Structural changes in the somatosensory system correlate with tic severity in Gilles de la Tourette syndrome

Gilles de la Tourette syndrome (GTS) is a neuropsychiatric disorder characterized by multiple motor and vocal tics. Previous structural MRI studies have identified regional abnormalities in grey matter, especially in the basal ganglia. These findings are consistent with the assumption of alterations in cortico-striato-thalamo-cortical circuits and dopaminergic neurotransmission playing a major role in the pathophysiology of GTS. Additionally, recent imaging studies suggested an involvement of sensory-motor cortices in the pathophysiology of GTS. However, little is known about the role of white matter changes in GTS. In this study, we aimed to examine whether GTS is associated with abnormalities in white matter microstructure and whether these changes are correlated with tic severity. In a morphometric study based on diffusion tensor MRI of the whole brain, we compared brain tissue diffusion characteristics between 15 unmedicated adults with GTS without psychiatric co-morbidity and 15 healthy age- and sex-matched controls. We performed voxel-based morphometry (VBM) of regional fractional anisotropy (FA) values to identify regional differences in white matter microstructure between the groups. We also tested for a linear relationship between regional FA values and clinical scores of tic severity. Probabilistic fibre tracking was applied to characterize anatomical connectivity of those areas showing differences in regional FA. Compared with healthy controls, GTS patients showed bilateral FA increases in white matter underlying the post- and precentral gyrus, below the left supplementary motor area, and in the right ventro-postero-lateral part of the thalamus. The peak increase in FA was located below the left postcentral gyrus. Probabilistic tractography identified transcallosal and ipsilateral cerebello-thalamo-cortical pathways of the somatosensory system passing through this subcortical region. In patients, regional FA in this region showed an inverse linear relationship with tic severity. These findings demonstrate, for the first time, structural alterations in somatosensory pathways in GTS. Changes of water diffusion characteristics point towards reduced branching in somatosensory pathways in GTS patients. The negative correlation between higher regional FA values and fewer tics suggests that these alterations of white matter microstructure represent adaptive reorganization of somatosensory processing in GTS.

Right hemisphere contributions to imitation tasks

Humans imitate biological movements faster than non-biological movements. The faster response has been attributed to an activation of the human mirror neuron system, which is thought to match observation and execution of actions. However, it is unclear which cortical areas are responsible for this behavioural advantage. Also, little is known about the timing of activations. Using whole-head magnetoencephalography we recorded neuronal responses to single biological finger movements and non-biological dot movements while the subjects were required to perform an imitation task or an observation task, respectively. Previous imaging studies on the human mirror neurone system suggested that activation in response to biological movements would be stronger in ventral premotor, parietal and superior temporal regions. In accordance with previous studies, reaction times to biological movements were faster than those to dot movements in all subjects. The analysis of evoked magnetic fields revealed that the reaction time benefit was paralleled by stronger and earlier activation of the left temporo-occipital cortex, right superior temporal area and right ventral motor/premotor area. The activity patterns suggest that the latter areas mediate the observed behavioural advantage of biological movements and indicate a predominant contribution of the right temporo-frontal hemisphere to action observation-execution matching processes in intransitive movements, which has not been reported previously.

Autosomal dominant myoclonus-dystonia and Tourette syndrome in a family without linkage to theSGCEgene

The objective of this study was to report clinical details and results of genetic testing for mutations in the epsilon-sarcoglycan (SGCE) gene, the Slit and Trk-like 1 (SLITRK1) gene and for linkage to the DYT15, DYT1, and DRD2 gene loci in a family with autosomal dominant myoclonus-dystonia (M-D) and Gilles de la Tourette syndrome (GTS). Fourteen family members, from three generations, underwent a detailed clinical assessment and donated DNA samples. The SGCE and the SLITRK1 gene were sequenced and investigated by gene dosage analysis in selected family members. Linkage to the SGCE, DYT15, DYT1, DRD2, and SLITRK1 loci was also tested.

