Below is a study about which part of the brain affects anxiety, depression, & obsessive-compulsive disorder.  It has to do with the part of the brain that relates to passive thoughts, as opposed to task thoughts used when you pay bills or make a shopping list.  Other studies describe how changing your thoughts can help change your mood thus relieving those same mental conditions.  All studies are cited below.

Something I read recently about St. Therese of Lisieux:  she suffered terribly from both scruples, a form of obsessive-compulsive disorder and depression.  One particular scruple (frequently recurring thought) that plagued St. Therese caused her to regularly refrain from communion (compulsive act to relieve the thought).  Her superior and biological sister Celine instructed the Saint to do the opposite and receive communion more frequently.  Many spiritual directors of Saints such as St. Francis de Sales to St. Jane de Chantal and St. Francis of Assisi to St. Clare of Assisi instructed the same.  Eventually the compulsion stops and the obsessive comes-out-of-nowhere thoughts are relieved.  Today cognitive therapists often instruct similarly.  So do priests when a penitent compulsively confesses such non-sins after they are tortured by, often sexual, frequently occurring intruding thoughts.  (Intruding passive thoughts are not sin—look it up in the catechism).

A useful task when passive ruminations of depression and anxiety strike is to have memorized key scripture passages to counteract such thoughts.   Such passages are Psalm 40:4, Matt 6:34, or Psalm 131.  Familiarity becomes a passive thought and thus may help lessen the effects of such brain influenced mental conditions.

Info about St. Therese of Lisieux:  The Context of Holiness by Marc Foley, O.C.D. Copyright WA Province of Discalced Carmelites Inc., 2008  pgs. 114-115.

Current study on parts of the brain regulating passive thoughts  https://www.sciencedaily.com/releases/2016/09/160928141901.htm?utm_source=feedburner&utm_medium=email&utm_campaign=Feed%3A+sciencedaily%2Fmind_brain%2Fdepression+%28Depression+News+–+ScienceDaily%29

Changing thoughts with cognitive-behavioral therapy affects changes in brain  https://www.sciencedaily.com/releases/2016/02/160202185552.htm   https://www.ncbi.nlm.nih.gov/pubmed/19622682

Photo by Jhonatan Alarcon.  Dedicated to public domain waiving all rights.  Found on Wikimedia Commons.