Lara Boyd

Professor

Research Classification

Plasticity / Neuronal Regeneration
Stroke
Learning and Memory
Physiology

Research Interests

Motor learning
Plasticity / Neuronal Regeneration
Motor System
Learning
Neurophysiology

Relevant Degree Programs

 

Research Methodology

Transcranial Magnetic Stimulation (TMS and rTMS)
Magnetic Resonance Imaging (MRI and fMRI)
Electroencephalography (EEG)

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2019
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Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2019)
Ready to move? The effect of stroke on attention and motor planning of voluntary leg movements. (2018)

Background: Many movements individuals perform in a given day are voluntary and goal directed, requiring the ability to focus attention and plan movements according to those goals. Altered cognition and motor impairments after stroke limit functional balance and walking performance. Little research has examined the neurophysiology of attention and planning for standing balance and walking after stroke. Examining the mechanisms underlying attention and planning of balance and walking is paramount to understanding the factors that may be important for stroke recovery and functional community living.Methods: The overarching objective of this thesis was to examine planning and attention for leg movements in individuals with stroke. There were two primary objectives: (1) to examine whether planning differs between the paretic and non-paretic legs measured at the muscle and brain, and (2) to determine how attention may alter, or gate, the processing of irrelevant somatosensory information used for planning. Summary of findings: In Chapters 2 and 3, contrary to expectations, no differences in planning were found between stepping with the non-paretic and paretic legs. However, individuals with greater motor impairment showed larger levels of muscle co-contraction during planning (Chapter 2), and greater cognitive effort and longer planning durations (Chapter 3) than individuals with less motor impairment. For attention, irrelevant somatosensory information was gated by attention during planning plantarflexion movements in young adults compared with rest (Chapter 4). In Chapter 5, the main finding was that some irrelevant somatosensory information was not gated by attention after stroke, while other information was gated. This indicates possible dysfunction in pathways connected to the somatosensory cortex after stroke that can be mediated by attention. More importantly, gating levels during early planning explained a significant amount of variability in a measure of community balance and mobility. Conclusions: This dissertation contributes new knowledge toward understanding the effects of stroke on planning and attention of leg movements. The findings suggest planning and attention are important factors in community levels of balance and mobility that require consideration in future development of targeted neurophysiological assessment and treatment of attention and planning after stroke, with potential impact on balance and walking performance.

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The behavioural and neural science of motor skill learning in healthy individuals and people with stroke (2017)

Background:Due to a high occurrence of motor impairment following stroke, motor learning is fundamentally important for stroke rehabilitation. Motor learning interventions can be difficult to deliver by clinicians and researchers due to individual differences in motor abilities that are compounded by neural changes associated with an ischemic insult. To enhance motor learning, interventions must be grounded in understanding patterns of performance change, both behaviourally and neurologically.Methods:In Chapters 2 and 3, performance data were fitted to exponential curves to measure performance change during skill acquisition. In Chapter 2, curve parameters were used to create an individualized learner-adapted algorithm to manipulate the level of difficulty of motor practice conditions. In Chapter 3, curve parameters were used to measure the rate of skill acquisition in healthy individuals and stroke populations. Chapter 4 explored the functional connectivity of motor-related brain networks before and after skill acquisition in healthy individuals and individuals with stroke. In Chapter 5, residual white matter in a motor-related brain network was used to examine individual responses following an intervention combining non-invasive brain stimulation and motor skill practice in individuals with stroke. Findings:In Chapter 2, skill acquisition under a learner-adapted algorithm, developed from curve parameters, showed that higher individualized levels of difficulty in practice were better for skill retention. In Chapter 3, individuals with stroke showed a slower rate of skill acquisition compared to healthy individuals, which was associated with worse motor performance-related change at a delayed retention test. In Chapter 4, individuals with stroke did not activate motor learning–related functional brain networks in the same manner as healthy individuals following motor skill practice. In Chapter 5, the integrity of white matter in the motor-related brain network was higher in individuals who positively responded to the intervention. Conclusions:The findings from this dissertation highlight the importance of modelling performance data to advance the evaluation of stroke rehabilitative interventions. This dissertation contributes new knowledge of a gray matter motor network associated with motor learning, and a white matter motor network biomarker that characterizes the response to non-invasive brain stimulation paired with motor practice in individuals with stroke.

