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Vintage etching of phrenological diagram with definitions of the various areas of the human skull.Cinders McLeod

Human brains like to be busy and are designed to be attentive to the slightest change – which makes them a worthy adversary for the challenges posed by the modern city.

That contest is increasingly urgent, as the process of urbanization kickstarted by the Industrial Revolution becomes the global norm. Studies say five million new people migrate or are born into cities each month in Africa, Asia and the Middle East. A dozen years from now, 60 per cent of the world will be urban; by 2100, cities will host three-quarters of humanity.

And as we cluster, the ancient animal brain we're born with has to contend with an urban norm where the rapidity of change could become overwhelming.

"Biological evolution is slow," says Jay Pratt, professor of psychology at the University of Toronto. "If you look at the acceleration of human cultural evolution, moving from villages to towns to cities, it's happened so much faster than any biological systems with our lifespan could keep up with."

Cities fascinate cognitive scientists because they constantly test our neurological range: There's no better place to entice, stimulate and distract the wandering mind.

The urban form is a metaphor for the mind – an interdependent network that has to make order out of a chaotic flow of non-stop sensory information.

"There's a tremendous flexibility and plasticity in the brain," says Prof. Pratt.

One of the most striking examples was demonstrated in a brain-scan study of London cab drivers by researchers at the city's University College: They were shown to have an enlarged posterior hippocampus, the part of the brain associated with navigation, presumably because of the advanced mental mapping required by their job.

Since the hippocampus seemed to grow over a cab-driver's career, this appears to be the brain adapting, not just the job attracting people with suitable skills.

That ability to extend memory and spatial processing is encouraging. But London cabbies are also clearly a special case. Memory is a very selective and specific component of experience.

Colin MacLeod, professor of psychology at University of Waterloo, points out the difference between driving your own car to a restaurant in a new part of town and taking a cab.

"If another person is driving," Prof. MacLeod says, "the next time you go to the same place, you may find you don't have a clue how to get there. Presumably you weren't encoding the relevant information – you weren't paying attention."

Yet attentiveness has a complicated relationship with memory. While the brain can't store all of the city's potential information at the level of instant accessibility, we realize as we navigate neighbourhoods that we've held onto knowledge we didn't realize we had – the location of a dry cleaner en route to work, the eerie feeling that a certain street is coming up on the right.

"There are arguments in cognitive literature that we encode sequence information virtually for free – that it's almost automatic even if it's of no immediate use to you," says Prof. MacLeod.

The mental Goldilocks dilemma: Too little stimulus or too much?

In this sense, our brains are hungry for what a city provides. "Humans enjoy being engaged," says Prof. Pratt. "We don't like living in sparse environments."

A straight, flat, empty path in some rural Arcadia frustrates our state of vigilance much more than a busy city intersection – it's hard to drift off in Times Square, whatever its other problems. But cities also impose tradeoffs between attentional capture and what psychologists call volitional control.

Our minds are remarkably good at driving a car through a complex urban intersection, carrying out a goal. Yet add a flashy advertising display by the side of a high-traffic street and the brilliant brain instinctively gets suckered in by the flickering brightness: For one dangerous moment, you look away.

In 2011, according to the U.S. National Highway Transportation Safety Board, 3,331 people in the United States were killed in crashes involving a distracted driver. Researchers at Carnegie Mellon found that the brain activity associated with driving is reduced by 37 per cent when you're using a cellphone.

From an evolutionary perspective, it's good to be twitchy: Any change to our sensory environment – a sudden sound, a jarring sight, an alien smell – represents a potential threat.

But cities take distraction to a level undreamed of on the primeval savannah, to the point where the urban form itself can be blamed for messing with our minds.

"In the 19th century," says Hillel Schwartz, a social historian and author of Making Noise: From Babel to the Big Bang and Beyond , "you get this notion that living in the city can be nerve-wracking, that the city itself can cause people to have a mental collapse."

Recent studies by scientists at Heidelberg University's Central Institute for Mental Health support this view. In trying to determine why rates of schizophrenia are much higher in people raised in the city, they showed that city types experience a heightened stress response in the amygdala, the part of the brain that processes emotion.

