Obsessive–compulsive disorder (OCD) is a mental and behavioral disorder in which an individual has intrusive thoughts and/or feels the need to perform certain routines repeatedly to the extent where it induces distress or impairs general function. As indicated by the disorders name, the primary symptoms of OCD are obsessions and compulsions. Obsessions are persistent unwanted thoughts, mental images, or urges that generate feelings of anxiety, disgust, or discomfort. Common obsessions include fear of contamination, obsession with symmetry, and intrusive thoughts about religion, sex, and harm. Compulsions are repeated actions or routines that occur in response to obsessions. Common compulsions include excessive hand washing, cleaning, counting, ordering, hoarding, neutralizing, seeking assurance, and checking things. Washing is in response to the fear of contamination. Ordering is the preference for tasks to be completed a specific way (e.g., organizing clothes a specific way). Hoarding is the collecting of unnecessary objects (e.g., collecting food wrappers). Neutralizing is the act of engaging in a ritual to make up for supposedly "bad behavior". Checking is the compulsion to check particular objects/places to ensure they are a certain way (e.g., checking to ensure the water is turned off). People with OCD tend to be overly cleanly, repeatedly count objects, and seek reassurance to avoid making a mistake. Many adults with OCD are aware that their compulsions do not make sense, but they perform them anyway to relieve the distress caused by obsessions.
The involvement of the orbitofrontal cortex and dorsolateral prefrontal cortex in OCD is shared with bipolar disorder, and may explain the high degree of comorbidity. Decreased volumes of the dorsolateral prefrontal cortex related to executive function has also been observed in OCD.People with OCD evince increased grey matter volumes in bilateral lenticular nuclei, extending to the caudate nuclei, with decreased grey matter volumes in bilateral dorsal medial frontal/anterior cingulate gyri. These findings contrast with those in people with other anxiety disorders, who evince decreased (rather than increased) grey matter volumes in bilateral lenticular/caudate nuclei, as well as decreased grey matter volumes in bilateral dorsal medial frontal/anterior cingulate gyri. Increased white matter volume and decreased fractional anisotropy in anterior midline tracts has been observed in OCD, possibly indicating increased fiber crossings. Cognitive models
Generally, two categories of models for OCD have been postulated. The first category involves deficits in executive dysfunction and is based on the observed structural and functional abnormalities in the dlPFC, striatum and thalamus. The second category involves dysfunctional modulatory control and primarily relies on observed functional and structural differences in the ACC, mPFC, and OFC.One proposed model suggests that dysfunction in the OFC leads to improper valuation of behaviors and decreased behavioral control, while the observed alterations in amygdala activations leads to exaggerated fears and representations of negative stimuli.Due to the heterogeneity of OCD symptoms, studies differentiating various symptoms have been performed.