You lie down with your head in a noisy MRI brain scanner. You agreed to take part in this experiment, and at first the psychologists in charge seemed nice.
They set you confusing maths problems to solve against the clock, and you do your best, but they aren't happy. "Can you please concentrate a little better?" they keep saying. Or: "You are among the worst performing individuals to have been studied in this laboratory." It is a relief when time runs out.
Few would enjoy this, and indeed volunteers who underwent it were monitored to make sure they had a stressful time. Their minor suffering, however, provided data for a major study. The researchers, led by Professor Andreas Meyer-Lindenberg of the Central Institute of Mental Health in Germany, were trying to find out more about how different people handled stress. They discovered that city dwellers' brains, compared with people who live in the countryside, seem not to handle it so well.
While Meyer-Lindenberg stressed out the subjects, he looked at two brain regions: the amygdalas and the perigenual anterior cingulate cortex (pACC). The amygdalas are known to be involved in assessing threats and generating fear, while the pACC helps regulate the amygdalas. In stressed city-dwellers, the amygdalas appeared more active; in people from small towns, less so; in people who lived in the countryside, least of all.
And something even more intriguing was happening in the pACC. Here the important relationship was not with where the subjects lived, but where they grew up. Those with rural childhoods showed the least active pACCs, those with urban ones the most. In the urban group, there seemed not to be the same smooth connection between the behaviour of the two brain regions observed in the others. An erratic link between the pACC and the amygdalas is often seen in those with schizophrenia. And schizophrenic people are much more likely to live in cities.
When the results were published in Nature, in 2011, media hailed the study as proof cities send us mad. Of course it proved no such thing - but it did suggest it. The results offered a tempting glimpse at the kind of urban warping of our minds that some people have linked to city life since Sodom and Gomorrah.
So why is it that the larger the settlement you live in, the more likely you are to become mentally ill? Another German researcher, Dr Mazda Adli, is a keen advocate of one theory, which implicates that most paradoxical urban mixture: loneliness in crowds. "Obviously our brains are not perfectly shaped for living in urban environments," Adli says. "In my view, if social density and social isolation come at the same time and hit high-risk individuals, then city-stress related mental illness can be the consequence."
A lot is at stake. Schizophrenia is one of the leading causes of disability worldwide, and its prevalence looks likely to increase. In 2010, the proportion of the world's population living in cities passed gently into the majority.
Many other possible impacts of city living on brain function are also being investigated. Researchers in the US and elsewhere have also found that exposure to nature seems to offer a variety of benefits to city dwellers.
Much of this research considers the question of "cognitive load", the wearying of a person's brain by stimulation, which is thought to weaken functions such as self-control. In terms of its impact on public health, Adli believes that urbanisation may even compare to climate change.
Yet this is a change we choose. As Adli himself is keen to emphasise, stress is only part of the impact cities have on our brains. "There's a lot of what we call urban advantage," he says, "When we live in cities there is a much richer environment. There is also better health care, better education, a better standard of living. All these are protective factors."
Indeed, for those at lower risk city life might even be indirectly beneficial for mental health. For instance, being cuddled, played with and well cared for by your parents is powerfully associated with fewer social and emotional problems later. But, as Professor Michael Marmot of University College London points out, "these various parenting activities do follow the social gradient. The lower the income the less likely you are to engage in them."
He also cites unemployment as very damaging. Choose to live in the countryside, in short, and it might actually damage your family's mental health if it makes you poorer. Perhaps Meyer-Lindenberg reminded his subjects of that when they collected their fee.