The use of functional neuroimaging to study reorganisation of the motor system during task performance following altered corticospinal excitability caused by repetitive transcranial magnetic stimulation

Repetitive transcranial magnetic stimulation (rTMS) in motor areas has been shown to induce transient and reversible changes in the corticospinal excitability of healthy individuals' brains. Furthermore, a number of studies have shown that this reorganisation can occur not only at the site of stimulation but also in other areas which might be anatomically or functionally connected to it. These effects are thought to depend on various parameters such as the protocol of stimulation, the site of stimulation and the behavioural paradigm chosen to study the effects. The aim of this thesis was to use functional neuroimaging in order to explore how the effects of long-lasting rTMS protocols (30-60min stimulation) on the motor system depend on specific conditioning parameters such as the frequency, the site and the pattern of stimulation. We examined how the pattern of activity in the brain reorganises depending on the anatomical and effective connectivity of the stimulated area, and on the behavioural task. Initial studies presented in this thesis used Positron Emission Tomography to compare the effects of high (5Hz) versus low (1Hz) frequency of rTMS on activity and motor network connectivity of the primary motor cortex (M1) during performance of a simple finger movement task or at rest. I found that task-related activity of motor area 4p within M1 and its connectivity with non-primary motor areas, such as the ipsilesional dorsal premotor (PMd) cortex could be modulated bidirectionally with low or high frequency rTMS over M1: compared to sham stimulation, 5Hz rTMS reduced task-related activity and network connectivity of that area. The opposite was true for 1Hz rTMS. A further study using the same task examined whether such reorganisation would be observed with 5Hz rTMS over the PMd. The effect of 5Hz rTMS on this site of stimulation revealed a different pattern of regional cerebral blood flow than rTMS over M1. However, a similar reorganisation in task-related activity and network connectivity was observed, particularly within the PMd area caudal to the stimulated site, which increased in activity after 5Hz rTMS. These two studies demonstrated that rTMS leads to widespread activity changes. However, changes in task-related activity and motor network connectivity occur in motor areas adjacent to the stimulated site. Given the lack of any behavioural effects in these studies, it can be hypothesised that these changes occur as compensatory mechanisms to the "virtual lesion" caused by rTMS. A functional magnetic resonance imaging (fMRI) study of the effects of rTMS over the lateral prefrontal cortex showed task-related changes in activity during performance of a cued choice reaction time task. Targeting the dorsolateral prefrontal cortex (DLPFC) with 5Hz rTMS led to task-related decreases in activity in the adjacent ventrolateral prefrontal cortex. This is reminiscent of the task-related decreases in activity of area 4p observed following 5Hz rTMS over M1 (described above). The side of lateral prefrontal conditioning affected behavioural performance in a further study which distinguished motor from spatial attention in the same behavioural paradigm. Left DLPFC stimulation led to a more prominent switch cost in the motor attention version of this task, confirming a left-lateralised dominance for switching motor responses, described in previous studies. These results provide new evidence that reorganisation in the brain observed following rTMS conditioning is very similar to reorganisation observed following lesions in patients. This reorganisation seems to depend on the task that is required to be performed and determines the pattern of activity obtained in functional neuroimaging studies. In addition, whether this will lead to a behavioural effect depends on the role of that particular area during task performance which is a function of its effective connectivity. These factors seem to determine whether the brain can compensate for the "virtual lesion" induced by rTMS

Transcranial Magnetic Stimulation and Neuroimaging Coregistration

The development of neuroimaging techniques is one of the most impressive advancements in neuroscience. The main reason for the widespread use of these instruments lies in their capacity to provide an accurate description of neural activity during a cognitive process or during rest. This important advancement is related to the possibility to selectively detect changes of neuronal activity in space and time by means of different biological markers. Specifically, functional magnetic resonance imaging (fMRI), positron emission tomography (PET), single-photon emission computed tomography (SPECT), and nearinfrared spectroscopy (NIRS) use metabolic markers of ongoing neuronal activity to provide an accurate description of the activation of specific brain areas with high spatial resolution. Similarly, electroencephalography (EEG) is able to detect electric markers of neuronal activity, providing an accurate description of brain activation with high temporal resolution. The application of these techniques during a cognitive task allows important inferences regarding the relation between the detected neural activity, the cognitive process involved in an ongoing task, and behaviour: this is known as a \u201ccorrelational approach\u201d

