L'imagerie motrice, ou représentation interne d'un mouvement, est une technique d'entraînement mental ayant la particularité d'impacter la plasticité cérébrale activité-dépendante. La pertinence de son intégration au sein des programmes de rééducation fonctionnelle a été validée par de nombreuses études expérimentales, tout particulièrement pour les personnes victimes d'un accident vasculaire cérébral ou d'une lésion de la moelle épinière. Les résultats obtenus par ce travail de thèse élargissent l'éventail des populations pouvant bénéficier de cette approche en validant la faisabilité et en précisant les conditions optimales de l'intégration de l'imagerie motrice dans le cadre de la rééducation de personnes amputées du membre inférieur. Nous rapportons également des résultats préliminaires prometteurs quant à ses effets sur le recouvrement de leurs capacités locomotrices. Les données montrent que le travail en imagerie motrice devrait être spécifique aux mouvements fonctionnels bilatéraux et que, sous réserve d'une prochaine validation auprès de cette population, l'imagerie motrice de tâches posturales ainsi que l'utilisation conjointe de la stimulation transcrânienne à courant continu, devraient permettre de maximiser son pouvoir d'action sur le recouvrement de la locomotion de personnes amputées du membre inférieur / Motor imagery, which refers to the internal representation of a movement, has the potential to impact activity-dependent plasticity. The relevance of motor imagery, as a technique allowing substantial motor performance gains and motor recovery in the field of rehabilitation, has been validated by numerous experimental studies in stroke patients and persons suffering from spinal cord injury. By highlighting the possibility of integrating motor imagery within the framework of lower-limb amputees’ rehabilitation programs, and revealing promising preliminary data regarding locomotion recovery, the results of this thesis broaden the range of people which might benefit from this technique. Data further suggest that motor imagery should specifically focus on functional and bilateral tasks, and that concomitant use of transcranial direct current stimulation should contribute to potentiate the impact of motor imagery on locomotion recovery of lower-limb amputees

Chez les patients hémiparétiques chroniques, la tDCS a été testée sur diverses tâches du membre inférieur, montrant des résultats intéressants, mais parfois contradictoires. Cependant, les effets de la tDCS sur la fonction motrice volontaire (extension de genou) et semi-automatique (locomotion) chez des patients hémiparétiques ne sont pas encore totalement connus. L’objet de ce travail était d’évaluer contre placébo les effets de la tDCS en polarité anodale sur les fonctions motrices volontaires et semi-automatiques, chez des patients hémiparétiques. L’étude comporte deux parties : la première a pour but d’évaluer l’effet contre placébo de la tDCS bilatérale sur la force volontaire maximale du quadriceps parétique par isocinétisme, la seconde a pour objectif d’étudier les effets contre placébo des effets de la tDCS unilatérale sur la marche par analyse tridimensionnelle du mouvement. Les résultats ont montré une absence d’effet de la tDCS sur les deux types de mouvements étudiés. De plus, la tDCS n’a pas eu d’effet sur la spasticité du quadriceps ni sur la performance aux tests fonctionnels des patients hémiparétiques. Cela suggère qu’il n’y a pas d’intérêt à l’utilisation de la tDCS bilatérale pour améliorer une performance motrice maximale (l’extension de genou) ni de la tDCS unilatérale pour améliorer un mouvement semi-automatique (la locomotion) chez les patients hémiparétiques chroniques. Les perspectives envisagées sont de reconduire ces expérimentations chez un groupe homogène de patients hémiparétiques en phase aigüe ou subaigüe afin de potentialiser les phénomènes plastiques post-lésionnels. Ceci permettra de renforcer ou non l’intérêt de l’utilisation de la tDCS chez les patients hémiparétiques afin d’améliorer leurs performances motrices. / In chronic hemiparetic patients, the use of tDCS has been tested on various lower limb tasks, showing interesting but sometimes contradictory results. However, the effects of tDCS on voluntary (knee extension) and semi-automatic (locomotion) motor function in hemiparetic patients are not yet fully known. The purpose of this work was to quantify the effects of tDCS in anodal polarity on the voluntary and semi-automatic motor functions in placebo-controlled studies. The study is divided into two parts: the first aims to evaluate the effect against placebo of bilateral tDCS on the maximal voluntary force of the paretic quadriceps by isokinetic assessment, the second aims to study the effects against placebo of unilateral tDCS on the gait of chronic hemiparetic patients, assessed by three-dimensional gait analysis. The results showed an absence of effect of tDCS on the two types of movements studied. In addition, tDCS had no effect on quadriceps spasticity or functional test performance in hemiparetic patients. This suggests that there is no interest in using bilateral tDCS to improve maximal motor performance (knee extension) or unilateral tDCS to improve semi-automatic movement (locomotion) in chronic hemiparetic patients. The envisaged perspectives are to continue these experiments in a homogeneous group of hemiparetic patients in acute or subacute stroke patients in order to potentiate the post-lesional plastic phenomena. This will strengthen or not the interest of the use of tDCS in hemiparetic patients to improve their motor performance.

