Exploring function and effective connectivity of the motor cortex and its role in Tourette syndrome

Tourette syndrome (TS) is a neurodevelopmental disorder characterised by vocal and motor tics. It is associated with cortical–striatal–thalamic–cortical (CSTC) circuit dysfunction and hyper-excitability of cortical motor regions. TS follows a developmental time course, in which tics often become increasingly more controlled during adolescence. Importantly, however, a substantial minority of patients continue to have debilitating tics into adulthood. This indicates that there may be important differences between adult TS patients and children and adolescents with the disorder. The first aim of my thesis was to explore the excitability of the primary motor cortex (PMC) at rest, during motor preparation, motor execution and the inhibition of action. In Chapters 3 and 4 I demonstrate that, in contrast to studies of adult patients, resting motor threshold (RMT) and the variability of motor-evoked potential (MEP) responses are increased in young people with TS, while the gain of motor excitability in reduced. Furthermore, these differences normalise with age over adolescence. I conclude that these effects are likely due to a developmental delay in the maturation of key brain networks in TS, consistent with recent brain imaging studies of structural and functional brain connectivity. Importantly, these findings suggest that the alterations in brain network structure and function associated with TS may be quite different in children and adult patients with the condition. In Chapter 4, I demonstrate that whilst there is evidence of reduced gain during motor execution in young people with TS relative to controls (Chapter 3), the reduction is likely driven by baseline differences and when corrected to baseline patients with TS show an increased ramping of motor excitability during motor execution. In fact, patients’ tic severity was inversely related to the modulation of motor excitability whereby those with the most severe tics were least able to increase excitability. Patients showed largely the same patterns of change in excitability during motor preparation and response inhibition. However, the extent to which patients could modulate excitability during motor preparation was related to phonic tic severity whereby those with the least severe tics had higher excitability change from baseline. In addition, those that were able to suppress motor excitability to a greater extent whilst inhibiting action had the least severe tics, likely engaging inhibitory mechanisms to a greater extent with the consequence of slower response times during the task. I conclude that the ability to modulate motor excitability is both related to pathology and adaptive compensatory mechanisms that may help in tic suppression. The second aim of this thesis was to explore effective connectivity, excitatory and inhibitory physiological mechanisms and the neurochemistry of PMC in young healthy adults. Subsequent experiments in Chapters 5 and 6 used various transcranial magnetic stimulation (TMS) techniques and proton magnetic resonance spectroscopy (1H-MRS) to investigate these issues. Chapter 5 explored interhemispheric facilitation and inhibition (IHI and IHF) in two directions between bilateral PMC. The results provided evidence for an asymmetry of interhemispheric interactions using dual site TMS (ds-TMS) whereby the left-to-right direction is more inhibitory than right-to-left. Furthermore, females appeared to show greater interhemispheric modulation than males and whilst there was robust evidence for IHI (in the left-to-right direction) IHF appeared to not be robust. Finally, Chapter 6 explored how TMS-induced measures of excitation and inhibition related to 1H-MRS measures of neurochemicals γ-aminobutyric acid (GABA), glutamate (Glu) and glutamine (Gln). GABA is the primary inhibitory neurotransmitter in the human brain and is critical for the regulation of neuronal excitability and the orchestration of neuronal networks and is critically important in neurodevelopmental disorders such as TS. GABA was not found to be related to measures of synaptic neurotransmission as assessed by TMS and neither was Gln. In contrast, Glu was found to be related to a hub of TMS measures, in particular, Glu was positively related to both intracortical facilitation (ICF) and long intracortical inhibition (LICI). Chapters 5 and 6 further uncovered relationships between ds-TMS, pp-TMS and 1H-MRS showing that these various measures likely have overlapping mechanisms. The final chapters extend our knowledge about the PMC and the methodologies used to assess its state. Chapter 5 extends our understanding of the communication between right and left PMC and highlights a normal asymmetry in communication. This is important for understanding neurodevelopmental disorders such as TS of which asymmetry in effective connectivity and brain volume have been implicated. Chapter 6 importantly shows that 1H-MRS measured GABA is likely irrelevant for assessing synaptic neurotransmission and thus its interpretations should be limited to non-synaptic levels of GABA. This is particularly important for TS research in which both changes in GABAA receptor activity is present in the PMC and abnormalities in GABA concentration have been shown

