The relative contribution of goal-directed and habit systems to psychiatric disorders.

Psychiatric disorders may be caused by underlying imbalances between goal-directed and habit systems in the brain. Numerous studies have aimed to establish whether this is because of a goal-directed system deficit, enhanced habit system, or both. This transdiagnostic approach to studying psychiatric disorders is increasingly popular. Maladaptive habitual behaviour is present in many disorders. It is the principal observation in disorders of compulsivity and is also present in other psychiatric disorders that are not primarily characterised by compulsive behaviour. The psychopathology that causes these disorders might be similar and could be targeted with specific treatment. Traditional categorical classification systems of psychiatric disorders do not reflect similarities in neurobiological dysfunction. The comorbidity and overlap between psychiatric disorders means that a dimensional classifications system based on underlying brain system dysfunction might be more appropriate. In this paper, the neural and neuromodulatory systems that contribute to goal-directed and habit systems are discussed. Account is taken of model-based and model-free computational learning mechanisms that are thought to give rise to goal-directed and habitual control respectively. Different psychiatric disorders that have a deficit in goal-directed behaviour or habit systems are then explored to see if there are similarities in the underlying neural systems despite differences in clinical presentation. It concludes that the relative contribution of goal-directed and habit systems in psychiatric disorders is not evenly distributed. Similar dysfunction of these systems might cause different psychiatric disorders. This neurobiological finding might influence classification systems and research into potential treatments.