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Researchers conducted a study which asked randomly selected participants to spend 50 minutes walking in either a natural or urban setting, and to submit to a series of psychological assessments before and after the walk. They found that volunteers who walked through a lush, green portion of Stanford campus showed improve cognitive function and mood compared to those who walked near heavy traffic for the same period of time. However, while this study showed that nature could have a positive effect on mental well-being, it did not examine the neurological mechanisms underlying this change.
http://www.collective-evolution.com/2015/09/03/how-walking-in-nature-changes-the-brain/