Increased GABA Contributes to Enhanced Control over Motor Excitability in Tourette Syndrome

Tourette syndrome (TS) is a developmental neurological disorder characterized by vocal and motor tics [1] and associated with cortical-striatal-thalamic-cortical circuit dysfunction [2, 3], hyperexcitability within cortical motor areas [4], and altered intracortical inhibition [4, 5, 6, 7]. TS often follows a developmental time course in which tics become increasingly more controlled during adolescence in many individuals [1], who exhibit enhanced control over their volitional movements [8, 9, 10, 11]. Importantly, control over motor outputs appears to be brought about by a reduction in the gain of motor excitability [6, 7, 12, 13]. Here we present a neurochemical basis for a localized gain control mechanism. We used ultra-high-field (7 T) magnetic resonance spectroscopy to investigate in vivo concentrations of γ-aminobutyric acid (GABA) within primary and secondary motor areas of individuals with TS. We demonstrate that GABA concentrations within the supplementary motor area (SMA)—a region strongly associated with the genesis of motor tics in TS [14]—are paradoxically elevated in individuals with TS and inversely related to fMRI blood oxygen level-dependent activation. By contrast, GABA concentrations in control sites do not differ from those of a matched control group. Importantly, we also show that GABA concentrations within the SMA are inversely correlated with cortical excitability in primary motor cortex and are predicted by motor tic severity and white-matter microstructure (FA) within a region of the corpus callosum that projects to the SMA within each hemisphere. Based upon these findings, we propose that extrasynaptic GABA contributes to a form of control, based upon localized tonic inhibition within the SMA, that may lead to the suppression of tics

Activity or Connectivity? Evaluating neurofeedback training in Huntington's disease

Non-invasive methods, such as neurofeedback training (NFT), could support cognitive symptom management in Huntington’s disease (HD) by targeting brain regions whose function is impaired. The aim of our single-blind, sham-controlled study was to collect rigorous evidence regarding the feasibility of NFT in HD by examining two different methods, activity and connectivity real-time fMRI NFT. Thirty-two HD gene-carriers completed 16 runs of NFT training, using an optimized real-time fMRI protocol. Participants were randomized into four groups, two treatment groups, one receiving neurofeedback derived from the activity of the Supplementary Motor Area (SMA), and another receiving neurofeedback based on the correlation of SMA and left striatum activity (connectivity NFT), and two sham control groups, matched to each of the treatment groups. We examined differences between the groups during NFT training sessions and after training at follow-up sessions. Transfer of training was measured by measuring the participants’ ability to upregulate NFT target levels without feedback (near transfer), as well as by examining change in objective, a-priori defined, behavioural measures of cognitive and psychomotor function (far transfer) before and at 2 months after training. We found that the treatment group had significantly higher NFT target levels during the training sessions compared to the control group. However, we did not find robust evidence of better transfer in the treatment group compared to controls, or a difference between the two NFT methods. We also did not find evidence in support of a relationship between change in cognitive and psychomotor function and NFT learning success. We conclude that although there is evidence that NFT can be used to guide participants to regulate the activity and connectivity of specific regions in the brain, evidence regarding transfer of learning and clinical benefit was not robust. Although the intervention is non-invasive, given the costs and absence of reliable evidence of clinical benefit, we cannot recommend real-time fMRI NFT as a potential intervention in HD

Activity or connectivity? A randomized controlled feasibility study evaluating neurofeedback training in Huntington's disease

