“Nature” Walks Change Your Brain?

This summer Gregory Bratiman and colleagues at Stanford published results from a study of brain activity in a urban settings. They compared measurements in two conditions, after a 90 minute walkin in a “natural” setting and a 90 minute walk in an urban setting.

They found less self reported “rumination” (often a flag for depression of other mental distress) and neuro imaginging to measure regional cerebral blood flow (rCBF). They conclude that “Our results indicate that nature experience reduced rumination and sgPFC activation.” (p. 8568 )

With the important he implication that:

Given the documented link between rumination and risk for depression and other psycho- logical illnesses, the reduction in rumination among those with the nature experience suggests one possible mechanism by which urbanization—which reduces opportunities for nature experi- ence—may be linked to mental illness.” (p. 8568 )

The authors note that this study fits into a body of literature that finds mental benefits to “natural” environments (e.g., ‘Blue Mind‘) and detriments form urban environments, and also a growing belief by some that these benefits reflect long term changes in brain functions.

This is hyped in the media, with bogus headlines such as, “How Walking in Nature Changes the Brain“.

I remain skeptical.

Let me be clear: I have no doubt at all that living in cities is stressful, and most likely psychologically harmful for many people. And I have not doubt that there are many no-urban environments that people like, and which are probably beneficial for people in many ways. (If nothing else, getting away from the city, well, gets you away from the city.)

But I have problems with this study and similar ones that are looking at single mechanisms—not coincidently, mechanisms we can measure with current technology—and telling stories about how these are the mediator between “urban life” and mental disorder.

With all of the things going on in a city, visual situation, noise, smell, crowds of people, and massive amounts of social interaction, We know that there are lots of “stressors”, dangers, traffic, and social signals that require attention and arousal almost constantly. There is little doubt that all parts of the brain are really active at all times.

So it is shooting fish in a barrel to find high levels brain activity, whatever tha means. For that matter, it’s not especially difficult to find patterns in self-report questions either.

So problem number one is that the variables measured are only a very limited slice of whatever is going on in people’s heads, and there is no particular reason that these are especially important.

But I have bigger problems with the study.

First of all, I have trouble with his terminology, consistently referring to urban green space as a “natural” setting. This is clearly a loaded phrase, and one that is not only arguable, but conceals a lack of clarity. What is the definition of “natural” or at least non-urban environment? If you are going to claim that there is psychological difference in these two environments, then what is it that distinguishes them?

His choice arbitrary, and both of the settings are urban, to my eyes. Actually, his “urban” setting is hellishly hyper-urban, while we have little clear picture of the “natural setting”. Are they walking on trails? Are there pavermed roads with cars passing? Can you hear traffic? Just how “non-urban” is this “natural” area?

A third issue is the subjects. In particular, other than the note that they are all city dwellers, there is no control for their own past experience and perception of “urban” or “natural” environments. This is an important point because it is well known that people habituate to environments, and have dramatically different concepts of what is a “natural” environment depending on their personal history.

It is extremely likely that such history could be a major factor in the enjoyment and benefits of urban green spaces in several ways. Past good experiences could predispose one to quickly relax and attend to nature. Past bad experiences could induce anxiety, perhaps different from the everyday tension, but far from calm. The situation could be familiar, or it could be novel, either of which could create a short term pleasant experience. (Similar arguments apply to the “urban” setting.)

For that matter, what would the comparable effect of any 90 minute break from ordinary routine? A game of golf? A walk through a quiet museum? A nap? And what were the participants taken away from? Were they getting time off from work, or were they giving up their own leisure time? There is so much more context to this walk besides the setting.

Regardless, I have to reserve judgment about the supposed effects of any 90 minute walk. Certainly, I would wonder if any such effects last long, or persist once the person returns to normal activities.

I also note that the participants were given phones and instructed to take pictures. In other words, this was a “tourism” task, and not unstructured strolling. The participants must have been thinking about taking pictures, which raises questions about what they were attending to in the environment.

It isn’t clear whether the participants had their own phones or were connected to the internet. I assume not—that would really throw a lot of confounding variables into the game. But even if they only had a phone to take pictures, they were attending to a small mobile screen. We know this is psychologically powerful, pulling attention from the environment, natural or not. And screen use itself may be liked to behavioral and neural changes.

In the end, I’m not especially convinced of the broader conclusions. The purported links between mental illness and these measures is no more than suggestive. The link between the environment and the measurements is very weak because of the numerous confounding factors mentioned above. I’m particularly skeptical of the entire notion that nebulously defined mental illnesses with some correlation to “living in a city” are mediated by specific neurological changes. “Living in a city” is a complicated and life long activity, and there are many, many things going on in the body and brain, all at once.

All that said, I certainly expect that spending time in urban green spaces (let’s avoid the loose term “natural”) makes people happier and healthier. We don’t really need to have this kind of alleged neurological connection to want more green space and nicer urban spaces, or to want to get out of the city for our own sanity.

  1. Gregory N. Bratman, J. Paul Hamilton, Kevin S. Hahn, Gretchen C. Daily, and James J. Gross, Nature experience reduces rumination and subgenual prefrontal cortex activation. Proceedings of the National Academy of Sciences, 112 (28):8567-8572, July 14, 2015 2015. http://www.pnas.org/content/112/28/8567.abstract

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