PROVIDENCE, R.I. [Brown University] — In exploring how serotonin affects how people learn and adapt to changes, researchers found that the neurotransmitter helps reduce "belief stickiness" — the tendency to get stuck on an old idea despite new contradicting evidence.

According to the researchers, the discovery holds important implications for the understanding and treatment of obsessive-compulsive disorder (OCD).

“These findings change the way we're studying the underlying drivers of OCD symptoms, which could therefore change the therapeutic approach,” said Frederike Petzschner, an assistant professor of cognitive and psychological sciences at Brown University. “The better we understand the mechanism of the disorder, the more effectively we can intervene within a psychotherapeutic intervention.”

Petzschner, who is affiliated with Brown’s Carney Institute for Brain Science, and collaborators at the University of Zurich, the Swiss Federal Institute of Technology in Zurich and the Universidade de Lisboa in Portugal published their findings in Nature Mental Health.

Researchers have known that the brain chemical serotonin can improve cognitive flexibility, but the way it did this hasn’t been clear. To get some answers, Petzschner and study co-author Vasco Conceição at the Universidade de Lisboa brought a computational psychiatry perspective to studying OCD.

They designed and conducted an experiment in which participants were given the selective serotonin reuptake inhibitor (SSRI) escitalopram, which increases serotonin, and asked to perform a task assessing belief stickiness.

“Belief stickiness, a term we came up with, is a characterization of a common phenomenon that involves difficulty detecting that the world has transitioned from one stage to another,” Petzschner said.

For the experiment, researchers gave 50 volunteers either a dose of escitalopram or a placebo. All participants played a computer game in which they had to collect different shells. The goal was to collect shells that provide pearls (which correlate to points) and avoid shells that contain dirt (which subtract points). As the game went on, the “seasons” would change: a shell that used to give pearls might start giving dirt. To win, players had to constantly infer which season a shell was currently in.

“A participant’s performance in the game served as evidence to the degree that they understand there are structures and dynamics in the environment that are reflecting something like a season, which is different from learning about an outcome through trial and error,” Petzschner said.

The researchers used computational models to compare task performance and correlated it with the level of escitalopram — and therefore, serotonin — in the participant’s blood.

The study showed that participants with sufficiently high escitalopram plasma levels had less belief stickiness, and therefore better inference about seasons (or the state of their world at that time), than participants who had been given the placebo.

Escitalopram, which is commonly known in the U.S. by the brand name Lexapro, is considered a front-line treatment for OCD. The inverse relationship of escitalopram with belief stickiness may explain the therapeutic effect of SSRIs on obsessive-compulsive disorder, the researchers concluded.

Understanding OCD: Habits vs. beliefs

Petzschner said the findings suggest a new approach to understanding OCD. She explained that an old theory posits that the repetitive behavior seen in OCD, such as obsessive hand-washing, is a habit.

“We thought that it has much more to do with not understanding that the state of the world has changed,” Petzschner said. “In other words, the person with OCD does not believe that the state of their hands, or the state of their world, has changed through hand-washing. They believe their hands are still dirty despite contradictory evidence.”

The study authors noted that exaggerated belief stickiness is exemplified by obsessions — “sticky” thoughts that persist despite contradicting evidence. None of the study participants had been diagnosed with OCD, but those whose test results showed that they had more obsessions had greater belief stickiness and worse state inference.

The team’s hypothesis was that SSRIs help OCD by allowing the person to update structure and understand more quickly that there have been changes in the environment.

“We were actually surprised at how strongly the results supported our hypothesis,” Petzschner said.

Petzschner said a shift in thinking could make OCD treatments more effective.

"If a single dose of an SSRI produces an acute boost in belief updating, the obvious next step is to schedule psychotherapy within that same window, when the brain is most receptive to revising old patterns," Petzschner said. She noted that pairing medication with therapy in a single session is an approach gaining traction in psychiatric care.

Funding for the study was provided by the René and Susanne Braginsky Foundation, the University of Zurich, Fundação para a Ciência e a Tecnologia, Portugal, the Tourette Association of America, and the Brainstorm Program at the Carney Institute.