Non-invasive methods, such as neurofeedback training, could support cognitive symptom management in Huntington’s disease by targeting brain regions whose function is impaired. The aim of our single-blind, sham-controlled study was to collect rigorous evidence regarding the feasibility of neurofeedback training in Huntington’s disease by examining two different methods, activity and connectivity real-time functional MRI neurofeedback training. Thirty-two Huntington’s disease gene-carriers completed 16 runs of neurofeedback training, using an optimized real-time functional MRI protocol. Participants were randomized into four groups, two treatment groups, one receiving neurofeedback derived from the activity of the supplementary motor area, and another receiving neurofeedback based on the correlation of supplementary motor area and left striatum activity (connectivity neurofeedback training), and two sham control groups, matched to each of the treatment groups. We examined differences between the groups during neurofeedback training sessions and after training at follow-up sessions. Transfer of training was measured by measuring the participants’ ability to upregulate neurofeedback training target levels without feedback (near transfer), as well as by examining change in objective, a priori defined, behavioural measures of cognitive and psychomotor function (far transfer) before and at 2 months after training. We found that the treatment group had significantly higher neurofeedback training target levels during the training sessions compared to the control group. However, we did not find robust evidence of better transfer in the treatment group compared to controls, or a difference between the two neurofeedback training methods. We also did not find evidence in support of a relationship between change in cognitive and psychomotor function and learning success. We conclude that although there is evidence that neurofeedback training can be used to guide participants to regulate the activity and connectivity of specific regions in the brain, evidence regarding transfer of learning and clinical benefit was not robust

Activation induced changes in GABA: functional MRS at 7 T with MEGA-sLASER

Functional magnetic resonance spectroscopy (fMRS) has been used to assess the dynamic metabolic responses of the brain to a physiological stimulus non-invasively. However, only limited information on the dynamic functional response of γ-aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the brain, is available. We aimed to measure the activation-induced changes in GABA unambiguously using a spectral editing method, instead of the conventional direct detection techniques used in previous fMRS studies. The Mescher-Garwood-semi-localised by adiabatic selective refocusing (MEGA-sLASER) sequence was developed at 7 T to obtain the time course of GABA concentration without macromolecular contamination. A significant decrease (−12±5%) in the GABA to total creatine ratio (GABA/tCr) was observed in the motor cortex during a period of 10 minutes of hand-clenching, compared to an initial baseline level (GABA/tCr = 0.11±0.02) at rest. An increase in the Glx (glutamate and glutamine) to tCr ratio was also found, which is in agreement with previous findings. In contrast, no significant changes in NAA/tCr and tCr were detected. With consistent and highly efficient editing performance for GABA detection and the advantage of visually identifying GABA resonances in the spectra, MEGA-sLASER is demonstrated to be an effective method for studying of dynamic changes in GABA at 7 T

Acute gabapentin administration in healthy adults. A double-blind placebo-controlled study using transcranial magnetic stimulation and 7T 1H-MRS

Gamma-aminobutyric acid (GABA) and glutamate are the primary neurotransmitters responsible for modulating excitatory and inhibitory signalling within the human brain. Dysfunctional GABAergic and glutamatergic signalling has been identified as a key factor in a range of neuropsychiatric conditions; hence measurement and modulation of these neurometabolites is important for improving our understanding of neuropsychiatric conditions and treatment options. Gabapentin (GBP) is one of several drugs developed to increase GABA levels and is routinely prescribed for conditions such as epilepsy and neuralgia. While animal and human studies indicate that GBP can elevate GABA levels, its exact mechanisms of action are not fully understood, although animal studies indicate that GBP does not have a direct effect upon GABAergic receptors.To investigate the impact of acute GBP administration in the human motor system we used two complimentary approaches – transcranial magnetic stimulation (TMS) and magnetic resonance spectroscopy (MRS). MRS and TMS measures of GABA have repeatedly been found to be uncorrelated and are likely to reflect different pools of synaptic and extra synaptic GABA, hence, measuring both within the same participants allows for an in-depth assessment of GBP effects.Despite significantly increased ratings of fatigue and tiredness within the GBP group, we failed to find any statistically significant changes in our MRS or TMS measures of GABA. Measures of MRS Glutamate (glu) and glutamine (gln) were also not affected by the administration of GBP. These findings are important as they run counter to previous work, and suggest that the effect of an acute dose of GBP is likely to be subject to substantial individual variation, with timing of measures particularly likely to impact observed effects. These findings have implications for the use of acute GBP dosing as a means to explore GABAergic function in health and disease

