What causes OCD? © Getty Images

What causes obsessive-compulsive disorder?

Dr Luke Norman of the University of Michigan combined data from multiple studies to find the brain networks involved with OCD.

What is obsessive-compulsive disorder (OCD)?

OCD has two primary symptoms. First are the obsessive thoughts, which often revolve around fears of harm occurring to the person with OCD or their loved ones. The second symptom is the compulsive behaviours, which are a way the person tries to regulate their anxiety.

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Compulsions can be linked with the obsessions – somebody who fears contracting a disease might keep washing their hands. But the compulsions can also be unrelated: the person with OCD might think an event is more likely to happen if they fail to perform an action a certain number of times, for instance. For diagnosis purposes, we typically say the illness should interfere for at least an hour a day and cause significant impairment.

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One circuit in OCD sufferers’ brains is enlarged and works like an accelerator for compulsive behaviours. Another circuit, which detects when greater self control needs to be exercised, serves as a brake, but doesn’t perform as well as the equivalent in healthy people. In essence, OCD sufferers know they need to slow down, but their brake pedal isn’t working as well as it needs to.

What causes OCD? © Getty Images

Why study brain scans?

It’s been hypothesised that brain networks involved in error processing and the ability to stop inappropriate behaviours – inhibitory control – are important in OCD. This is often measured in experimental tests like the stop signal task: participants are asked to press a button every time they see a picture on a screen, unless they hear a sound after the image is displayed. Previous studies that used this type of task within a functional MRI scanner to look at abnormalities in brain activation have provided inconsistent findings, possibly due to small sample sizes.

We collected data from 10 studies and put them together in a meta-analysis with a combined sample of 484 participants.

Which brain networks are involved?

OCD is a disorder of specific brain circuits. We think there are two main ones. First the ‘orbito-striato-thalamic circuit’, involved particularly in habits – it’s physically enlarged in OCD and over-activated when patients are presented with pictures or videos related to their fears, so it works like an accelerator pedal on compulsive behaviours.

The second is the ‘cingulo-opercular network’, which is involved in detecting when you need to engage greater self-control over your behaviour. In our meta-analysis, we found that patients showed increased activation in this brain network, but they performed more poorly during the inhibitory control task itself. The cingulo-opercular network is like a brake on ongoing behaviour: while patients with OCD show more activation in this brain network, it’s not bringing about the subsequent changes in behaviour that we would normally see in healthy people.

What have you found out about treatments for OCD?

Psychotherapy is very important for OCD, particularly cognitive behavioural therapy. This involves getting patients to gradually approach the things they’re afraid of and learn that bad things don’t happen when they’re exposed to OCD triggers. We’re doing a big study on that now, looking at brain scans before and after treatment, to examine whether the two brain networks show more normal activation patterns as patients get better. Scientists have also been exploring repetitive transcranial magnetic stimulation to target the cingulo-opercular network. It does appear to have quite good efficacy for OCD.

© Sam Falconer

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