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Mental fatigue decreases motivation. I tested whether applying electricity to a self-control region of the brain would replenish some of the motivation normally lost during mental fatigue. 224 people participated in this study. Each person received real or placebo brain stimulation while undergoing activities that increased mental fatigue. The dependent variable was a task where participants had to perform work by clicking a computer mouse repeatedly. Before performing this task, participants indicated how hard they were planning to work on this motivation task. Participants who received real brain stimulation were able to perform more work, but only if they also indicated they planned to work hard on the task.

Simulation of the electromagnetic response of the human body relies heavily upon efficient computational CAD models or phantoms. The Visible Human Project (VHP)-Female v. 3.1 - a new platform-independent full-body electromagnetic computational model is revealed. This is a part of a significant international initiative to develop powerful computational models representing the human body. This model’s unique feature is full compatibility both with MATLAB and specialized FEM computational software packages such as ANSYS HFSS/Maxwell 3D and CST MWS. Various mesh processing algorithms such as automatic intersection resolver, Boolean operation on meshes, etc. used for the development of the Visible Human Project (VHP)-Female are presented. The VHP - Female CAD Model is applied to two specific low frequency applications: Transcranial Magnetic Stimulation (TMS) and Transcranial Direct Current Stimulation (tDCS). TMS and tDCS are increasingly used as diagnostic and therapeutic tools for numerous neuropsychiatric disorders. The development of a CAD model based on an existing voxel model of a Japanese pregnant woman is also presented. TMS for treatment of depression is an appealing alternative to drugs which are teratogenic for pregnant women. This CAD model was used to study fetal wellbeing during induced peak currents by TMS in two possible scenarios: (i) pregnant woman as a patient; and (ii) pregnant woman as an operator. An insight into future work and potential areas of research such as a deformable phantom, implants, and RF applications will be presented.

tDCS brain stimulation does not always work in the intended direction. It has been found to sometimes worsen behavior rather than improve it. A preliminary study shows that people high on sensation-seeking and lack of premeditation were prone to reverse effects of tDCS on performance on a Stop Signal Task. Both of these constructs are related to dopamine levels. Study 2 seeks to intentionally cause a reverse effect of tDCS by increasing participants’ dopamine levels via caffeine. There was not a significant interaction between tDCS and caffeine on errors on the Stop Signal Task in this study. However, other factors interacted with tDCS and caffeine including lack of premeditation. This two study package suggests the effects of tDCS are variable across individuals, with personality and neurochemistry both affecting behavioral outcomes of tDCS.