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The neurophysiology of sensorimotor integration in healthy aging and chronic stroke (2017)

Background: General decline in motor performance is often cited in healthy aging, and motor deficits persist into the chronic phase of stroke recovery. Abnormalities in sensorimotor integration may underlie these motor deficits; however, the effect of aging and chronic stroke on sensorimotor integration neurophysiology is not well understood. Further, investigation into the plasticity of sensorimotor integration is important to establish in populations experiencing sensorimotor decline.Methods: The overall objective of this thesis was to comprehensively understand the neurophysiology of sensorimotor integration, including the influence of aging and chronic stroke on sensorimotor integration and the reliability of common neurophysiological measures. The first research chapter (Chapter 2) explores age and stroke-related differences in measures of indirect sensorimotor integration. Chapters 3 and 4 investigate baseline differences in measures of direct sensorimotor integration induced by aging and chronic stroke, respectively. Further, they use an intervention to index plasticity of sensorimotor integration in these populations. The final chapter (Chapter 5) addresses the reliability of a variety of neurophysiological methods used to examine somatosensory and motor cortical excitability, as well as sensorimotor integration.Summary of Findings: In Chapter 2, older individuals and individuals with chronic stroke had reduced short-latency afferent inhibition, compared to younger individuals suggesting the difference is largely driven by age-related factors. Greater disinhibition post-stroke related to worse motor function and impairment. Chapter 3 showed that measures of direct sensorimotor integration are less susceptible to age-related changes than measures of indirect sensorimotor integration. Sensory training altered direct but not indirect sensorimotor integration, suggesting independent modulation of separate anatomical pathways of sensorimotor integration. Chapter 4 highlighted differences in direct sensorimotor integration between individuals with chronic stroke and older individuals such that vibration had less of an impact on baseline motor cortex excitability in individuals post-stroke and the intracortical response to sensory training was reduced. Chapter 5 showed high reliability in transcranial magnetic stimulation thresholds, the amplitudes of evoked potentials elicited at high stimulation intensities, and latency-based measures.Conclusions: This dissertation contributes new knowledge to the impact of aging and chronic stroke on sensorimotor integration and the reliability of the measures used to quantify sensorimotor integration.

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The effects of acute high-intensity aerobic exercise on motor cortical plasticity and motor learning (2016)

Background: Motor learning is mediated by plasticity of neural circuits involved in movement. As such, there is great interest in the development of strategies that maximize brain plasticity to promote motor learning. Aerobic exercise has emerged as an intervention with robust effects on the nervous system, including interactions with mechanisms of neuroplasticity and memory. Yet, the effects of acute aerobic exercise on plasticity and learning in the motor system are not well understood. Methods: The overall objective of this thesis was to examine the effects of a single bout of high-intensity aerobic exercise on motor cortical (M1) plasticity and motor learning for an upper limb muscle in young healthy individuals. The first three research chapters describe experiments evaluating the effects of acute high-intensity cycling on: M1 plasticity (Chapter 2), continuous motor sequence learning (Chapter 2), activity in cerebello-motor circuits (Chapter 3) and discrete motor sequence learning (Chapter 4). In Chapter 5, a study exploring relationships of genetic and epigenetic variation with acute aerobic exercise effects on M1 plasticity and motor learning is described. Summary of findings: In Chapter 2, M1 plasticity induced by paired associative stimulation was facilitated when preceded by exercise, compared to a period of rest. Further, continuous motor sequence learning was enhanced when exercise was performed prior to task practice. Transcranial magnetic stimulation assessments utilized in Chapter 3 suggested that modulation of activity in cerebello-motor circuits may contribute to exercise-induced facilitation of M1 plasticity. In Chapter 4, exercise prior to discrete motor sequence task practice enhanced the rate of improvement in task performance at a 24-hour retention test, suggesting an effect of aerobic exercise on motor memory retrieval. Finally, in Chapter 5 genetic variants and DNA methylation patterns impacting brain-derived neurotrophic factor and dopamine signaling pathways were associated with inter-individual variability in exercise effects on M1 plasticity and motor learning. Conclusions: This dissertation contributes new knowledge towards understanding the effects of acute high-intensity aerobic exercise on plasticity and learning in the motor system. The findings have implications for development of strategies to prime neuroplasticity and motor learning with acute aerobic exercise in sport or rehabilitation settings.

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Effect of repetitive transcranial magnetic stimulation over supplementary motor area in individuals with Parkinson disease (2011)

