For the most part, our brains didn't evolve in cities. But in a few decades, almost 70 per cent of the world's people will live in urban environments. Despite the prosperity we associate with cities, urbanization presents a major health challenge. Cities, with their accelerated pace of life, can be stressful. The results are seen in the brains and behaviour of those raised in cities or currently living in one.

On the upside, city dwellers are on average wealthier and receive better health care, nutrition and sanitation than rural residents. On the downside, they experience an increased risk of chronic disease, a more demanding and stressful social environment and greater levels of inequity. In fact, city dwellers have a 21 per cent greater risk for anxiety disorders and a 39 per cent increased likelihood of mood disorders.

A study published in Nature links city living with sensitivity to social stress. MRI scans show greater exposure to urban environments can increase activity in the amygdala, a brain structure involved in emotions such as fear and the release of stress-related hormones. According to the study, the amygdala "has been strongly implicated in anxiety disorders, depression, and other behaviours that are increased in cities, such as violence."

The researchers also found people who lived in cities for their first 15 years experienced increased activity in an area of the brain that helps regulate the amygdala. So if you grew up in the city, you're more likely than those who moved there later in life to have permanently raised sensitivity to stress.

Author and professor David Gessner says we're turning into "fast twitch" animals. It's like we have an alarm clock going off in our brains every 30 seconds, sapping our ability to concentrate for longer periods of time. The demands of urban life include a constant need to filter information, dodge distractions and make decisions. We give our brains little time to recover.

How do we slow things down? Nature seems to be the answer. Cognitive psychologist David Strayer's hypothesis is that "being in nature allows the prefrontal cortex, the brain's command centre, to dial down and rest, like an overused muscle."

Research shows even brief interactions with nature can soothe our brains. Stanford's Gregory Bratman designed an experiment in which participants took a 50-minute walk in either a natural or an urban environment. People who took the nature walk experienced decreased anxiety, brooding and negative emotion and increased memory performance. Bratman's team found walking in natural environments can decrease rumination, the unhealthy but familiar habit of thinking over and over about causes and consequences of negative experiences. Their study also showed neural activity in an area of the brain linked to risk for mental illness was reduced in participants who walked through nature compared with those who walked through an urban environment.

Korean researchers investigated the differences in brain activity when volunteers just looked at urban versus natural scenery. For those viewing urban images, MRI scans showed increased blood flow to the amygdala region. In contrast, areas of the brain associated with empathy and altruism lit up for those who viewed natural scenes.

In Japan, scientists found people spending time in nature -- shinrin-yoku or "forest bathing" -- inhale "beneficial bacteria, plant-derived essential oils and negatively-charged ions" which interact with gut bacteria to strengthen the body's immune system and improve both mental and physical health.

Spending time in nature regularly is not a panacea for mental health but it's an essential component of health and psychological resilience. Nature helps us withstand and recover from life's challenges. Even city dwellers can find nearby nature -- a garden, local park or trail -- to give their overworked brains a break.

Every spring, the David Suzuki Foundation challenges Canadians to spend more time outside for health and mental well-being. The 30×30 Nature Challenge asks people to commit to spending at least 30 minutes a day in nature for 30 days in May. When you take the 30×30 pledge at 30x30.davidsuzuki.org/, you'll receive the latest research on the health benefits of spending time outdoors along with practical tips on how to add green time to your daily routine.

Let's show our brains -- and bodies -- some love. Get outside!

Written with contributions from David Suzuki Foundation Senior Public Engagement Specialist Aryne Sheppard.

Tiré de : http://www.huffingtonpost.ca/david-suzuki/nature-calms-brain-heals-body_b_9674740.html