On the surface, Tourette’s syndrome and obsessive-compulsive disorder (OCD) seem to have little in common. Tourette’s is characterized by repetitive involuntary facial or vocal tics, whereas OCD sufferers have all-consuming thoughts and overwhelming urges to perform certain actions. But 50 to 70 percent of people with Tourette’s also have OCD, and recent studies suggest that the same genetic roots may underlie both conditions [see “Obsessions Revisited,” by Melinda Wenner Moyer; Scientific American Mind, May/June 2011]. Now a new study published in Neurology may help scientists further understand how the disorders overlap and differ by revealing several key differences in the brain activity of Tourette’s patients with and without OCD.

Andrew Feigin and his colleagues at North Shore LIJ Health System in Manhasset, N.Y., scanned the brains of 12 unmedicated Tourette’s patients—some of whom also had OCD—and 12 healthy subjects using positron-emission tomography, which reveals patterns of brain activity. Compared with healthy controls, those who had Tourette’s exhibited more activity in the premotor cortex and cerebellum, regions that handle motor control, and less activity in the striatum and orbitofrontal cortex, areas involved in decision making and learning. These findings support the idea that the symptoms of the disorder may arise from the brain’s inability to suppress abnormal actions using decision-making skills.

When the researchers compared the Tourette’s patients who had OCD with those who did not, they found that the patients who had both disorders exhibited greater activity in the primary motor cortex and precuneus, an area that plays a role in self-awareness. Previous research has suggested that in patients who suffer from both disorders, OCD might show up more in the form of compulsions than obsessions, and these findings support that idea: the increased activity of the precuneus may reflect individuals’ efforts and ability to resist obsessive thought, and the motor cortex may be more active because OCD is manifesting itself more physically than mentally.

Although the neural networks that seem to be affected in Tourette’s and OCD are distinct, they nonetheless involve brain regions with similar functions—in particular, motor activity and decision making. The discovery isn’t all that surprising, Feigin says, given that tics are actually quite similar to compulsions—both, after all, involve extra involuntary movements.