Obsessive-compulsive disorder (OCD) is an anxiety-related mental condition that affects 1-2% of the population. It is severely debilitating and can strike anyone regardless of age, gender and cultural background. The majority of us will be familiar with its overt symptoms, such as obsessive checking, compulsive washing or performing routines ritualistically, but less is known about how the disorder manifests in the thought processes behind the compulsions.

Our research, recently published in Neuroimage: Clinical, set out to discover what happens in the brain when people experience a spurious fusion between negative thoughts and actions. We found that the worse the condition was, the greater the amount of high frequency brain activity in the precuneus, a part of the cerebral cortex (the outer part of the brain). This is a key area because it has extensive connections to widespread brain regions and acts as a hub for the default network, a group of brain areas activated when we are internally focused with self-referential thoughts.

At some point in our life we all experience negative unwanted thoughts, like the thought or image of a loved one coming to harm. While the majority of us are capable of brushing away these thoughts without much trouble, an OCD sufferer can’t. They’re likely to misinterpret the thought as being important; a dangerous one that requires preventive action.

One variety of misinterpretation that OCD sufferers experience is Thought-Action Fusion, or TAF. It is the belief that thoughts and actions are inextricably linked, and that thoughts, mostly negative, can have real-world consequences. For example, an OCD sufferer who has a thought of her child becoming ill would feel that by merely having the thought, she has increased the likelihood of her child becoming ill in reality. This leads to an inflated sense of responsibility for harm, which causes a great deal of anxiety and guilt.

It also comes with the urge to do something to avert the catastrophe. Sufferers may then engage in compulsive behaviours or mental rituals to neutralise the imminent threat. While this behaviour will provide some relief, by reducing anxiety, the effect is short-lived. Eventually it only serves to reinforce the beliefs that the compulsions are working and that leads to a vicious circle of obsessions and compulsions.

Learning more about the biological basis of this underlying cognitive bias may help in the search for new treatments for people with OCD so we induced thought-action fusion in two groups of participants – one with high levels of obsessive-compulsive symptoms (high-OC) and another with low levels of symmptoms (low-OC) – and recorded the accompanying brain activity using electroencephalography (EEG), which measures electrical activity along the scalp.

We found that the high-OC group experienced a significantly greater magnitude of thought-action fusion than the low-OC group, and reported far higher levels of anxiety, guilt, and the urge to neutralise. When we looked at the accompanying brain activity we found that the greater magnitude was correlated with more high-frequency brain activity in the precuneus.

Now we have found what appears to be a neural signature for thought-action fusion, our next step is to explore ways to modify its activities which may help break the vicious circle in the OCD thinking process.