Deep brain stimulation helps some people with obsessive-compulsive disorder (OCD), but no one was quite sure why it is effective. A new study offers an explanation: the stimulation has surprisingly pervasive effects, fixing abnormal signalling between different parts of the brain.
A small number of people with difficult-to-treat OCD have had electrodes permanently implanted deep within their brain. Stimulating these electrodes reduces their symptoms.
To work out why stimulation has this effect, Damiaan Denys and Martijn Figee at the Academic Medical Center in Amsterdam, the Netherlands, and colleagues recorded neural activity in people with electrodes implanted into a part of the brain called the nucleus accumbens. This region is vital for conveying motivational and emotional information to the frontal cortex to guide decisions on what actions to take next. In some people with OCD, feedback loops between the two get jammed, leading them to do the same task repeatedly to reduce anxiety.
The researchers took fMRI scans as participants rested. In 13 people with OCD and implanted electrodes, there was continuous and excessive exchange of signals between the nucleus accumbens and the frontal cortex that was not seen in 11 control subjects. When the electrodes were activated, though, the neural activity of both brain regions in the people with OCD became virtually identical to that in the controls.
The researchers also used EEGs to monitor electrical activity in the brain as the 13 people with OCD viewed images linked with their obsessions, such as cleaning toilets. This time, the team observed excessive activity in the frontal cortex – and again, this activity disappeared when the electrodes were activated.
"The most striking thing is that stimulation doesn’t just affect the nucleus accumbens, but the whole network linked up with the cortex," says Figee.
The study suggests that the electrodes do more than normalise brain activity at the site where they are implanted, as had been assumed. Rather, they appear to repair entire brain circuits that had been faulty. “It resets and normalises these circuits,” says Figee.
Thomas Schlaepfer at the University of Bonn, Germany, points out that such work may allow researchers to use deep brain stimulation to learn about the causes of OCD as they treat it. “It will serve as a research platform informing us about the underlying neurobiology of such disorders,” he says.
(Image courtesy: Michael S. Okun)