Neuroplastic Changes Following Brain Ischemia and their Contribution to Stroke Recovery: Novel Approaches in Neurorehabilitation

Ischemic damage to the brain triggers substantial reorganization of spared areas and pathways, which is associated with limited, spontaneous restoration of function. A better understanding of this plastic remodeling is crucial to develop more effective strategies for stroke rehabilitation. In this review article, we discuss advances in the comprehension of post-stroke network reorganization in patients and animal models. We first focus on rodent studies that have shed light on the mechanisms underlying neuronal remodeling in the perilesional area and contralesional hemisphere after motor cortex infarcts. Analysis of electrophysiological data has demonstrated brain-wide alterations in functional connectivity in both hemispheres, well beyond the infarcted area. We then illustrate the potential use of non-invasive brain stimulation (NIBS) techniques to boost recovery. We finally discuss rehabilitative protocols based on robotic devices as a tool to promote endogenous plasticity and functional restoration

Resting state fMRI study of brain activation using rTMS in rats

Background and purpose: Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive neuromodulation technique used to treat many neurological and psychiatric conditions. However, not much is known about the mechanisms underlying its efficacy because human rTMS studies are mostly non-invasive while most animal studies are invasive. Invasive animal studies allow for cellular and molecular changes to be detected and hence, have been able to show that rTMS may alter synaptic plasticity in the form of long-term potentiation. This is the first rodent study using non-invasive resting state functional magnetic resonance imaging (rs-fMRI) to examine the effects of low-intensity rTMS (LI-rTMS) in order to provide a more direct comparison to human studies. Methods: rs-fMRI data were acquired before and after 10 minutes of LI-rTMS intervention at one of four frequencies—1 Hz, 10 Hz, biomimetic high frequency stimulation (BHFS) and continuous theta burst stimulation (cTBS)—in addition to sham. We used independent component analysis to uncover changes in the default mode network (DMN) induced by each rTMS protocol. Results: There were considerable rTMS-related changes in the DMN. Specifically, (1) the synchrony of resting activity of the somatosensory cortex was decreased ipsilaterally following 10 Hz stimulation, increased ipsilaterally following cTBS, and decreased bilaterally following 1 Hz stimulation and BHFS; (2) the motor cortex showed bilateral changes following 1 Hz and 10 Hz stimulation, an ipsilateral increase in synchrony of resting activity following cTBS, and a contralateral decrease following BHFS; and (3) in the hippocampus, 10 Hz stimulation caused an ipsilateral decrease while 1 Hz and BHFS caused a bilateral decrease in synchrony. There was no change in the correlation of the hippocampus induced by cTBS. Conclusion: The present findings suggest that LI-rTMS can modulate functional links within the DMN of rats. LI-rTMS can induce changes in the cortex, as well as in remote brain regions such as the hippocampus when applied to anaesthetised rats and the pattern of these changes depends on the frequency used, with 10 Hz stimulation, BHFS and cTBS causing mostly ipsilateral changes in synchrony of activity in the DMN and 1 Hz stimulation causing bilateral changes in synchrony, with the contralateral changes being more prominent than ipsilateral changes. Hence, combined rTMS-fMRI emerges as a powerful tool to visualise rTMS-induced cortical connectivity changes at a high spatio-temporal resolution and help unravel the physiological processes underlying these changes in the cortex and interconnected brain regions