Introduction: Parkinson Disease (PD) is a characterized by bradykinesia along with altered oscillations in the basal ganglia (BG) and cortex resulting from loss of dopaminergic neurons in the substantia nigra. In addition, PD is linked to decreased activation of the Supplementary Motor Area (SMA) attributed to altered cortico-basal ganglia pathways resulting in poverty of movement. Purpose: The main goal of the study was to determine whether 5 Hz rTMS over SMA improved motor control - indexed by a serial targeting task (STT) and a handwriting task in individuals with PD. Secondary aims were to 1) record change in cortical oscillations using electroencephalogram (EEG) during STT performance, and 2) to consider the role of the SMA in motor imagery in individuals with PD as indexed by a break test. Methods: In a cross-over design, individuals with PD were randomized to receive either 5 Hz or control rTMS over the SMA. Three experimental tasks were administered prior to and immediately after the rTMS intervention with right hand; the STT, a handwriting task, and a break test. EEG was collected during STT and cortical excitability was indexed by assessing resting motor threshold (RMT). Results: Participants showed an improved motor performance in STT post 5 Hz rTMS. There was concurrent reduction in θ and α oscillations during the reaction time phase of the task in frontocentral and central EEG channels along with lowered RMT in M1 post 5 Hz rTMS. In the handwriting task, participants showed improvement in stroke size, axial pressure after 5 Hz rTMS. Both groups’ (5 Hz and control) performance on the break test was similar.Conclusion: The findings of this thesis suggest that 5 Hz rTMS is beneficial at least in the short-term for individuals with PD. 5 Hz rTMS improved motor performance, shifted cortical oscillations and cortical excitability. However, these effects were noted for only a single session. The results of this thesis may contribute to future research related to development of rTMS as a therapeutic option for individuals with PD.

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Neural and behavioural correlates of motor performance in children with developmental coordination disorder (2010)

Introduction: With a prevalence of 5-6%, developmental coordination disorder (DCD) is one of the most common disorders of childhood. Children with DCD struggle to learn new motor skills, but the neurological mechanisms underlying the disorder are essentially unknown.Purpose: The purpose of this thesis was three-fold: (1) to present a synopsis of current literature examining the potential neural correlates of DCD; (2) to determine if patterns of brain activity differed between children with and without DCD while performing a fine-motor task; and (3) to investigate whether children with DCD are able to demonstrate improved motor learning as evidenced by increased accuracy on a fine-motor task and/or shifts in patterns of brain activation.Methods: A comprehensive literature review of possible neural correlates of DCD was conducted, which provided the background for the two studies included in this thesis. Both of these studies employed a block design and used functional magnetic resonance imaging to map patterns of brain activation associated with motor performance (Chapter 3) and motor learning (Chapter 4) of a fine-motor task. Seven children who met the diagnostic criteria for DCD (ages 8-12 years) and seven typically-developing (TD), closely age-matched children participated in the studies.Results: The literature review implicated the cerebellum as a likely source of dysfunction associated with DCD. Chapter 3 showed that, despite similar levels of behavioural motor performance, substantial differences in patterns of brain activity were noted between children with DCD and TD children. Differences in motor behaviour emerged in Chapter 4, with the DCD group showing little change in tracing accuracy compared to the improvements noted in the TD group. Neuroimaging results from Chapter 4 suggest that children with DCD may have a deficit in updating internal models of movement through under-activation of the cerebellum and/or the cerebello-thalamo-cortical pathway. Conclusion: Findings from this thesis have made several important and novel contributions to our understanding of children with DCD. This work has suggested support for several hypotheses related to the mechanisms underlying DCD and provided some of the first neuroimaging evidence to suggest possible explanations for findings of previous research in children with DCD.

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Master's Student Supervision (2010 - 2018)
Characterizing cortical-cerebellar neurophysiology during visuomotor adaptation in chronic middle cerebral artery stroke (2018)

The acquisition of complex motor skills shapes our behavior and ability to adapt in everyday life. The underlying biological processes are complex, involving multiple neuroanatomic pathways and efficient communication within functional brain networks. Understanding the neural underpinnings of motor learning and adaptation within the context of health and disease is fundamental, and particularly relevant to brain injury, recovery, and rehabilitation. Following middle cerebral artery (MCA) stroke, individuals often experience upper limb motor impairment that continues into the chronic phase. Motor recovery is influenced by the reorganization of brain networks compensating for tissue damage and by well-designed motor rehabilitation programs that promote use of the remaining motor output. However, predicting the outcome of rehabilitation has been difficult and recovery is often incomplete. Impairments in cortical-cerebellar pathways may underlie motor deficits and influence motor function, yet, to date, little work has examined the influence of cortical-cerebellar relationships in relation to motor adaptation in chronic MCA stroke. The overall objective of the present thesis was to investigate the impact of MCA stroke on cortical-cerebellar neurophysiology and motor adaptation using transcranial magnetic stimulation (TMS). TMS was employed to investigate cortical-cerebellar excitability at baseline, and then during a sensorimotor adaptation task in a group of chronic MCA stroke and a group healthy older controls. Both groups had similar cortical-cerebellar excitability before and during the adaptation task. However, individuals with MCA stroke are impaired compared to healthy age matched controls in sensorimotor adaptation. This suggests that adaptation deficits after MCA stroke may be influenced by motor network substrates beyond the cerebellum. The present thesis contributes new knowledge towards understanding the impact of chronic MCA stroke on cortical-cerebellar pathways and sensorimotor adaptation. The finding of sufficient cortical-cerebellar connectivity suggests that it may be a candidate pathway for TMS stimulation to modulate motor related networks for stroke rehabilitation.

