Study participants who went on a 90-minute walk through a natural environment reported lower levels of rumination (repetitive thought focused on negative aspects of the self) than participants who walked through an urban environment. Their brains showed a corresponding change in activity when scanned.
Bratman, Gregory N., J. Paul Hamilton, Kevin S. Hahn, Gretchen C. Daily, and James J. Gross. (2015). Nature experience reduces rumination and subgenual prefrontal cortex activation. Proceedings of the National Academy of Sciences, 201510459.