Transcranial direct current stimulation (tDCS), together with speech therapy, is known to relieve the symptoms of aphasia. Knowledge about amount of current to apply and stimulation location is needed to ensure the best result possible. Segmented tissues are used in a finite element method (FEM) simulation and by creating a mesh, information to guide the stimulation is gained. Thus, correct segmentation is crucial. Manual segmentation is known to produce the most accurate result, although it is not useful in the clinical setting since it currently takes weeks to manually segment one image volume. Automatic segmentation is faster, although both acute stroke lesions and nectrotic stroke lesions are known to cause problems. Three automatic segmentation routines are evaluated using default settings and two sets of tissue probability maps (TPMs). Two sets of stroke patients are used; one set with acute stroke lesions (which can only be seen as a change in image intensity) and one set with necrotic stroke lesions (which are cleared out and filled with cerebrospinal fluid (CSF)). The original segmentation routine in SPM8 does not produce correct segmentation result having problems with lesion and paralesional areas. Mohamed Seghier’s ALI, an automatic segmentation routine developed to handle lesions as an own tissue class, does not produce satisfactory result. The new segmentation routine in SPM8 produces the best results, especially if Chris Rorden’s (professor at The Georgia Institute of Technology) improved TPMs are used. Unfortunately, the layer of CSF is not continuous. The segmentation result can still be used in a FEM simulation, although the result from the simulatation will not be ideal. Neither of the automatic segmentation routines evaluated produce an acceptable result (see Figure 5.7) for stroke patients. Necrotic stroke lesions does not affect the segmentation result as much as the acute dito, especially if there is only a small amount of scar tissue present at the lesion site. The new segmentation routine in SPM8 has the brightest future, although changes need to be made to ensure anatomically correct segmentation results. Post-processing algorithms, relying on morphological prior constraints, can improve the segmentation result further.

BACKGROUND: Parkinson’s disease (PD) is a common debilitating disorder that largely effects the aging population. It is associated with a loss of dopamine-producing brain cells, which leads to abnormal brain activity and ultimately, a loss of locomotor control. Transcranial direct current stimulation (tDCS) is a technology that effectively modulates brain excitability by sending low electric current through the scalp. It has been demonstrated to improve working memory, intelligence, learning ability, as well as relieving symptoms of depression, Alzheimer’s and schizophrenia (Kekic, Boysen, Campbell, & Schmidt, 2015; Khedr et al., 2014; Manor et al., 2015). tDCS may thus serve as an effective therapeutic strategy for this vulnerable PD population. OBJECTIVE: The primary purpose of this study was to examine the acute effects of single sessions of tDCS targeting different brain networks on locomotor control metrics and other outcomes in patients with PD. DESIGN: A pilot, double-blinded, sham-controlled study. METHODS: A total of 15 older adults between the ages of 40-85 with a physician diagnosis of PD will be recruited. Participants are screened with questionnaires to determine eligibility. If eligible, participants will undergo a dual task assessment and a freezing of gait (FOG) provoking protocol prior to, as well as immediately after, a 20-minute session of tDCS. The acute effects of each stimulation session will be observed. There will be three different stimulation conditions that each target different areas of the brain: the motor cortex (M1), the motor cortex and the dorsolateral prefrontal cortex (DLPFC), and a sham (i.e., control) condition. Multiple aspects of locomotion (i.e., FOG, gait speed, stride time variability, percent of each walking stride spent with both feet on the ground) and cognition are assessed. RESULTS: This study began enrolling participants on March 3rd, 2016. To date, one participant has been enrolled and completed baseline testing as well as all three tDCS visits. This 42-year-old participant was diagnosed with PD two years ago and symptoms are mild. No side effects were observed during tDCS and the participant was unable to decipher between the M1 and the sham stimulation, but was able to tell the difference between sessions when receiving multi-focal stimulation. DISCUSSION: In this case study, tDCS was well tolerated by the patient and double-blinding procedures were effective. Thus, while tDCS did not induce significant improvements in gait or cognition in this relatively high functioning patient, the developed study protocol and tDCS intervention are highly feasible in the PD population.

