Numerous studies have shown that it is HUMAN EXPERIENCE that brings about changes in brain chemicals and also result structural changes in the brain. Mice subjected to various psychological stresses (e.g. being restrained) clearly show many changes in the brain [see: Popoli M, et al. (2012), The stressed synapse: the impact of stress and glucocorticoids on glutamate transmission, Nature reviews Neuroscience. 2011;13(1):22-37.] - these changes are reversible through psychological means (e.g. when stressed, restrained animals are released as described in the same article). Other studies have shown that as taxi drivers do their jobs (psychological causes), the brain changes. Jugglers gain more grey matter in certain areas as a result of engaging in juggling. Various psychological habits, choices, etc., can also change the brain. For example, research has also shown that impulsivity trait results in reductions in gray matter. On the other hand, mindfulness practices (that result in reductions in impulsivity) are known to change the structure and function of the brain in positive ways (e.g. increases in gray matter and cortical thickness as well as white matter connectivity).
So, in these types of studies it is important to get the ‘causal direction’ right.