Humans possess the ability to generate an incredible degree of complex, highly skilled, and coordinated movements. Although much is known about the anatomical and physiological components of upper limb movement, the exact means by which these different areas coordinate is still far from understood. The ability to perform symmetrical, bimanual tasks with ease suggest a default coupling between mirror motor regions – a default coupling that is perceptible in unilateral movements. During intended unimanual movement in the upper limbs, bilateral changes to motor cortex output occur. The purpose of this study was to investigate the neural underpinnings of these bilateral changes and investigate the involvement of intracortical inhibitory circuits. Previous studies have shown that transcallosal connections between cortical representations of the intrinsic muscles of the hands are relatively sparser than the more proximal muscles of the upper limbs. It was hypothesized that differential responses in overall motor output or intracortical inhibition to ipsilateral muscle activation between the FDI and ECR could infer the involvement of transcallosal pathways; although interhemispheric transfer was not directly investigated in this thesis. Two studies used focal transcranial magnetic stimulation (TMS), specifically paired-pulse protocols, to investigate changes in short-interval intracortical inhibition (SICI) and long-interval intracortical inhibition (LICI) in response to contraction of contralateral homologous muscle groups to the inactive test muscle. Also, the response to activation of a non-homologous, but spatially close, muscle was investigated. Lastly, two muscle groups were investigated, a distal, intrinsic muscle of the hand (first dorsal interosseous) and a relatively more proximal muscle of the upper limb (extensor carpi radialis). These studies revealed that at low levels of force generation, unilateral isometric contractions facilitate ipsilateral mirror motor representations and reduce local GABA¬A receptor mediated inhibition. Notably, while similar facilitation occurred in both the distal and proximal effectors, decreases in SICI were much more robust in the ECR. Findings from this thesis provides insight into the neural mechanisms governing bilateral changes with unilateral movement and is important in the guiding the focus of future research.

Transcranial magnetic stimulation (TMS) is a non-invasive technique that can be used to assess the integrity neuronal circuits in the motor cortex, both at the intrahemispheric and interhemispheric level. In the present study, TMS was used to examine age-related modulation of corticospinal function. Participants underwent hand function testing to examine possible links between TMS measures and manual ability. Participants consisted of healthy young (n=13) and senior (n=17) right-handed individuals. Hand function testing consisted of a battery of tests administered bilaterally to assess each participant’s dexterity, strength, movement speed and reaction time. The following TMS measures were assessed bilaterally: resting motor threshold, recruitment curve and silent periods of the contralateral and ipsilateral hand. Both young and senior subjects showed significant intermanual differences in most behavioral measures, favoring their dominant right hand. There was an age-related difference in TMS measures indicating a decline in intrahemispheric excitability and interhemispheric inhibition. A general trend linking specific TMS measures in the active state with age-related changes in hand function on the dominant hand was found. Our results suggest that TMS markers of corticospinal excitability can be used to predict declining hand function with age and thus could provide an early diagnosis of pathological aging.

This thesis consists of four chapters. In this thesis we explored associative plasticity of human motor cortex with the use of noninvasive transcranial magnetic stimulation (TMS). Paired Associative Stimulation (PAS) has grown in popularity because of its potential clinical applications. We used TMS techniques in combination with electromyographic (EMG) measurements to study cortical excitability and kinematic features of arm movement. This work has focused in a cohesive approach to answer certain fundamental questions about a) the rules of cortical plasticity and mechanism of PAS, b) the interaction between the state of neuronal excitability at the targeted cortical network and the effects of PAS, and c) translation of these effects into obvious measurable kinematic changes starting from network level changes and ending up with the behavioral modulation of arm movement. First we explored the role of GABAergic intracortical networks and intracortical facilitation on modulation of cortical excitability by showing for the first time that PAS can be conditioned by these inhibitory and facilitatory intracortical networks. Next, using standard indirect approaches utilizing peripheral EMG measures, we showed a graded excitability response for the PAS technique and showed that interactions of PAS with motor learning depends on the degree as well as the state of cortical excitability. Rules governing the interactions of brain stimulation techniques and motor learning are important because brain stimulation techniques can be used to modify, improve or disrupt motor adaptation and skill learning with great potential for clinical applications such as facilitation of recovery after stroke. TMS provide us with a unique opportunity to study the rules of plasticity at a systems level, which is a combination of synaptic and nonsynaptic (metaplastic) changes. These changes can occur either in the direction to limit the physiological range of neuronal functioning (homeostatic) or against the direction established state of neurons.

