Disorders of compulsivity share a common pattern of decision-making and brain structure
New research conducted at the University of Cambridge suggests people affected by binge eating, substance abuse, and obsessive compulsive disorder all share a common pattern of decision making and similarities in brain structure. In this study, primarily funded by the Wellcome Trust and published in the journal Molecular Psychiatry, researchers found that people who are affected by disorders of compulsivity have lower grey matter volumes, or fewer nerve cells, in caudate and medial orbitofrontal cortex involved in keeping track of goals and rewards.
In our everyday life we make decisions based on habits or aimed at achieving specific goals, or “goal-directed” decision making. Cambridge researchers theorise that decision-making arises from two computational learning mechanisms, model-based and model-free. For instance, when driving home from work, we tend to follow habitual choices—our “autopilot” mode (model-free mechanism)—as we know the road well. However, if we move to a nearby street, we will initially follow a “goal-directed” choice (model-based mechanism) to find our way home, unless we slip into “autopilot” and revert to driving back to our old home. We cannot always control the decision-making process and make systematically thought-out choices, even when we know they are bad for us. In many cases this will be relatively benign, such as being tempted by a cake whilst on a diet, but in extreme cases it can lead to disorders of compulsivity.
In order to understand what happens when our decision-making processes are compromised, a team of researchers led by the Department of Psychiatry at the University of Cambridge compared almost 150 individuals with disorders including methamphetamine dependence, obesity with binge eating and obsessive compulsive disorder, while a control group was represented by healthy volunteers of the same age and gender.
The study participants firstly took part in a computerised task to test their ability to make choices aimed at receiving a reward over and above making compulsive choices. In the second study, the researchers compared brain scans taken using magnetic resonance imaging (MRI) in healthy individuals and a subset of obese individuals with or without binge eating disorder (a subtype of obesity in which the person binge eats large amounts of food rapidly).
The investigators showed that all of the disorders demonstrated a shift from goal-directed behaviours towards automatic habitual choices. The MRI scans revealed that obese subjects with binge eating disorder have lower grey matter volumes—a measure of the number of neurons—in the orbitofrontal cortex and striatum of the brain compared to those who do not binge eat. These brain regions are involved in keeping track of goals and rewards. Moreover, in healthy volunteers lower grey matter volumes were also associated with a shift towards more habitual choices.
Dr Valerie Voon, the leading researcher of the study, claims: “Seemingly diverse choices – drug taking, eating quickly despite weight gain, and compulsive cleaning or checking – have an underlying common thread: rather that a person making a choice based on what they think will happen, their choice is automatic or habitual… Compulsive disorders can have a profoundly disabling effect of individuals. Now that we know what is going wrong with their decision making, we can look at developing treatments, for example using psychotherapy focused on forward planning or interventions such as medication which target the shift towards habitual choices.”
Voon, V., Derbyshire, K., Rück, C., Irvine, M. A., Worbe, Y., Enander, J., Schreiber, L. R. N., Gillan, C., Fineberg, N. A., Sahakian, B. J., Robbins, T. W., Harrison, N. A., Wood, J., Daw, N. D., Dayan, P., Grant, J. E., Bullmore, E. T. Disorders of compulsivity: a common bias towards learning habits. Molecular Psychiatry, 2014; DOI: 10.1038/mp.2014.44