RESULTS

We included three healthy and 11 affected family members with M-D (n = 3), dystonia alone (n = 2), GTS (n = 1), tics (n = 1) or a combination of these with obsessive compulsive disorder (OCD) (M-D + OCD: n = 2; dystonia+OCD: n = 1; M-D + GTS + OCD: n = 1). There was no linkage to the SGCE, DYT15, DYT1 or DRD2 loci. No changes were found in the SLITRK1 gene. The presence of both M-D and GTS in one family, in which all known M-D loci and a recently discovered GTS locus were excluded, suggests a novel susceptibility gene for both M-D and GTS.

Laterality of interhemispheric inhibition depends on handedness

There is some evidence that handedness is related to lateralisation of excitability in the motor system. We investigated lateralisation of interhemispheric inhibition (IHI), motor thresholds and short interval intracortical inhibition (SICI) and facilitation (SICF) in relation to handedness in 12 right (RH) and 13 left handed (LH) subjects. Because there is some controversy as to the optimal localisation to produce IHI we also compared IHI induced by conditioning the dorsal premotor cortex (dPM) versus primary motor cortex (M1) in ten RH. IHI was stronger following conditioning the motor dominant as compared to the motor non-dominant hemisphere in RH and LH. Motor thresholds were higher when elicited over the right hemisphere than over the left in both RH and LH, while SICI and SICF showed no differences between hemispheres or dependency from handedness. We hypothesize that IHI is a function of handedness perhaps reflecting predominant usage of the dominant hand, while lateralisation of thresholds and intracortical excitability are determined by other factors.

Do simple intransitive finger movements consistently activate frontoparietal mirror neuron areas in humans?

The posterior inferior frontal gyrus (pIFG) and anterior inferior parietal lobule (aIPL) form the core regions of the human "mirror neuron system" that matches an observed movement onto its internal motor representation. We used event-related functional MRI to examine whether simple intransitive finger movements evoke "mirror activity" in the pIFG and aIPL. In separate sessions, participants either merely observed visuospatial stimuli or responded to them as quickly as possible with a spatially compatible finger movement. A picture of a relaxed hand with static dots on the tip of the index and little finger was continuously presented as high-level baseline. Four types of stimuli were presented in a pseudorandom order: a color change of a dot, a moving finger, a moving dot, or a simultaneous finger-dot movement. Dot movements were spatially and kinematically matched to finger movements. Participants were faster at imitating a finger movement than performing the same movement in response to a moving dot or a color change of a dot. Though imitative responses were facilitated, fMRI revealed no additional "mirror activity" in the pIFG and aIPL during the observation or imitation of finger movements as opposed to observing or responding to a moving dot. Mere observation of a finger movement alone failed to induce significant activation of the pIFG and aIPL. The lack of a signature of "mirror neuron activity" in the inferior frontoparietal cortex is presumably due to specific features of the task which may have favored stimulus-response mapping based on common spatial coding. We propose that the responsiveness of human frontoparietal mirror neuron areas to simple intransitive movements critically depends on the experimental context.

Abnormal plasticity of the sensorimotor cortex to slow repetitive transcranial magnetic stimulation in patients with writer's cramp

Previous studies demonstrated functional abnormalities in the somatosensory system, including a distorted functional organization of the somatosensory cortex (S1) in patients with writer's cramp. We tested the hypothesis that these functional alterations render S1 of these patients more susceptible to the "inhibitory" effects of subthreshold 1 Hz repetitive transcranial magnetic stimulation (rTMS) given to S1. Seven patients with writer's cramp and eight healthy subjects were studied. Patients also received rTMS to the motor cortex hand area (M1). As an outcome measure, short-latency afferent inhibition (SAI) was tested. SAI was studied in the relaxed first dorsal interosseous muscle using conditioning electrical stimulation of the index finger and TMS pulses over the contralateral M1. Baseline SAI did not differ between groups. S1 but not M1 rTMS reduced SAI in patients. rTMS had no effects on SAI in healthy subjects. Because SAI is mediated predominantly at a cortical level in the sensorimotor cortex, we conclude that there is an abnormal responsiveness of this area to 1 Hz rTMS in writer's cramp, which may represent a trait toward maladaptive plasticity in the sensorimotor system in these patients.