and policymakers could then apply those insights. Also open for investigation are whether city dwellers are vulnerable specifically to social stress, or other forms as well.

With a growing majority of humanity now firmly urbanized, "the fact that we will be living mostly in cities seems inescapable," wrote Kennedy and Adolphs. "This highlights the importance of understanding the effects that such living conditions will have on human mental health."

Image: Chris Isherwood/Flickr

See Also:

• Poverty Goes Straight to the Brain
• Digital Overload Is Frying Our Brains
• Scanning Dead Salmon in fMRI Machine Highlights Risk of Red Flags
• Antidepressants May Thwart Quest for True Love

Citations: "City living and urban upbringing affect neural social stress processing in humans." By Florian Lederbogen, Peter Kirsch, Leila Haddad, Fabian Streit, Heike Tost, Philipp Schuch, Stefan Wust, Jens C. Pruessner, Marcella Rietschel, Michael Deuschle & Andreas Meyer-Lindenberg. Nature, Vol. 474 No. 7352, June 23, 2011.

"Stress and the city." By Daniel Kennedy and Ralph Adolphs. Nature, Vol. 474 No. 7352, June 23, 2011.