There have been many studies into the links between brain structure and depression. Particular emphasis has been placed on the limbic system, specifically the hippocampus and amygdala, which controls emotion and mood regulation. A U.S. study of 64 people compared people with depression, psychotic depression and healthy controls using brain scans to measure the size of their hippocampi and amygdalas. The researchers found that the participants with psychotic depression had smaller amygdalas than those with depression and the control group whose amygdalas were of similar size. The smaller people's amygdala the earlier was the age of onset of their psychotic depression.
Keller, Jennifer ... [et al] - Hippocampi and amygdalar volumes in psychotic and nonpsychotic unipolar depression American Journal of Psychiatry July 2008, 165(7), 872-880