Examining the neural antecedents of tics in Tourette syndrome using electroencephalography
Tourette syndrome (TS) is a neurological disorder of childhood onset that is characterised by the occurrence of motor and vocal tics. TS is associated with cortical-striatal-thalamic-cortical circuit [CSTC] dysfunction and hyper-excitability of cortical limbic and motor regions that are thought to lead to the occurrence of tics. Importantly, individuals with TS often report that their tics are preceded by premonitory sensory/urge phenomena (PU) that are described as uncomfortable bodily sensations that precede the execution of a tic and are experienced as a strong urge for motor discharge. While tics are most often referred to as involuntary movements, it has been argued that tics should be viewed as voluntary movements that are executed in response to the presence of PU and bring temporary relief from the uncomfortable bodily sensations that are associated with PU. This issue remains unresolved but has very important implications for the design of clinical interventions for TS. To investigate this issue further, we conducted a study using electroencephalography (EEG). Specifically, we recorded movement-related EEG (mu and beta band oscillations) during (a) the immediate period leading up to the execution of voluntary movements by a group of individuals with TS and a group of matched healthy control participants, and (b) the immediate period leading up to the execution of a tic in a group of individuals with TS. We demonstrate that movement-related mu and beta band oscillations are not observed prior to tics in individuals with TS. We interpret this effect as reflecting the greater involvement of a network of brain areas, including the insular and cingulate cortices, basal ganglia nuclei, and the cerebellum, in the generation of tics in TS. We also show that beta-band desynchronization does occur when individuals with TS initiate voluntary movements, but, in contrast to healthy controls, desynchronization of mu-band oscillations is not observed during the execution of voluntary movements for individuals with TS. We interpret this finding as reflecting a dysfunction of physiological inhibition in TS, thereby contributing to an impaired ability to suppress neuronal populations that may compete with movement preparation processes.