There has been a lot of research recently using brain scanning to look at differences in the brains of people with mental-health problems. Because brain scanning is an expensive and time-consuming business studies are often quite small meaning that samples are more likely to be affected by chance factors and extreme cases. One way around this is to pool the results from several different studies together in what researchers call a meta-analysis. Psychologists from the Institute of Psychiatry, King's College London did exactly this with studies on brain structure and obsessive-compulsive disorder. They looked at 12 different studies comparing to people with OCD to 376 people without. They found that although there was no difference overall in the amount of grey matter in participants' brains those with OCD had more grey matter in the bilateral lenticular nuclei and caudate nuclei and less in the bilateral dorsal medial frontal/anterior cingulate gyri. People with more severe OCD were more likely to have increased grey matter in their basal ganglia. Taking antidepressants did not seem to have any effect on grey matter in people with OCD.
Radua, Joaquim and Mataix-Cois, David - Voxel-wise meta-analysis of grey matter changes in obsessive-compulsive disorder British Journal of Psychiatry November 2009, 195(5), 393-402