Brains smokestackCould neuroscience someday get to the root of what cities do to our brains — and our mental health?Photo: Stuart HerbertScientists have been doing studies for years that show you are more likely to suffer from mental illness if you live in a city. What they haven’t figured out is why.

Now, researchers in Germany have conducted experiments that they believe might begin to get at the neuroscience behind the crazy-making nature of urban areas.

Publishing in the journal Nature, a group led by Andreas Meyer-Lindenberg of the University of Heidelberg’s Central Institute of Mental Health in Mannheim, Germany, looked at how social stress affected the minds of subjects, some city-dwellers and some not. They did this by asking volunteers to solve a series of increasingly difficult math problems while their brains were being imaged — and essentially making them feel shamed and stupid for getting the answers wrong.

The imaging results showed that people who lived at any point in urban areas, as opposed to small towns or rural places, had heightened response to the social stress created by the questions.

From Nature News:

This ‘social stress’ activated many brain areas, two of them specifically correlated with the volunteers’ history of urban living. The amygdala, which processes emotion, was activated only in people currently living in a city. And the cingulate cortex, which helps to regulate the amygdala and processes negative emotions, responded more strongly in those brought up in cities than in those who grew up in towns or rural areas.

I called Dr. Meyer-Lindenberg to ask him more about the implications of his experiment, and what he thought might be the cause of heightened sensitivity to social stress among urban dwellers.

“On the neural level, we find two things,” he told me. “A, the neural effects are completely dissociated, so current urban living only affects the amygdala, urban upbringing only affects the cingulate. And B, these areas are associated with these illnesses. The amygdala is sort of a danger center, and it’s critically important for fear. And it is clear that the amygdala is a major player in depression. The cingulate is a prefrontal area regulating negative emotion, and it’s known to be one of the earliest areas affected by schizophrenia.”

So what accounts for the hyperactivity of the amygdala-cingulate circuit in urban dwellers? “That exact circuit that we found hyperactive has also shown to be activated when someone comes too close to you and crowds your personal space,” Meyer-Lindenberg told me.

But he cautioned against inferring that mere density of population is at fault. “It’s still speculation,” he said. “There could be myriad components of the urban experience that might or might not be bad for you from the point of your risk for mental illness. No one really knows. People are annoyed by noise or by traffic, or it could also be lead, or air pollution, but there’s no evidence base to say this is an important factor, this is not an important factor. Therefore there’s no basis for urban planning that’s grounded in human biology, at least with regard to mental illness.”

In a way, it’s like the situation in 19th-century London, when cholera was endemic, but city officials didn’t understand how it was spread. The changes they initially made to infrastructure to combat the disease actually worsened the problem, because they were based on intuition rather than hard scientific data. The story of how science finally solved the cholera epidemic is told in Steven Johnson’s fascinating book The Ghost Map.

Rigorous epidemiological research that might help us understand the root causes of metal illness in cities is scant, according to Meyer-Lindenberg. “What I find is a lot of theorizing but very little data,” he said.

And that just won’t cut it when the public-health stakes are so high. “Just to give you an idea of the size of the issues from the point of psychiatry, it has been estimated that 30 percent of all schizophrenias could be avoided if everyone were born in a rural environment,” he told me.

Meyer-Lindenberg sees all of this from a scientific standpoint: He wants more research and more facts. How does proximity to urban green space affect mental illness rates? How about immigrant status? Those are just two possible avenues for exploration, said Meyer-Lindenberg. Mapping these factors is increasingly easy, thanks to open data sources such as satellite photography.

He said that some research has suggested that the very opportunities available in cities might be part of the problem. Being surrounded by a lot of other people who have wealth, when your own income is sketchy, could increase social anxiety.

“It could easily be that some of the things that draw people to cities, some of the stimulating aspects, lots of input and so on, might also be problematic from the point of mental illness,” he said. “It might have to do more with how many options you see around you but cannot easily reach that might be a factor here, rather than the absolute amount of wealth you have.

“Social status is closely linked to socioeconomic variables. What we found in our imaging studies is that if your social status becomes labile, and especially if you are in danger of losing it, a very similar brain circuit becomes active. There is a convergence of socially relevant risk factors on that circuit.”

Different types of urban environments might also affect people in different ways. “It’s very different if you live in Manhattan and you sort of live in a series of overlapping villages, if you will, or if you live in a city like Sao Paolo, in which no such microstructure is immediately available to you, or if you live in a large spread-out area,” said Meyer-Lindberg.

The social connections that are fostered in more walkable neighborhoods could help city dwellers from losing it. “A previous study found that that the size of your social support network is actually correlated to the size of the exact brain circuit we found in this study,” Meyer-Lindenberg said. “So that’s a protective factor. The more friends you have, the bigger those brain structures are.”

Already, more than half the human population lives in cities. That proportion will only increase. New cities are springing up all over the developing world, some built to order, some completely unplanned. The form they take could be crucial.

More knowledge about what exactly drives people could lead to concrete solutions that would make for better mental health — the same way the discovery of how disease was spread by waterborne germs finally ended the scourge of cholera in London.

“I think it would be important to make cities better, given that we can’t escape cities, given the dynamics of urbanization,” said Meyer-Lindenberg. “That’s a reality that we’re not going to get rid of.”