What's Happening in the Brain
Obsessive behaviours can emerge for the first time in people with dementia, they can be the exaggeration of a person's prior behaviour, or they can get worse in a person who has always had an obsessive behavioural disorder. Either way, they should share some fundamental brain features, although this is not yet fully understood.
At present, obsessive-compulsive disorder, as the psychiatrists use the term, is thought of as a developmental problem. Several parts of the brain are implicated, with disruption of connections between brain structures known as the anterior cingulate, the basal ganglia, the thalamus and the frontal lobes (and within them, what are known as the orbitofrontal cortices).
Still, compulsive-like disorders can occur in adults in many settings. For example, they have been associated with several medical disorders, perhaps as a physiologic consequence, or a psychological reaction, or even from treatment. They are also seen in schizophrenia (especially the paranoid subtype) in association with dementia, in bipolar disorders, or as an exaggeration of lifelong traits. In general, randomized, placebo-controlled trials suggest that many patients respond to selective serotonin reuptake inhibitors while others respond to antipsychotic drugs. This suggests that the brain chemicals serotonin and dopamine are involved in these disorders.