Cities are not only transforming the way people live, but new research shows they are also altering how the brain responds to stress.
The changes may be helping push vulnerable city dwellers "over the edge," says Dr. Jens Pruessner, a Montreal brain researcher and co-author of an international study to be published Thursday in the journal Nature.
The findings point to brain mechanisms that link urban living to the risk of mental illness. And Pruessner and his co-authors say they could help explain why anxiety and mood disorders are more common among people in cities and schizophrenia is almost twice as high for people born and raised in urban areas.
The study is the first to investigate how city life affects the brain's ability to deal with stress.
It looked at how 92 healthy volunteers, aged 18 to 80, in Germany responded to a devilish math test known as the Montreal Imaging Stress Task.
The test is meant to stress people out and make them fail, says Pruessner, who designed it with colleagues at the Douglas Mental Health University Institute at McGill University. It is impossible to get the right answers to the computerized test "even if you're a genius," Pruessner said in a telephone interview Wednesday.
Making it even more stressful, the research subjects had to complete the tests under time pressure and with disapproving feedback from researchers exhorting them to do better. Meanwhile investigators monitored the participants' brain activity using functional magnetic resonance.
The study revealed that activation of the brain's amygdala, which is involved with emotion and moods, to the stress test was greater in individuals living in big cities than in people from towns and rural areas.
It could be that the extra and repeated stress of city life sensitizes the amygdala making it "hyper-responsive," says Pruessner.
The study also found that individuals raised in big cities showed stronger activation in the perigenual anterior cingulate cortex, another region involved in regulation of emotion, mood and stress.
"If you've been raised and born in the big city then this structure seems to be more sensitive to responding to stress," says Pruessner.
The researchers say the risk of anxiety disorders is 21 per cent higher for people from cities, and 39 per cent more for mood disorders, while the incidence of schizophrenia is almost double for individuals who are born and brought up in cities.
It may be the "stronger response to stress" pushes people already at risk or predisposed to mental illness "over the edge," says Pruessner.
Cities' impact on mental health in a pressing issue as the human landscape is changing so dramatically. In 1950, just 30 per cent of the world's population lived in cities. Today more than 50 per cent of people are urban dwellers, a figure that is only expected to climb.
"De-urbanization" is not really an option, says Pruessner, but there are things people can do to guard against the ill effect of city living.
"It is important to take time off," he says. "Find time for recreation as that is known to bring stress levels down and help you cope."