The human primary motor cortex (M1) undergoes structural and functional change throughout life by a process known as neuroplasticity. Techniques which artificially induce neuroplastic changes are seen as potential adjunct therapies for neurological conditions reliant on neuroplasticity for recovery of function. Unfortunately, the reported improvements in function when these techniques have been used in combination with regular rehabilitation have so far been inconsistent. One reason attributed to this is the large variability in effectiveness of these techniques in inducing neuroplastic change. This thesis has investigated factors influencing the effectiveness and reproducibility of neuroplasticity induction in human M1 using several experimental paradigms. The effectiveness and reproducibility of inducing neuroplasticity in human M1 using two variants of a paired associative stimulation (PAS) protocol was investigated in the first set of experiments (Chapter 2). Both protocols repeatedly paired a peripheral electrical stimulus to the median nerve of the left wrist with single-pulse transcranial magnetic stimulation (TMS) delivered 25 ms later to the contralateral M1. Neuroplastic changes were quantified by comparing the amplitude of the muscle evoked potential (MEP) recorded in abductor pollicis brevis (APB) muscle by suprathreshold TMS prior to and following PAS. With both protocols, neuroplasticity induction was more effective, and the responses across sessions more reproducible, if the experiments were performed in the afternoon compared to the morning. Subsequent experiments confirmed the time of day modulation of PAS-induced neuroplasticity by repeatedly testing twenty-five subjects on two separate occasions, once in the morning (8 am), and once in the evening (8 pm) (Chapter 3). Time of day was also shown to modulate GABAergic inhibition in M1. In a further set of experiments, a double-blind, placebo-controlled study demonstrated that artificially elevated circulating cortisol levels (with a single oral dose of hydrocortisone) inhibits PAS-induced neuroplasticity in the evening (8 pm), indicating that the time of day modulation of neuroplasticity induction with PAS is due, at least in part, to differences in circulating cortisol levels (Chapter 3). The cortical circuits that are modulated by PAS have also been shown to be important in motor learning. Therefore, the final set of experiments, described in Chapter 4, investigated whether motor-training-related changes in motor performance (and cortical excitability) following a ballistic motor training task are also modulated by time of day. Twenty-two subjects repeatedly abducted their left thumb with maximal acceleration for thirty minutes during two experimental sessions (morning (8 am) and evening (8 pm)) on separate occasions. Motor training improved motor performance, and increased cortical excitability, however these changes were independent of time of day. It may be that the motor training task and/or outcome measures used were not sufficiently sensitive to detect a subtle time of day effect of motor training on motor performance. Alternatively, the normally functioning motor system may be able to compensate for changes in cortical excitability to maintain optimal motor performance. These findings have important implications for therapies reliant on neuroplasticity for recovery of function, and indicate that rehabilitation may be most effective when circulating cortisol levels are low. / Thesis (Ph.D.) - University of Adelaide, School of Molecular and Biomedical Science, 2009

