It appears that a specific neural circuit connecting the orbito-frontal cortex (OFC) and the basal ganglia may be responsible for the obsessive and intrusive thoughts and compulsive behaviours that characterize OCD. This has become known as the worry circuit.

The OFC has been shown to be associated with decision-making, yet research also shows that it is linked to urges for sex, aggression, and bodily excretion. This helps explain why this brain region may be linked to overwhelming drives to enact certain behaviours over which people feel they have no control.

The worry circuit links the OFC and a region called the caudate nucleus, which is part of the basal ganglia. This region sends the strongest impulses to the thalamus, which triggers further thinking about these impulses. A series of behaviours may then be conducted, thus reducing the impulse - for example, washing our hands if we feel we might have touched something contaminated to help us to feel clean and reduce anxiety. Generally speaking, the caudate nucleus is associated with the initiation and subsequent inhibition of movements, especially those that result in positive outcomes. It is also involved with cognition, emotion, attention, and habit formation.

A dysfunction in this circuit has been proposed as an explanation of OCD, where behaviours linked to certain urges or impulses fail to reduce the urge, and even seem to reinforce it - for example, triggering repetitive sequences of behaviors, such as checking that electrical appliances are switched off or that doors are locked over and over again.

Thus, overactivity in the neural pathway connecting the OFC and caudate nucleus could trigger compulsive behaviour.  Abnormalities of the caudate nucleus could mean that once initiated, goal-directed behaviour fails to be inhibited, leading to relentless cycles of compulsive behaviour.

One strength of this neurological explanation of compulsive behaviour is that it is supported by research evidence.

For example, Thobois et al. (2004) conducted a case study on a man who underwent brain surgery for a blood clot on his caudate nucleus, and subsequently suffered from compulsions but without obsessive thinking.

This study provides important evidence for the localization of repetitive behaviour to the caudate nucleus, as the patient experienced no such behaviour before the surgery.

The research by Thobois et al (2004) has been supported by other case studies. Katz and Flemming (2014) reported the case of a 31-year-old woman with lesions on the caudate nucleus, also caused by internal bleeding. She developed an obsession with germs and contamination that required hospitalization. She was successfully treated by a combination of behavioural therapy and medication.

Gillian et al (2014) carried out a quasi-experiment with both patients with OCD and a control group.  Both groups received fMRI scans while they were asked to press a pedal to avoid shock to the wrist over an extended time. In the final stage of testing, the shock block was removed to assess whether the pedal-pressing behaviors of the patients could be discontinued.

The researchers found that patients with OCD were less capable of stopping their pedal-pressing habits than the control group. In addition, individuals in the OCD cohort were more likely to have hyperactivation in the caudate nucleus after the shock block was removed, compared with those without OCD.

Although the physiology of the worry circuit is fairly well understood, the reason why people develop abnormalities in this region is still open to debate. As these pathways are rich in serotonin receptors, it has been argued that deficiencies of this neurotransmitter may be responsible and this may result from both genetic and environmental factors. For example, serotonin production is inhibited by high levels of the stress hormone cortisol, helping to explain why traumatic life events may trigger OCD-like symptoms. Yet such explanations are arguably over-simplified; many people who have faced such life events do not develop symptoms, and this is no doubt mediated by protective factors including social support and biochemical factors that underpin resilience.

As many as 230 genes have been linked to OCD, making it a polygenic disorder. As noted above, both serotonin and dopamine may be linked in some way to OCD.  The enzyme which breaks down dopamine is called catechol-O-methyltransferase [COMT] and the gene that codes for this enzyme is called the COMT gene. Karayiorgou et al. 1997 identified a specific version of this gene called the ‘low activity’ allele which is more common in people with OCD. Low levels of this enzyme could lead to a buildup of dopamine in the synapse, which could be linked to the repetitive behaviour cycles common to people with OCD.

Another set of genes that have been the focus of more recent attention are the SLITRK genes - which are responsible for synapse regulation. Specifically, the SLITRK5 gene plays an important role in learning from our experiences as it has links with neuronal growth and neuroplasticity. This gene is involved in coordinating processes controlled by BDNF (brain-derived neurotrophic factor) which is essential to the healthy growth and function of our neural networks.

In terms of practical applications of genetic explanations of OCD, research on the role of the SLITRK family of genes suggest that treatment designed to increase BDNF may be beneficial for people with OCD. Diet, exercise, time spent with others, and even being outdoors in the sunshine have all been shown to increase this essential brain chemical.

One strength of the genetic explanation of compulsive behaviour is that there is strong support from animal experiments such as Schmelkov et al. (2010).

Although having certain genes may make one vulnerable to a disorder, environmental experiences, especially those linked to chronic stress and trauma, have a central role to play in gene expression. This means that OCD may result from an interaction between genetic vulnerability and a person’s life experiences. For example, Timpano et al. (2007) found that 54% of a sample of 265 people living with OCD had experienced at least one traumatic life event and there was a positive correlation between the number of such events and the severity of symptoms. However, the study demonstrated that people who exhibited compulsive checking and obsessions with order were most likely to have experienced traumatic life events than other types of compulsions.

Checking for understanding