This thesis investigates the neurobiological pathways that underpin learning of visual categories, and the behaviour associated with these neural systems. The work contains two strands. The first assesses the neural and behavioural predictions of the COmpetition between Verbal and Implicit Systems (COVIS) account of category learning. The second aims to examine the brain regions implicated in the prototype effect after transcranial Direct Current Stimulation (tDCS) to the left dorsolateral prefrontal cortex (DLPFC). COVIS predicts there are separate explicit and implicit category learning systems. According to COVIS, the explicit system optimally learns rule-based (RB) categories and relies upon the frontal lobes for working memory (WM) and executive functioning processes, and the medial temporal lobes (MTL) to store decision boundaries. In contrast, the implicit system employs the basal ganglia to procedurally learn information-integration (II) categories through stimulus-response associations. Experiment 1 found little evidence of separable implicit or explicit systems in an fMRI study that investigated category decision making processes during RB and II category learning using conditions matched in difficulty, category separation and number of relevant stimulus dimensions. Contrary to the predictions of COVIS, the MTL was more active during the II condition compared to the RB condition, an area that should be more engaged by the explicit system. There was also extensive neural activation overlap found between RB and II learning. Experiments 2 and 3 aimed to generalise these neural findings to activation during feedback processing in RB and II conditions. Experiment 2 was a behavioural study which showed that adding a feedback delay necessary for fMRI data analysis did not differentially impact RB or II learning. Experiment 3, including this feedback delay, found the same neural pattern of results as Experiment 1 offering further support that the MTL is more engaged in II learning than RB learning. There was also again considerable overlap in the regions involved in the two tasks. Taken together, Experiments 1 to 3 found no evidence for the neurally dissociable category learning systems predicted by COVIS. Experiments 4, 5 and 6 investigated the behavioural dissociation reported by Smith et al. (2014) that deferring feedback to the end of a six trial block selectively impairs II learning compared to a unidimensional RB condition. Experiment 4 replicated this result. However, when equating the number of dimensions relevant for RB and II learning in Experiment 5, both conditions were hindered by deferring feedback, with Experiment 6 confirming that conjunctive RB learning was impaired by deferred feedback compared to immediate feedback. I concluded that the dissociation reported by Smith et al. is attributed to the use of a unidimensional category as a comparison for II performance, and that when the number of relevant stimulus dimensions between conditions are controlled there is little evidence for the separable systems of COVIS. Experiment 7 used tDCS to investigate if RB or II learning was differentially affected by anodal stimulation to the left DLPFC. Although there was no significant difference in learning between category conditions, during anodal stimulation participants improved less across blocks than those receiving sham stimulation. While the results suggest that the effect of tDCS on RB and II learning may be more tangible during stimulation, the numerical pattern of the data warrants further research into the possibility that RB participants are more affected by tDCS than II participants after stimulation to the left DLPFC. Strand 2 of this thesis aimed to further previous work that suggests anodal stimulation to the DLPFC during a prototype distortion task induced a prototype effect (better responding to unseen prototype trials than other category exemplars derived from this prototype) that was not present in sham participants. Contrary to this past work, Experiments 8 and 9 found that anodal stimulation to the left DLPFC inhibited a prototype effect that was present in sham participants. Experiment 10 implemented a combined tDCS and fMRI task and found that anodal participants engaged the stimulated DLPFC and the MTL more than sham participants in measures of the prototype effect. Based on these findings, this thesis argues that anodal tDCS to the left DLPFC inhibits perceptual learning by disrupting error prediction processes. Anodal participants are also considered to use generalization more than sham participants when perceiving category exemplars, a process attributed to the MTL.

It has been estimated that 15 million individuals around the world experience the ill effects of neural disabilities every year. Neural disabilities can affect motor control, such as Locked in Syndrome or Amyotrophic Lateral Sclerosis, whereas other affect working memory, such as schizophrenia, Alzheimer's and Parkinson's. However, recent research has show that mental rehearsal of physical movement tasks may remain intact following higher centre damage, and as such represents a new opportunity to accessing the motor system and using it to control devices. Brain Computer Interfaces (BCI) captures the brain's electrical activity and translates it into real time electrical outputs, independent of the orthodox output pathways of peripheral nervous system and muscles. Utilising the brain's electrical activity BCI has the potential to significantly enhance the lives of many individuals suffering from neurological disorders. Unfortunately, the electrical activity associated with motor activity in these individuals can be lower than normal, with acute cortical infarcts decreasing the alpha wave oscillations for the affected pericentral sensorimotor areas. This has brought into doubt whether the intensity of brain signals in these individuals can be large enough to be used as a BCI system control signal for biofeedback training. This thesis aims to examine both if alternative EEG signal can be used and if externally applied neuromodulation can facilitate the process.