The human primary motor cortex (M1) undergoes structural and functional change throughout life by a process known as neuroplasticity. Techniques which artificially induce neuroplastic changes are seen as potential adjunct therapies for neurological conditions reliant on neuroplasticity for recovery of function. Unfortunately, the reported improvements in function when these techniques have been used in combination with regular rehabilitation have so far been inconsistent. One reason attributed to this is the large variability in effectiveness of these techniques in inducing neuroplastic change. This thesis has investigated factors influencing the effectiveness and reproducibility of neuroplasticity induction in human M1 using several experimental paradigms. The effectiveness and reproducibility of inducing neuroplasticity in human M1 using two variants of a paired associative stimulation (PAS) protocol was investigated in the first set of experiments (Chapter 2). Both protocols repeatedly paired a peripheral electrical stimulus to the median nerve of the left wrist with single-pulse transcranial magnetic stimulation (TMS) delivered 25 ms later to the contralateral M1. Neuroplastic changes were quantified by comparing the amplitude of the muscle evoked potential (MEP) recorded in abductor pollicis brevis (APB) muscle by suprathreshold TMS prior to and following PAS. With both protocols, neuroplasticity induction was more effective, and the responses across sessions more reproducible, if the experiments were performed in the afternoon compared to the morning. Subsequent experiments confirmed the time of day modulation of PAS-induced neuroplasticity by repeatedly testing twenty-five subjects on two separate occasions, once in the morning (8 am), and once in the evening (8 pm) (Chapter 3). Time of day was also shown to modulate GABAergic inhibition in M1. In a further set of experiments, a double-blind, placebo-controlled study demonstrated that artificially elevated circulating cortisol levels (with a single oral dose of hydrocortisone) inhibits PAS-induced neuroplasticity in the evening (8 pm), indicating that the time of day modulation of neuroplasticity induction with PAS is due, at least in part, to differences in circulating cortisol levels (Chapter 3). The cortical circuits that are modulated by PAS have also been shown to be important in motor learning. Therefore, the final set of experiments, described in Chapter 4, investigated whether motor-training-related changes in motor performance (and cortical excitability) following a ballistic motor training task are also modulated by time of day. Twenty-two subjects repeatedly abducted their left thumb with maximal acceleration for thirty minutes during two experimental sessions (morning (8 am) and evening (8 pm)) on separate occasions. Motor training improved motor performance, and increased cortical excitability, however these changes were independent of time of day. It may be that the motor training task and/or outcome measures used were not sufficiently sensitive to detect a subtle time of day effect of motor training on motor performance. Alternatively, the normally functioning motor system may be able to compensate for changes in cortical excitability to maintain optimal motor performance. These findings have important implications for therapies reliant on neuroplasticity for recovery of function, and indicate that rehabilitation may be most effective when circulating cortisol levels are low. / Thesis (Ph.D.) - University of Adelaide, School of Molecular and Biomedical Science, 2009

A Neuronavegação é uma técnica de visualização computacional da localização de instrumentos em relação às estruturas neuronais. A estimulação magnética transcraniana (EMT) é uma ferramenta para estimulação cerebral não-invasiva, que tem sido utilizada em aplicações clínicas, para o tratamento de algumas patologias, e também em pesquisas. Entretanto, a EMT é uma técnica altamente dependente de parâmetros como o posicionamento e orientação da bobina de estimulação em relação às estruturas neuronais. Para auxiliar no posicionamento da bobina, uma combinação entre neuronavegação e EMT é utilizada, chamada de EMTnavegada (EMTn). Essa técnica permite o monitoramento em tempo real da bobina de EMT em relação às neuroimagens. Porém, a utilização da EMTn ainda é pouco explorada, tanto na pesquisa quanto no ambiente clínico, devido ao alto custo, exigência da imagem de ressonância magnética, complexidade e baixa portabilidade dos sistemas de EMTn comerciais. O neuronavegador de código aberto e livre, InVesalius Navigator, vem sendo desenvolvido para ajudar a suprir essa necessidade. Assim, o objetivo desta dissertação foi desenvolver ferramentas para o sistema de neuronavegação InVesalius Navigator. As funcionalidades adicionadas foram: i) suporte para três tipos de rastreadores espaciais; ii) sincronização da EMT com o neuronavegador; iii) guia para o reposicionamento da bobina. Além disso, com intuito de contornar a necessidade de utilizar a imagem de ressonância magnética foram realizados estudos para a substituição por uma imagem padrão. Na parte de desenvolvimento, experimentos de caracterização foram realizados para validação das ferramentas. O sistema de neuronavegação apresentou-se intuitivo e de fácil portabilidade. Além disso, a precisão obtida foi semelhante à de sistemas comerciais. Os erros de localização foram inferiores a 3 mm, considerados aceitáveis para aplicações clínicas. Na segunda parte, procedimentos que não exigem extrema precisão, como a localização e digitalização do hotspot, a variabilidade foi considerada aceitável. Portanto, a utilização da imagem média mostrou-se uma possível alternativa para as imagens de ressonância magnética. / Neuronavigation is a computer image-guided technique to locate surgical instruments related to brain structures. The transcranial magnetic stimulation (TMS) is a non-invasive brain stimulation method, it has been used for clinical purposes, treating neurological disorders, and also for research purpose, studying cortical brain function. However, the use of TMS is highly dependent on coil position and orientation related to brain structures. The navigated TMS (nTMS) is a combined technique of neuronavigation system and TMS, this technique allows tracking TMS coil by image guidance. Yet, nTMS is not widely used, either in research and in the clinical environment, due to the high cost, magnetic resonance imaging requirement, complexity, and low portability of commercial TMS systems. Thus, the aim of this dissertation was to develop tools for the neuronavigator system InVesalius Navigator, such as: i) support for three types of spatial trackers; ii) synchronization of the TMS with the neuronavigator; iii) guide for coil repositioning. In addition, in order to overcome the magnetic resonance imaging requirement, studies were made to replace it with a standard brain image. In the development part, characterization experiments were done to validate the new functionalities. Therefore, the accuracy obtained was similar to commercial systems. Localization errors were less than 3 mm considered acceptable for clinical applications. In the second part, for procedures that do not require extreme accuracy, such as the location and scanning of the hotspot, the variability was considered acceptable. Therefore, the use of the standard brain image was a possible alternative for magnetic resonance imaging.