Studying the cortical state with transcranial magnetic stimulation

Cortical excitability and connectivity describe the state of the cerebral cortex. They reflect the ability of neurons to respond to input and the way information flows in the neuronal networks. These properties can be assessed with transcranial magnetic stimulation (TMS), which enables direct and noninvasive modulation of cortical activity. Electrophysiological or hemodynamic recordings of TMS-evoked activity or behavioral measures of the stimulation effect characterize the state of the cortex during and as a result of the stimulation. In the research reported in this Thesis, the ability of TMS to inform us about the cortical state is studied from different points of view. First, we examine the relationships between different measures of cortical excitability to better understand the physiology behind them; we show how cortical background activity is related to motor cortical excitability and how the evoked responses reflect the excitability. Second, this study addresses the questions whether the TMS-evoked responses include stimulation-related artifacts, how these artifacts are generated, and how they can be avoided or removed. Specifically, we present a method to remove the artifacts from TMS-evoked electroencephalographic (EEG) signals arising as a result of cranial muscle stimulation. The use of TMS-EEG has been limited to relatively medial sites because of these artifacts, but the new method enables studying the cortical state even when stimulating areas near the cranial muscles, especially lateral sites. Finally, this work provides new information about brain function. The mechanisms how the brain processes visually guided timed motor actions are elucidated. Moreover, we show that cortical excitability as measured with TMS-evoked EEG increases during the course of wakefulness and decreases during sleep, which contributes to our understanding of what happens in the brain during wakefulness that makes us feel tired and why the brain needs sleep. The study also shows the sensitivity of the TMS-EEG measurement to changes in the state of the cortex. Accordingly, we demonstrate the power of TMS in studying the cortical state

Transcranial magnetic stimulation combined with functional magnetic resonance imaging: From target identification to prediction of therapeutic effects in stroke patients

