Inhibitory control and impulsive responses in neurodevelopmental disorders

Several neurodevelopmental disorders are characterized by impulsive conduct. Patients with these disorders tend to act on a whim, performing unintentional actions that are inappropriate to the context and have undesirable and risky consequences. Impulsivity has been frequently ascribed to deficient ‘inhibitory control’ (impairment in the ability to suppress a planned or already initiated action). However, this deficit is unlikely to explain different clinical traits, such as in Tourette syndrome (TS), obsessive-compulsive disorder (OCD), or attention-deficit/hyperactivity-disorder (ADHD). In fact, evidence from behavioral studies is exceptionally mixed. This is likely because both impulsivity and inhibitory control are complex constructs. Notably, response inhibition has at least two neuropsychological domains: (1) reactive inhibition (the ability to stop a response outright when a stop instruction is presented); and (2) proactive inhibition (the ability to adapt movement according to the context the individual is in). Clinical characteristics are strikingly different across conditions, depending on motor inhibitory control impairments. For instance, both reactive and proactive domains are severely compromised in patients with OCD who have not previously been exposed to therapy/treatment. Patients with ADHD show a selective impairment of reactive inhibition, while patients with TS without previous exposure to therapy/treatment do not show any impairment of inhibitory control. Overall, there are many reasons why urges cannot always be controlled and they cannot be credited to one general impairment of motor inhibition. Therefore, identifying specific inhibitory impairments in urge control disorders might be extremely important in defining the disease and suggesting the most appropriate treatment. As the features of complex neurodevelopmental disorders are unlikely to focus exclusively on inhibitory control, further studies are urgently needed to explore how it interacts with other mental skills such as working memory and the ability to shift between tasks.