The neuroscience of obsessive-compulsive disorder
As awareness of obsessive-compulsive disorder (OCD) has grown over the years, so has the degree to which the disorder is misunderstood. For example, one common misperception is that individuals who suffer from OCD all engage in repetitious rituals like hand-washing, repeatedly checking the locks, or flicking the light switch on and off a specific number of times. While some people with OCD do experience ritualistic compulsions, this is not a necessary component of an OCD diagnosis. In fact, up to 60% of OCD cases don't display clear compulsive behavior; instead, these "primarily obsessive" individuals are plagued only by obsessive thinking--although that persistent obsessive thinking in and of itself can be very disruptive to their lives.
The neurobiology of OCD is still being sorted out, but current perspectives suggest that at the root of OCD are problems with communication between three brain areas: the cortex, striatum, and thalamus (i.e. cortico-striato-thalamic pathways). The pathways that connect these areas are involved in both the initiation and termination of behavior; researchers believe that an imbalance within these pathways may cause individuals with OCD to get stuck in repetitive loops of thought and behavior. Because selective serotonin reuptake inhibitors (SSRIs) can be helpful in treating OCD, it has been hypothesized that serotonin may play an important role in the disorder. Cortico-striato-thalamic pathways, however, also utilize dopamine heavily, so some have suggested that an imbalance between serotonin and dopamine levels may be at the root of the disorder.
In a study recently published in Nature Neuropscyhopharmacology, an international team of investigators (Radua et al.) focused on how white matter tracts in the brain might be affected in OCD. While the cell bodies of neurons make up what is known as grey matter, white matter is mostly made up of myelinated axons. Axons are the long extensions of neurons that carry information away from the cell body, making it possible for that signal to then be passed to an adjacent neuron. Myelin is a white insulatory material that covers axons and facilitates the conduction of a signal down the axon.
Radua et al. compiled the results of 34 studies that used methods like diffusion tensor imaging to examine white matter integrity in OCD patients. They found that widespread abnormalities in the white matter of OCD patients (as compared to non-OCD patients) had been reported. Aberrations were most frequently seen in tracts that make up the corpus callosum and cingulum. The corpus callosum is a large fiber bundle that connects the left and right hemispheres of the brain, while the cingulum is a group of fibers that connects different structures of the limbic system. Some of the reported abnormalities were related to the cortico-striato-thalamic pathways typically associated with OCD, but there were also published reports of anomalies beyond these pathways.
The implications of these findings aren't completely clear, but they reinforce the view that OCD is a disorder that results from faulty communication between multiple brain areas. They also, however, support hypotheses that suggest that the pathophysiology of OCD expands beyond the cortico-striato-thalamic loops mentioned above. Thus, our current perspectives on OCD are probably only beginning to scratch the surface of what is likely an irregular and complex interplay between several areas of the brain.
Radua, J., Grau, M., van den Heuvel, O., Thiebaut de Schotten, M., Stein, D., Canales-Rodríguez, E., Catani, M., & Mataix-Cols, D. (2014). Multimodal Voxel-Based Meta-Analysis of White Matter Abnormalities in Obsessive–Compulsive Disorder Neuropsychopharmacology, 39 (7), 1547-1557 DOI: 10.1038/npp.2014.5