Repetitive transcranial magnetic stimulation (rTMS), particularly theta-burst stimulation (TBS), can be applied to modulate cortical excitability beyond the period of stimulation (Huang et al., 2005). Consequently, rTMS is regarded to have high therapeutic potential for treatment of various psychiatric and neurological diseases related to cortical hypo- or hyperexcitability such as stroke (Ridding & Rothwell, 2007). Whether rTMS induced effects are sufficiently robust to be useful in clinical settings is currently under intense investigation. The most challenging problem appears to be considerably high variability in rTMS induced effects both, across studies (Hoogendam et al., 2010) and individual patients (Ameli et al., 2009). Hence, the major goal of the present thesis was to improve rTMS intervention strategies in stroke patients suffering from chronic motor hand deficits by multimodal uses of (repetitive) TMS with state-of-the-art neuroimaging techniques. Sources of variance across studies are likely to be methodological in origin. They might result from different strategies to identify the cortical rTMS target position. Individual functional magnetic resonance (fMRI) data have been demonstrated to yield best spatial approximations of the most excitable TMS position compared to other techniques (Sparing et al., 2008). However, there is still a considerably large spatial mismatch between the cortical position showing highest movement-related fMRI signal and the cortical position yielding highest muscle responses when stimulated with TMS of up to 14 mm (Bastings et al., 1998; Boroojerdi et al., 1999; Herwig et al., 2002; Krings et al., 1997; Lotze et al., 2003; Sparing et al., 2008; Terao et al., 1998). The underlying cause of this spatial mismatch is unknown. Hence, the aim of the first study (Study I) of the present thesis was to test the hypothesis that the spatial mismatch between positions with highest fMRI signal change and positions with highest TMS excitability might be caused by the widely-used Gradient-Echo blood oxygenation level dependent (GRE-BOLD) fMRI technique. GRE-BOLD signal has been demonstrated to occur further downstream from the site of neural activity in large veins running on the cerebral surface (Uludag et al., 2009). Consequently, we tested the hypothesis that alternative fMRI sequences may localize neural activity (i) closer to the anatomical motor hand area, i.e. Brodmann Area 4 (BA4), and (ii) closer to the optimal TMS position than GRE-BOLD. The following alternative fMRI techniques were tested: (i) Spin-Echo (SE-BOLD) assessing blood oxygenation level dependent signal changes with decreased sensitivity for the macrovasculature at high magnetic fields (≥ 3 Tesla, Uludag et al., 2009) and (ii) arterial spin labelling (ASL), assessing local changes in cerebral blood flow (ASL-CBF) which have been shown to occur in close proximity to synaptic activity (Duong et al., 2000). GRE-BOLD, SE-BOLD, and ASL-CBF signal changes during right thumb abductions were obtained from 15 healthy young subjects at 3 Tesla. In 12 subjects, brain tissue at fMRI peak voxel coordinates was stimulated with neuronavigated TMS to investigate whether spatial differences between fMRI techniques are functionally relevant, i.e. impact on motor-evoked potentials (MEPs) recorded from a contralateral target muscle, which is involved in thumb abductions. A systematic TMS motor mapping was performed to identify the most excitable TMS position (i.e. the TMS hotspot) and the centre-of-gravity (i.e. the TMS CoG), which considers the spatial distribution of excitability in the pericentral region. Euclidean distances between TMS and fMRI positions were calculated for each fMRI technique. Results indicated that highest SE-BOLD and ASL-CBF signal changes occurred in the anterior wall of the central sulcus (BA4), whereas highest GRE-BOLD signal changes occurred significantly closer to the gyral surface where most large draining veins are located. fMRI techniques were not significantly different from each other in Euclidean distances to optimal TMS positions since optimal TMS positions were located considerably more anterior (and slightly surprisingly in premotor cortex (BA6) and not BA4). Stimulation of brain tissue at GRE-BOLD peak voxel coordinates with TMS resulted in significantly higher MEPs (compared to SE-BOLD and ASL-CBF coordinates). This was probably the case because GRE-BOLD positions tended to be located at the gyral crown, which was slightly (but not significantly) closer to the TMS hotspot position. Taken together, findings of Study I suggest that spatial differences between fMRI and TMS positions are not caused by spatial unspecificity of the widely-used GRE-BOLD fMRI technique. Hnece, other factors such as complex interactions between brain tissue and the TMS induced electric field (Opitz et al., 2011), could be the underlying cause. Identification of the cortical rTMS target position is particularly challenging in stroke patients since reorganization processes after stroke may shift both, fMRI and TMS positions in unknown direction and extend (Rossini et al., 1998). In the second study (Study II) of the present thesis, we therefore tested whether findings obtained from healthy young subjects in Study I do also apply to chronic stroke patients and older (i.e. age-matched) healthy control subjects. In this study, arterial spin labelling (ASL) was used to assess CBF and BOLD signal changes simultaneously during thumb abductions with the affected/non-dominant and the unaffected/dominant hand in 15 chronic stroke patients and 13 age-matched healthy control subjects at 3 Tesla. Brain tissue at fMRI peak voxel coordinates was stimulated with neuronavigated TMS to test whether spatial differences are functionally relevant and impact on MEPs. Systematic TMS motor mappings were performed for both hemispheres in overall 12 subjects (6 stroke patients and 6 healthy subjects). Euclidean distances between fMRI and TMS positions were calculated for each hemisphere and fMRI technique. In line with results of Study I, highest ASL-CBF signal changes were located in the anterior wall of the central sulcus (BA4), whereas highest ASL-BOLD signal changes occurred significantly closer to the gyral surface. In contrast to Study I, there were no significant differences between ASL-CBF and ASL-BOLD positions in MEPs when stimulated with neuronavigated TMS, which suggests that spatial differences (in depth) were not functionally relevant for TMS applications. In line with Study I, there were no significant differences between fMRI techniques in Euclidean distances to optimal TMS positions, since optimal TMS positions were located considerably more anterior than fMRI positions (in premotor cortex, i.e. BA6). Stroke patients showed overall larger displacements (between fMRI and TMS positions) on the ipsilesional (but not the contralesional) hemisphere compared to healthy subjects. However, none of the fMRI techniques yielded positions significantly closer to the optimal TMS position. Hence, functional reorganization may impact on spatial congruence between fMRI and TMS, but the effect is similar for ASL-CBF and ASL-BOLD. Pathomechanisms underlying stroke induced motor deficits are still poorly understood but a simplified model of hemispheric competition has been suggested, which proposes relative hypoexcitability of the ipsilesional hemisphere and hyperexcitability of the contralesional hemisphere leading to pathologically increased interhemispheric inhibition from the contralesional onto the ipsilesional hemisphere during movements of the paretic hand (Duque et al., 2005; Grefkes et al., 2008b, 2010; Murase et al., 2004). In line with the model of hemispheric competition, both increasing excitability of the ipsilesional hemisphere (Khedr et al., 2005; Talelli et al., 2007) as well as decreasing excitability of the contralesional hemisphere (Fregni et al., 2006; Di Lazzaro et al., 2008a) have been demonstrated to normalize cortical excitability towards physiological levels and/or ameliorate motor performance of the stroke affected hand. However, there is considerably high inter-individual variance and some patients may even show deteriorations of motor performance after rTMS (Ameli et al., 2009). Therefore, the aim of the third study (Study III) was to identify reliable predictors for TBS effects on motor performance of the affected hand in stroke patients, which appears essential for successful implementation of TBS in neurorehabilitation. Overall, 13 chronic stroke patients with unilateral motor hand deficit and 12 age-matched healthy control subjects were included in the study. All patients received 3 different TBS interventions on 3 different days: (i) intermittent TBS (iTBS, facilitatory) over the primary motor cortex (M1) of the ipsilesional hemisphere, (ii) continuous TBS (cTBS, inhibitory) over M1 of the contralesional hemisphere, and (iii) either iTBS or cTBS over a control stimulation site (to control for placebo effects). Motor performance was measured before and after each TBS session with 3 different motor tasks and an overall motor improvement score was calculated. All subjects participated in an fMRI experiment, in which they performed rhythmic fist closures with their affected/non-dominant and unaffected/dominant hand. A laterality index (LI), reflecting laterality of fMRI signal in cortical motor areas was calculated. Effective connectivity, i.e. the direct or indirect causal influence that activity in one area exerts on activity of another area (Friston et al., 1993a), was inferred from fMRI data by means of dynamic causal modelling (DCM). Due to relatively high inter-individual variance, neither iTBS nor cTBS was significantly different from control TBS in terms of average behavioural (or electrophysiological) changes over the group of patients. However, beneficial effects of iTBS over the ipsilesional hemisphere were predicted by a unilateral fMRI activation pattern during movements of the affected hand and by the integrity of the cortical motor network. The more pronounced the promoting influence from the ipsilesional supplementary motor area (SMA) onto ipsilesional M1 and the more pronounced the inhibitory effect originating from ipsilesional M1 onto contralesional M1, the better was the behavioural response to facilitatory iTBS applied to the ipsilesional hemisphere. No significant correlations were found for behavioural improvements following cTBS or behavioural changes of the unaffected hand. Taken together, Study III yielded promising results indicating that laterality of fMRI signal and integrity of the motor network architecture constitute promising predictors for response to iTBS. In patients in whom the connectivity pattern of the ipsilesional motor network resembled physiological network connectivity patterns (i.e. preserved inhibition of the contralesional hemisphere and supportive role of the SMA of the ipsilesional hemisphere), beneficial effects of iTBS over the ipsilesional hemisphere could be observed. In contrast, patients with severely disturbed motor networks did not respond to iTBS or even deteriorated