Comparing GABA-­dependent physiological measures of inhibition with proton magnetic resonance spectroscopy measurement of GABA using ultra-­high-­field MRI

Imbalances in glutamatergic (excitatory) and GABA (inhibitory) signalling within key brain networks are thought to underlie many brain and mental health disorders, and for this reason there is considerable interest in investigating how individual variability in localised concentrations of these molecules relate to brain disorders. Magnetic resonance spectroscopy (MRS) provides a reliable means of measuring, in vivo, concentrations of neurometabolites such as GABA, glutamate and glutamine that can be correlated with brain function and dysfunction. However, an issue of much debate is whether the GABA observed and measured using MRS represents the entire pool of GABA available for measurement (i.e., metabolic, intracellular, and extracellular) or is instead limited to only some portion of it. GABA function can also be investigated indirectly in humans through the use of non-invasive transcranial magnetic stimulation (TMS) techniques that can be used to measure cortical excitability and GABA-mediated physiological inhibition. To investigate this issue further we collected in a single session both types of measurement, i.e., TMS measures of cortical excitability and physiological inhibition and ultra-high-field (7 Tesla) MRS measures of GABA, glutamate and glutamine, from the left sensorimotor cortex of the same group of right-handed individuals. We found that TMS and MRS measures were largely uncorrelated with one another, save for the plateau of the TMS IO curve that was negatively correlated with MRS-Glutamate (Glu) and intra-cortical facilitation (10ms ISI) that was positively associated with MRS-Glutamate concentration. These findings are consistent with the view that the GABA concentrations measured using MRS largely represent pools of GABA that are linked to tonic rather than phasic inhibition and thus contribute to the inhibitory tone of a brain area rather than GABAergic synaptic transmission

Alterations in the microstructure of white matter in children and adolescents with Tourette syndrome measured using tract-based spatial statistics and probabilistic tractography

Tourette syndrome (TS) is a neurodevelopmental disorder characterised by repetitive and intermittent motor and vocal tics. TS is thought to reflect fronto-striatal dysfunction and the aetiology of the disorder has been linked to widespread alterations in the functional and structural integrity of the brain. The aim of this study was to assess white matter (WM) abnormalities in a large sample of young patients with TS in comparison to a sample of matched typically developing control individuals (CS) using diffusion MRI. The study included 35 patients with TS (3 females; mean age: 14.0 ± 3.3) and 35 CS (3 females; mean age: 13.9 ± 3.3). Diffusion MRI data was analysed using tract-based spatial statistics (TBSS) and probabilistic tractography. Patients with TS demonstrated both marked and widespread decreases in axial diffusivity (AD) together with altered WM connectivity. Moreover, we showed that tic severity and the frequency of premonitory urges (PU) were associated with increased connectivity between primary motor cortex (M1) and the caudate nuclei, and increased information transfer between M1 and the insula, respectively. This is to our knowledge the first study to employ both TBSS and probabilistic tractography in a sample of young patients with TS. Our results contribute to the limited existing literature demonstrating altered connectivity in TS and confirm previous results suggesting in particular, that altered insular function contributes to increased frequency of PU

Exploring function and effective connectivity of the motor cortex and its role in Tourette syndrome