Putting the brain on park

If there are specific aspects of the city that are hard on the brain, urban design can be used to reduce these pressures. Studies by Marc Berman, a researcher at the Rotman Research Institute in Toronto, suggest that an environment along the lines of a Stanley Park or Mont Royal can have a beneficial effect: Urbanites who took a short break in a natural setting of trees and water performed better on memory and attention tests than those who wandered a busy city street.

The brain has a remarkable ability to focus, shutting out extraneous stimuli to find a place of relaxation and creativity. When the detective story you're reading is sufficiently gripping, you will go temporarily deaf because you're inhabiting another world. That magical ability just doesn't transfer well to a city intersection.

Focus has its limitations, even in the animal world: A street-smart cat will suddenly dart into the path of a car when he's fixated on turf battles with his rivals. And so, too, a shopper intent on picking out a favourite brand of cereal in the densely packed, attention-craving supermarket lanes will miss the imminent collision from an oncoming shopping cart – steered by another granola-seeking brain.

The processing capacity of the brain is limited, since there's a tradeoff between increased skull size and the advanced human ability to walk upright. So the scenes of the world we think we're partaking in are actually a highly partial composite of reality based on 200,000 rapid eye movements every day – snapshots of the exceedingly tiny bits we're interested in.

Our peripheral vision may be reassuringly extensive and Gretzky-like – 150 degrees in scope – but the amount we actually see at a single moment, with any degree of detail and colour and sharp-edged focus, is less than three degrees.

"Most of the time, over most of the visual field, we're mostly blind," says Prof. Pratt.

The fact that a store shelf can stretch us to our limits is no cause for existential despair. From the brain's point of view, says Prof. Pratt, "it's actually an amazingly complex visual scene."

The key thing for the urbanite is to recognize this complexity, and the demanding effect it has on our brain, for better and for worse. Since the organ in our heads is designed to be on a constant alert for differences, the city becomes a place of hyperalert vigilance.

"This leads to two entirely different theories of how we encounter the city," says Mr. Schwartz. "One, people need to be stimulated and the city is a learning machine. Or two, the city is a place where people are overstimulated, where we have too many things thrown at us."

The greedy gears of perception

Both versions could be true, of course. Some people can carry on an intelligent conversation in the boisterous restaurant of the moment. Other people hear nothing but ambient noise and can't wait to escape.

Prof. Nilli Lavie of University College, London, is fascinated by a paradox that must occur to anyone who watches their fellow citizens go in and out of mental focus: "How is it that we are sometimes extremely distracted by information we really wish to ignore and at other times we are so focused that we can't take in important events right in front of us?"

Her theory is that the brain will process every bit of information it can handle within its capacity: Our neural machinery can't be shut down at will, and if the task we're engaged in doesn't stretch us far enough, we'll keep increasing our load.

Walking down a city street, for example, seems so simple. And so, confusing the basic act of locomotion with the hazardous scene we're traversing, we add a smartphone to the mix.

Big mistake: "The brain has a limited processing capacity," says Prof. Lavie, "and when it's flooded with high information load, it shuts out any information that is not considered task-relevant. And then there's simply no capacity left for other stimuli like the cars, people and street poles that we bump into."

"We make very bad assumptions about what we're good at," says Prof. MacLeod. "We think we're 'multitaskers' and really we're not. Our brain allows us to switch attention fairly quickly. But we don't always realize we're switching. We think we're doing two things at once, and so we get fooled."

This reluctance to accept our mental limits means that everyday neurological issues are often discussed as if they were problems of civility – how dare you message-sending hordes walk straight into me, or obnoxiously talk so loudly on your cellphone? While it's true that we're not always thinking about the people all around us, it's not because we mean to be rude. It's that our brains are somewhere else.

For the massed city-dwellers of the 21st century, everyday life will offer a constant series of such brain-testing temptations, and how well we respond depends on our willingness to understand that the brain's strengths are often its limits.

It's a neurological feat that we can walk or drive and talk on the phone at the same time. But that doesn't mean that we can do both things well – let alone incorporate the instant alerts a city sends our way, without the benefit of a personalized ring tone.