Exploring Brain Circuitry: Simultaneous Application of Transcranial Magnetic Stimulation and Functional Magnetic Resonance Imaging

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Development of Low-Frequency Repetitive Transcranial Magnetic Stimulation as a Tool to Modulate Visual Disorders: Insights from Neuroimaging

Repetitive transcranial magnetic stimulation (rTMS) has become a popular neuromodulation technique, increasingly employed to manage several neurological and psychological conditions. Despite its popular use, the underlying mechanisms of rTMS remain largely unknown, particularly at the visual cortex. Moreover, the application of rTMS to modulate visual-related disorders is under-investigated. The goal of the present research was to address these issues. I employ a multitude of neuroimaging techniques to gain further insight into neural mechanisms underlying low-frequency (1 Hz) rTMS to the visual cortex. In addition, I begin to develop and refine clinical low-frequency rTMS protocols applicable to visual disorders as an alternative therapy where other treatment options are unsuccessful or where there are simply no existing therapies. One such visual disorder that can benefit from rTMS treatment is the perception of visual hallucinations that can occur following visual pathway damage in otherwise cognitively healthy individuals. In Chapters 23, I investigate the potential of multiday low-frequency rTMS to the visual cortex to alleviate continuous and disruptive visual hallucinations consequent to occipital injury. Combining rTMS with magnetic resonance imaging techniques reveals functional and structural cortical changes that lead to the perception of visual hallucinations; and rTMS successfully attenuates these anomalous visual perceptions. In Chapters 45, I compare the effects of alternative doses of low-frequency rTMS to the visual cortex on neurotransmitter levels and intrinsic functional connectivity to gain insight into rTMS mechanisms and establish the most effective protocol. Differential dose-dependent effects are observed on neurotransmitter levels and functional connectivity that suggest the choice of protocol critically depends on the neurophysiological target. Collectively, this work provides a basic framework for the use of low-frequency rTMS and neuroimaging in clinical application for visual disorders