By Emma Young

At the heart of obsessive-compulsive disorder (OCD) are the intrusive, often distressing, thoughts. My skin is dirty… I must have left the gas on and my house will burn down… But why do some obsessive thoughts compel the person to act on them, while others don’t? And how are some people with OCD able to control the compulsion to act – to repeatedly wash their hands, or to go home to check appliances, for example – while others can’t?

As the authors of a new study on OCD, published in Clinical Psychological Psychotherapy, point out: “A single negative intrusive thought may result in an avalanche of compulsive behaviour, whereas even hours of intrusive thought may prompt little or no compulsive behaviour in some individuals.”

An earlier internet-based pilot study of people diagnosed with OCD, by the same team, suggested that if the obsessive thoughts are accompanied by matching illusory sensory perceptions (preoccupation with the idea of one’s house burning down accompanied by the smell of smoke, for example), this makes the experience more intense, and is more likely to lead to compulsive behaviour.

To investigate more thoroughly, Steffen Moritz at the University of Hamburg, Germany, and his colleagues, recruited 34 people diagnosed with OCD, and who were being treated at Hamburg University’s medical centre. Interviews established the severity of the participants’ obsessive and compulsive symptoms, and they took a battery of tests, including the research team’s own Sensory Properties of Obsessions Questionnaire.

Around three quarters of the patients reported experiencing obsessive thoughts with sensory properties. Obsessions concerning contamination – with having dirt, or perhaps blood, on the skin, for example – were particularly associated with sensory experiences, especially involving vision or touch, such as a perception of a tingling in the fingers.

The researchers also found that the more a patient’s thoughts were accompanied by these sensory experiences, the less control they had over their compulsions. “If [the finding is] replicated, the extent of sensory experiences may explain why some patients act upon obsessional beliefs,” they write.

Why did some patients report sensory experiences during obsessional thoughts, when others didn’t? It’s possible that it stems from an overall more vivid imagination, the researchers suggest.

If imagination is shown by future work to be an important factor, then in theory, this might open up new approaches to treatment. Perhaps teaching patients how to use vivid mental imagery to counteract undesirable perceptions might help. However, as the researchers also note, there may a risk that this would also amplify negative images.

—If it is absurd, then why do you do it? The richer the obsessional experience, the more compelling the compulsion

Emma Young (@EmmaELYoung) is Staff Writer at BPS Research Digest