Fatigue is defined as an exercise-induced decrease in maximal voluntary force produced by a muscle. Fatigue may arise from central and/or peripheral mechanisms. Supraspinal fatigue (a component of central fatigue) is defined as a suboptimal output from the motor cortex and measured using transcranial magnetic stimulation (TMS). Reductions in O2 supply (hypoxia) exacerbate fatigue and as the severity of hypoxia increases, central mechanisms of fatigue are thought to contribute more to exercise intolerance. In study 1, the feasibility of TMS to measure cortical voluntary activation and supraspinal fatigue of human knee-extensors was determined. TMS produced reliable measurements of cortical voluntary activation within- and between-days, and enabled the assessment of supraspinal fatigue. In study 2, the mechanisms of fatigue during single-limb exercise in normoxia (arterial O2 saturation [SaO2] ~98%), and mild to severe hypoxia (SaO2 93-80%) were determined. Hypoxia did not alter neuromuscular function or cortical voluntary activation of the knee-extensors at rest, despite large reductions in cerebral oxygenation. Maximal force declined by ~30% after single-limb exercise in all conditions, despite reduced exercise time in severe-hypoxia compared to normoxia (15.9 ± 5.4 vs. 24.7 ± 5.5 min; p < 0.05). Peripheral mechanisms of fatigue contributed more to the reduction in force generating capacity of the knee-extensors following single-limb exercise in normoxia and mild- to moderate-hypoxia, whereas supraspinal fatigue played a greater role in severe-hypoxia. In study 3, the effect of constant-load cycling exercise to the limit of tolerance in hypoxia (SaO2 ~80%) and normoxia was investigated. Time to the limit of tolerance was significantly shorter in hypoxia compared to normoxia (3.6 ± 1.3 vs. 8.1 ± 2.9 min; p < 0.001). The reductions in maximal voluntary force and knee-extensor twitch force at task-failure were not different in hypoxia compared to normoxia. However, the level of supraspinal fatigue was exacerbated in hypoxia, and occurred in parallel with reductions in cerebral oxygenation and O2 delivery. Supraspinal fatigue contributes to the decrease in whole-body exercise tolerance in hypoxia, presumably as a consequence of inadequate O2 delivery to the brain.

