Neuroimaging Applications in Tourette's Syndrome.

Tics are neurodevelopmental hyperkinetic symptoms typically associated with unpleasant sensory experiences called premonitory urges. Tourette syndrome (TS) is the primary chronic tic disorder for which medical surveillance is most frequently required, and is associated with a complex phenotypical spectrum encompassing different types of abnormal behaviors. Animal models of tics support their link to phasic activity changes throughout the sensorimotor loop of the cortico-basal ganglia-thalamo-cortical network. Event-related functional magnetic resonance imaging (fMRI) studies on patients with TS showed that the supplementary motor area relays preparatory signals related to tics to the primary motor area and other cortical regions relevant to action monitoring, following which cortico-basal ganglia-thalamo-cortical activation leads to the manifestation of tics. Despite their methodological heterogeneity, structural MRI studies highlighted the existence of anatomical markers of distinct sub-phenotypes of the TS spectrum. Initial evidence suggests that combining MRI structural methods and functional intrinsic connectivity assessed during resting state could even discriminate between TS patients and control groups. MR-spectroscopy and positron emission tomography studies suggest that TS may be related to a complex interplay between different neurotransmitters (particularly dopamine, GABA and glutamate), but discrepancy across studies prevents firm conclusions. Recent volumetric, cortical thickness and fMRI studies results showed an association between premonitory urges and somatosensory and insular cortical regions, involved in the processing of interoceptive and enteroceptive stimuli and motor output modulation. Finally, both structural and functional MRI studies have provided important support to the subtyping of the TS spectrum with respect to behavioral co-morbidities, in line with a "dimensional approach" to the classification of neuropsychiatric disorders, which is based on the identification of neurocognitive endophenotypes and of their anatomical substrate.