Tics are a hyperkinetic movement disorder. They are defined as movements or sounds that typically resemble voluntary actions, but appear repetitive, often exaggerated in intensity and are not bound to a certain social context. Tics are typically preceded by a phenomenally strong sensory experience known as the premonitory urge. Crucially, and different from most other hypekinesias, tics can be voluntarily inhibited on demand. Tics are prevalent in many disorders, including neurodevelopmental syndromes and neurometabolic or neurodegenerative conditions, but are most commonly documented in primary tic disorders. Tourette syndrome (TS) is the prototypical primary tic disorder encountered in clinics and affects up to 1% of school-age children.
The predominant pathophysiological models of tic disorders view the abnormal motor behaviours as the result of pathological gain increase of neuromotor signals within the circuitry that also generates voluntary actions. Crucially, they introduce a concept of voluntary tic inhibition, as the capacity to exert top-down inhibitory control to temporarily suppress the pathologically increased neuromotor activity. In-depth understanding of voluntary tic inhibition may, thereby, have important implications for the regulatory control of tics, including the development and implementation of more efficient treatment interventions.
In the medical literature, the term of voluntary tic inhibition has also been synonymously used with tic control. However, the latter term has also been used to describe a range of different processes related to tic reduction not relevant to the specific designation that is suggested here. Voluntary tic inhibition (or voluntary tic control) denotes a specific cognitive effortful process of tic suppression. The inhibitory process is an intentional and goal-directed activity, which is specifically decided and initiated by the patients themselves, and, thereby, is also amenable to reinforcement.
In a progression of five consecutive studies, we here explore the behavioural associations and neural correlates of voluntary tic inhibition in adolescents and adults with tic disorders and TS. First, we explore the putative association between premonitory urges and voluntary tic inhibition. Importantly, we provide experimental evidence to support the view that the two processes are not directly related. We then examine the characteristic capacity of voluntary tic inhibition to specifically suppress tic movements without affecting motor performance for voluntary actions. We assess the relation between the voluntary motor system and voluntary tic inhibitory control and introduce the concept of neuromotor noise in the pathophysiology of tic disorders to explain our findings. We also discuss the spatial characteristics of voluntary tic inhibition and provide a model of somatotopic-specificity of voluntary inhibitory tic control. Finally, we review the pathophysiological concept of deficient action control underlying the manifestation of tics and provide experimental evidence against it. We also examine the neural correlates of inhibitory control both over voluntary actions and involuntary tic behaviours. Current limitations in the research of voluntary tic inhibition are discussed and future directions of the scientific study of voluntary tic inhibitory control are suggested.
