The majority of recent scientific papers on OCD deal with the biomedical aspect of the illness. Research in this area is so impressive that the United States Congress has seen fit to include OCD among a small group of mental disorders covered by the Mental Health Equitable Treatment Act. Thus, OCD has been officially recognized as being just as “biologically based” as diabetes or heart disease, and therefore deserving of the same insurance coverage.

The occurrence of obsessions and compulsions has been irrefutably linked to chemical changes in the brain. Studies of neurotransmitters—chemicals that serve as messengers  between brain cells—reveal that one called serotonin is abnormally active in individuals who suffer from obsessive-compulsive disorder. When concentrations of this neurotransmitter are altered by medications, the severity of the disorder changes dramatically.

Studies of brain anatomy demonstrate that OCD sufferers possess a larger number of working brain cells than other people. This interesting finding makes intuitive sense, since their problem is that they think too much. As it turns out, all of us are born with an excess of brain cells. Their absolute numbers decrease through the years as our brains become more streamlined and efficient, a process referred to as “pruning.” There is speculation that obsessive-compulsive disorder may reflect a failure in this aspect of brain maturation.

Studies of brain physiology provide the most dramatic evidence for obsessive-compulsive disorder’s biochemical roots. Advanced brain-scanning techniques, such as positron emission tomography (PET) and magnetic resonance imaging (MRI), allow researchers to see where brain cells are actively at work. For instance, if a person is scanned while studying a book, the visual center in the back of the brain lights up on the screen. If a person is solving a puzzle, the frontal lobe stands out. In people suffering from obsessions, it is the basal ganglia, a grape like cluster of cells located deep in the brain, that lights up abnormally. Aberrations in the basal ganglia are apparent in obsessional patients at rest, and they increase when patients are exposed to situations that intensify their obsessional fears. Some experts now even refer to OCD as a “basal ganglia disease.” Stroke, head injury, and carbon monoxide poisoning can all cause the occurrence of obsessions and compulsions when they damage the nerve cells of this region. Even streptococcal infections (“strep throat”) can lead to obsessive-compulsive disorder through the production of antistreptococcal antibodies that damage the basal ganglia.

Why would abnormalities in the basal ganglia cause obsessions? Two decades ago, all that was known for sure about the basal ganglia was that it coordinated involuntary movements, those body motions that are done without the need of our thinking about them, such as walking. Recent research shows, however, that the basal ganglia coordinates not only movements, but also our awareness of our thoughts.

Nobel Prize–winning researcher Gerald Edelman has done some of the key work in this area. In his recent book A Universe of Consciousness, written with Giulio Tononi, Edelman concludes that conscious awareness results from the electrical discharges of elaborate, ever-changing meshes of rapidly firing brain cells called “dynamic cores.” While the basal ganglia does not contain these cores, Edelman posits, its cells are arranged in a manner that allows them to direct what goes into and comes out of them. In certain situations when the basal ganglia is defective, he notes, “obsessions are forced upon one’s consciousness.”

Harvard’s Roger Pitman suggests a helpful analogy: The basal ganglia works like a computer using a match-mismatch mechanism to identify certain thoughts that are of special importance for survival—specifically, those conveying a danger message. In the basal ganglia, an image or idea is compared against preexisting wishes, fears, and memory traces. If there is a match, the thought harmonizes with associations about the self and the world, and it is dismissed from consciousness. If there is not a match, the idea is returned to the cortical areas of the brain for further conscious appraisals. Normally, an idea is dismissed when it should be—when, through conscious reflection, an individual realizes that danger does not lurk. In obsessive-compulsive disorder, however, the idea is kept in strong conscious awareness long after the individual himself knows that it should be dismissed.