The Neural Substrates of Tics vs. Obsessive-compulsive Behaviours in Tourette Syndrome

Tourette Syndrome (TS) is a chronic neuropsychiatric disorder characterized by the presence of tics and is often comorbid with obsessive-compulsive symptoms (OCS). Tics and OCS share many phenomenological similarities as they are both characterized by repetitive behaviours that are often preceded by sensory phenomena; however, it not known how tics and OCS in TS differ neurobiologically from one another. Accordingly, 40 TS patients with varying severities of tics and OCS, as well as 20 healthy controls, completed an fMRI scan comprised of three functional tasks to probe different aspects of TS+OCD symptomatology. Firstly, to elucidate the neural correlates responsible for the development and modulation of urges, participants completed an emotional blink suppression paradigm. During the emotional inhibition of blinks, tic severity was found to be associated with activity in the superior frontal gyrus, whereas there were no significant associations with OCS severity. Secondly, participants completed an OCS provocation paradigm with stimuli from the checking, washing and symmetry symptom dimensions to determine the neurobiology of OCS in TS. In the patient group, negative associations between OCS severity and activity in the supramarginal gyrus, inferior frontal gyrus, sensorimotor cortex and precuneus were common among all the provocation conditions. Conversely, tic severity was only associated with activity in the anterior cingulate cortex for the symmetry condition. Finally, resting state functional connectivity analyses were conducted with seeds placed in the supplementary motor area (SMA), orbitofrontal cortex, insula, caudate and putamen. Within the patient group, tic severity was associated with greater connectivity between the putamen and the sensorimotor cortex; OCD severity was associated with less connectivity between the SMA and thalamus and between the caudate and precuneus. In general, the studies revealed that patients with greater symptom severities exhibited less activity in regions responsible for cortical control over cognitive and motor processing, and the activation of areas that may be more directly responsible for the occurrence of tics and OCS. The results suggest that tics and OCS are characterized by partially overlapping neural substrates that may explain why they are highly comorbid and phenomenologically similar, but also why they are still considered distinct symptom domains.Ph.D