In the latest issue of the journal Nature, science writer Alison Abbott surveys recent research that tests the link between urban stress and psychosis. The association makes sense — cities are stressful places, and stress plays a known role in mental health problems — but remains hard to isolate for obvious reasons. "It is difficult to study whether something as complex as a 'city environment' has an impact on the brain," writes Abbott.
A number of studies have narrowed in on the city-psychosis relationship in the past 20 years. Scientists have found that rates of schizophrenia are higher in people brought up in the city compared to the country, that people simply born with a higher "degree of urbanization" showed an increased risk, and that family history for the illness makes those in a city environment even more vulnerable. Some researchers feel comfortable enough suggesting a "causal association" between cities and psychosis: the more early years you spend in cities, the greater your risk for mental disorder.
Of course that should mean schizophrenia rates are rising over time, as cities around the world get larger. Many epidemiologists have found that's not the case, broadly speaking, but some small local studies have indicated the trend. In work published in 2003, British researchers found that the incidence of schizophrenia doubled in southeast London between 1965 and 1997. The rate went from seven to 11 cases per 100,000 people in the early years of the long-term study (depending on the type of diagnosis used), to 15 to 21 cases per 100,000 — without a concurrent rise in population.
Still it's remained unclear exactly which element of urban life has the greatest impact on the proposed city-psychosis link. A breakthrough study on the role of stress appeared in Nature last summer [full PDF]. The work, led by Andreas Meyer-Lindenberg of the Central Institute for Mental Health in Germany, found that living in or being from a city affected the way people processed stress compared to those from someplace else.
Meyer-Lindenberg and colleagues reached their conclusions through a series of brain imaging studies. They put healthy subjects into a scanner and stressed them out by having them perform a difficult math task under a time constraint while receiving negative feedback from the researchers. Then they compared the results of test subjects from the city (more than 100,000 population), from a town (more than 10,000) or from a rural region.
Those subjects who lived in a city showed increased activity in their amygdala and those brought up in the city had increased activity in their anterior cingulate cortex — two regions associated to stress regulation — compared to the subjects from towns or rural areas. No other brain regions showed any clear difference during the test. When the researchers ran a second test with a different stress task, this time a simultaneous math and spatial problem with negative feedback, the results were the same.
To make sure it was stress that created the different activation and not the performance of a cognitive task itself, Meyer-Lindenberg's group ran a third experiment involving a working memory and face recognition task. This time they found no connections between the urban test subjects and increased amygdala or anterior cingulated activation. The study had its limitations — among other things it wasn't causal, and these differences must be found in mentally ill subjects to mean something more — but it did "link the urban environment for the first time to social stress processing."
As Abbott reports, in the current Nature, the Meyer-Lindenberg work has inspired a wave of subsequent studies involving stress, cognition, and the city. The Dutch researcher Jim van Os is planning a $2.6 million field investigation that uses smart-phone apps to identify just which aspects of city life are the most stressful. (May we suggest the StressSense app?) Meyer-Lindenberg has proposed a similar effort to track people as they move through Heidelberg and give them spot cognitive tests depending on their location in the city.
Ultimately this type of work might help us design healthier cities — especially if it's combined with psychological findings about the restorative properties of urban parks. Indeed, city planners have already contacted Meyer-Lindenberg, according to Abbott. It is certainly difficult to study whether and how the complexity of the city impacts the brain, but that also makes it all the more important.