L'empathie nous permet de comprendre et de réagir aux sensations des autres individus. Regarder une situation douloureuse peut induire des comportements de type prosociaux orientés vers les autres ou des réponses d'évitement comme celles enregistrées en réponse à une menace. Le but principal de cette thèse était d'étudier les comportements d'approche/évitement et freezing qui se produisent soit en observant la douleur des autres, soit pendant l'anticipation de la douleur. Deux tâches manipulant la prise de perspective ont permis d'enregistrer des cotations supérieures concernant le niveau de douleur, des temps de réaction inférieurs (expérience 1) et des index de réponses d'évitement plus grands (expérience 2) pour la perspective consistant à imaginer que le sujet représenté dans une condition douloureuse était la personne la plus aimée. Dans la troisième expérience, nous avons analysé le comportement du freezing au niveau du système corticospinal du participant : un effet du freezing spécifique fut rapporté uniquement lorsque de la présentation des stimuli douloureux en perspective du première personne. Dans une quatrième expérience, l'effet du freezing, normalement présent en réponse aux stimuli douloureux fut aussi rapporté dans le cadre de l'anticipation de la douleur pour soi-même. Nos études suggèrent que ce sont principalement les mécanismes cognitifs de prise de perspective qui modulent la réponse empathique et que la perspective de la personne la plus aimée induise la réponse empathique la plus forte. Au contraire les réponses du freezing des modulations corticospinales sont principalement observées lorsque le sujet adopte une perspective en première personne / Empathy allows us to understand and react to other people feelings. Regarding empathy for pain, a witness looking at a painful situation may react to other-oriented and prosocial-altruistic behaviors or self-oriented withdrawal responses. The main aim of this thesis was to study approach/avoidance and freezing behavioral manifestations that co-occurring along with both others’ pain observation and during the anticipation of pain. In two perspective-taking tasks, we investigated the influence of the type of relationship between the witness and the target in pain. Results showed that higher pain ratings, lower reactions times (experiment 1) and greater withdrawal avoidance postural responses (experiment 2) were attributed when participants adopted their most loved person perspective. In experiment 3, we analyzed the freezing behavior in the observer’s corticospinal system while subject was observing painful stimuli in first-and third-person perspectives. Results showed the pain-specific freezing effect only pertained to the first-person perspective condition. An empathy for pain interpretation suggests empathy might represent the anticipation of painful stimulation in oneself. In experiment 4 results, we found that the freezing effect present during a painful electrical stimulation was also present in the anticipation of pain. In conclusion, our studies suggest that cognitive perspective-taking mechanisms mainly modulate the empathic response and the most loved person perspective seems to be prevalent. In addition, more basic pain-specific corticospinal modulations are mainly present in the first-person perspective and it seems to not be referred to the empathy components

Transcranial magnetic stimulation (TMS) is a non-invasive technique that can be used to assess the integrity neuronal circuits in the motor cortex, both at the intrahemispheric and interhemispheric level. In the present study, TMS was used to examine age-related modulation of corticospinal function. Participants underwent hand function testing to examine possible links between TMS measures and manual ability. Participants consisted of healthy young (n=13) and senior (n=17) right-handed individuals. Hand function testing consisted of a battery of tests administered bilaterally to assess each participant’s dexterity, strength, movement speed and reaction time. The following TMS measures were assessed bilaterally: resting motor threshold, recruitment curve and silent periods of the contralateral and ipsilateral hand. Both young and senior subjects showed significant intermanual differences in most behavioral measures, favoring their dominant right hand. There was an age-related difference in TMS measures indicating a decline in intrahemispheric excitability and interhemispheric inhibition. A general trend linking specific TMS measures in the active state with age-related changes in hand function on the dominant hand was found. Our results suggest that TMS markers of corticospinal excitability can be used to predict declining hand function with age and thus could provide an early diagnosis of pathological aging.

Theta-burst stimulation (TBS) is a form of repetitive transcranial magnetic stimulation (TMS) developed to induce neuroplasticity. TBS usually consists of 50 Hz bursts at 5 Hz intervals. When applied intermittently, it can lead to facilitation of motor evoked potentials (MEPs), although these effects can be variable between individuals. Here, we aimed to determine whether a version of intermittent TBS (iTBS) consisting of 30 Hz bursts at 6 Hz intervals would produce less variable modulation. Nineteen healthy adults underwent single-pulse TMS to assess corticomotor excitability at baseline as reflected in MEP amplitude. 30 Hz iTBS was then administered and MEP amplitude was reassessed at 5-, 20- and 45-mins after the iTBS protocol. Compared to baseline, MEPs were significantly facilitated up to 45-min post-iTBS and most participants exhibited the expected facilitation. These observations suggest that 30 Hz/6 Hz iTBS may provide a sound alternative to induce consistent neuromodulatory effects over the commonly used 50 Hz/5 Hz protocol.