OCD: Fear: PET: Amygdala activity in obsessive-compulsive disorder with contamination fear: a study with oxygen-15 water positron emission tomography.

Entrez PubMed: "Previous imaging studies of obsessive-compulsive symptom states have implicated frontal-striatal and limbic regions in the pathophysiology of obsessive-compulsive disorder (OCD). Functional imaging studies, however, have yielded inconsistent results, presumably due to methodological differences (patient inclusion criteria, stimulus paradigm, imaging technique, and absence of control groups). . . Main effects of stimulus type (contamination vs. neutral) were found in bilateral occipital cortex in both groups. A significant group interaction effect was observed in the left amygdala reflecting enhanced activity in response to contamination stimuli in OCD patients. Sensitization effects were observed in the right amygdala in the OCD group; these paralleled an increase in levels of distress and obsessionality as well as a decrease in dorsolateral prefrontal activity. The findings of the present study are consistent with the hypothesis of decreased frontal-striatal control of limbic structures, specifically the amygdala, resulting in an inadequate fear response in OCD patients with contamination fear."

Amygdala activity in obsessive-compulsive disorder with contamination fear: a study with oxygen-15 water positron emission tomography.
van den Heuvel OA, Veltman DJ, Groenewegen HJ, Dolan RJ, Cath DC, Boellaard R, Mesina CT, van Balkom AJ, van Oppen P, Witter MP, Lammertsma AA, van Dyck R.
Psychiatry Res. 2004 Dec 30;132(3):225-37.