Tourette syndrome (TS) is a neurodevelopmental disorder characterised by vocal and motor tics. It is associated with cortical–striatal–thalamic–cortical (CSTC) circuit dysfunction and hyper-excitability of cortical motor regions. TS follows a developmental time course, in which tics often become increasingly more controlled during adolescence. Importantly, however, a substantial minority of patients continue to have debilitating tics into adulthood. This indicates that there may be important differences between adult TS patients and children and adolescents with the disorder. The first aim of my thesis was to explore the excitability of the primary motor cortex (PMC) at rest, during motor preparation, motor execution and the inhibition of action. In Chapters 3 and 4 I demonstrate that, in contrast to studies of adult patients, resting motor threshold (RMT) and the variability of motor-evoked potential (MEP) responses are increased in young people with TS, while the gain of motor excitability in reduced. Furthermore, these differences normalise with age over adolescence. I conclude that these effects are likely due to a developmental delay in the maturation of key brain networks in TS, consistent with recent brain imaging studies of structural and functional brain connectivity. Importantly, these findings suggest that the alterations in brain network structure and function associated with TS may be quite different in children and adult patients with the condition. In Chapter 4, I demonstrate that whilst there is evidence of reduced gain during motor execution in young people with TS relative to controls (Chapter 3), the reduction is likely driven by baseline differences and when corrected to baseline patients with TS show an increased ramping of motor excitability during motor execution. In fact, patients’ tic severity was inversely related to the modulation of motor excitability whereby those with the most severe tics were least able to increase excitability. Patients showed largely the same patterns of change in excitability during motor preparation and response inhibition. However, the extent to which patients could modulate excitability during motor preparation was related to phonic tic severity whereby those with the least severe tics had higher excitability change from baseline. In addition, those that were able to suppress motor excitability to a greater extent whilst inhibiting action had the least severe tics, likely engaging inhibitory mechanisms to a greater extent with the consequence of slower response times during the task. I conclude that the ability to modulate motor excitability is both related to pathology and adaptive compensatory mechanisms that may help in tic suppression. The second aim of this thesis was to explore effective connectivity, excitatory and inhibitory physiological mechanisms and the neurochemistry of PMC in young healthy adults. Subsequent experiments in Chapters 5 and 6 used various transcranial magnetic stimulation (TMS) techniques and proton magnetic resonance spectroscopy (1H-MRS) to investigate these issues. Chapter 5 explored interhemispheric facilitation and inhibition (IHI and IHF) in two directions between bilateral PMC. The results provided evidence for an asymmetry of interhemispheric interactions using dual site TMS (ds-TMS) whereby the left-to-right direction is more inhibitory than right-to-left. Furthermore, females appeared to show greater interhemispheric modulation than males and whilst there was robust evidence for IHI (in the left-to-right direction) IHF appeared to not be robust. Finally, Chapter 6 explored how TMS-induced measures of excitation and inhibition related to 1H-MRS measures of neurochemicals γ-aminobutyric acid (GABA), glutamate (Glu) and glutamine (Gln). GABA is the primary inhibitory neurotransmitter in the human brain and is critical for the regulation of neuronal excitability and the orchestration of neuronal networks and is critically important in neurodevelopmental disorders such as TS. GABA was not found to be related to measures of synaptic neurotransmission as assessed by TMS and neither was Gln. In contrast, Glu was found to be related to a hub of TMS measures, in particular, Glu was positively related to both intracortical facilitation (ICF) and long intracortical inhibition (LICI). Chapters 5 and 6 further uncovered relationships between ds-TMS, pp-TMS and 1H-MRS showing that these various measures likely have overlapping mechanisms. The final chapters extend our knowledge about the PMC and the methodologies used to assess its state. Chapter 5 extends our understanding of the communication between right and left PMC and highlights a normal asymmetry in communication. This is important for understanding neurodevelopmental disorders such as TS of which asymmetry in effective connectivity and brain volume have been implicated. Chapter 6 importantly shows that 1H-MRS measured GABA is likely irrelevant for assessing synaptic neurotransmission and thus its interpretations should be limited to non-synaptic levels of GABA. This is particularly important for TS research in which both changes in GABAA receptor activity is present in the PMC and abnormalities in GABA concentration have been shown