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The effects of acute aerobic exercise on motor skill learning and neurophysiology in healthy older adults (2018)

Aging is associated with a reduced ability to perform motor tasks, as well as a decreased capacity for neuroplastic change and motor skill learning. Evidence in young healthy adults suggests a single session of aerobic exercise can facilitate motor learning and changes in movement-related neurophysiological circuits. However, we do not know whether these effects extend to healthy older adults. The objectives of the present thesis were to examine whether a single session of moderate-intensity aerobic exercise 1) facilitates motor skill acquisition and consolidation, and 2) modulates motor cortical and intracortical circuitry, in healthy older adults. Twenty-two participants (55-75 years old) completed a maximal exercise stress test at least two days prior to all other sessions. Next, participants practiced a motor sequence task after 20-minutes of moderate-intensity cycling or 20-minutes of seated rest, on separate occasions. To assess motor learning, participants performed the motor task 24 hours later at a no-exercise retention test. On a separate day, neurophysiological measures using transcranial magnetic stimulation were obtained at two time-points prior to and two time-points following an acute bout of moderate-intensity cycling. Performing aerobic exercise immediately prior to task practice did not yield any statistically significant differences in measures of motor skill acquisition or consolidation. However, we observed a non-significant trend towards improvements in motor memory consolidation, such that under the exercise condition there were greater improvements in repeated sequences compared to rest. Additionally, we found that after exercise there was an increase in long-interval intracortical inhibition (LICI), which returned to near baseline levels within 30 minutes post-exercise. Overall, these findings suggest that a single bout of moderate-intensity aerobic exercise transiently modulates GABA-B mediated intracortical inhibition in healthy older adults, however, these exercise-induced neurophysiological effects may not necessarily translate to changes in motor behaviour.This work is the first to investigate the efficacy of an acute bout of aerobic exercise in facilitating motor performance and learning, as well as modulating motor cortical and intracortical circuits, in healthy older adults. Further understanding of how exercise influences motor learning and neurophysiology in the aging brain will be critical for the development of potential rehabilitation strategies. 

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Characterizing the impact of type-2 diabetes on cortical sensorimotor pathways in chronic stroke : a multimodal neuroimaging study (2016)

Individuals with type-2 diabetes have an increased incidence of ischemic stroke, and experience higher rates of disability after stroke than non-diabetics. Recent research suggests that type-2 diabetes has adverse effects on neuronal integrity and function, however, to date very little work has examined the interactions of diabetes with chronic stroke recovery. The goal of the present thesis is to address this gap in the literature by employing multimodal magnetic resonance imaging (MRI) techniques to examine the impact of type-2 diabetes on the integrity of surviving sensorimotor neural tissue in individuals with chronic hemiparesis as a result of ischemic stroke. We employ volumetric MRI, diffusion tractography, and magnetic resonance spectroscopy (MRS) to explore the structure of motor and sensory cortex grey matter and white matter projections. We found individuals with chronic stroke and diabetes had lower regional cortical thickness in primary somatosensory cortex, and primary and secondary motor cortices. Contralesional primary and secondary motor cortex thicknesses were negatively related to motor outcomes of the paretic upper-limb in the diabetes group. MRS revealed stroke survivors with diabetes had bilaterally reduced creatine levels in sensorimotor cortex. Diabetes status did not impact gross cortical volumes, white matter volumes, or white matter microstructure in projections from the primary motor and sensory cortex. These results suggest that type-2 diabetes alters cerebral metabolic function, which may result in thinning to sensorimotor grey matter. This work provides preliminary evidence for differential profiles of cerebral recovery from stroke in individuals with diabetes. Given the worldwide increase in the prevalence of diabetes it is critical that we examine the mechanisms of increased post-stroke disability in type-2 diabetes to inform targeted therapies for this population.

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Corticospinal and strength adaptations following unilateral strength training after stroke (2016)

Unilateral strength training of the less affected (LA) limb has been shown to improve strength bilaterally. This improved strength is referred to as cross-education in the literature. This intervention has the potential to be beneficial for individuals who cannot train both sides of the body due to post-stroke hemiparesis. To date only one group has researched cross-education in the upper limb in stroke, with varied results. The main purpose of our work was to determine if strength training of the LA forearm would change patterns of cortical excitability bilaterally after stroke, and additionally affect changes in strength and function bilaterally. Twenty-four participants with chronic (> 6 months) stroke-related hemiparesis engaged in three baseline sessions separated by 4-7 days. During these sessions individuals’ forearm strength, motor function, and motor impairment were tested, along with a TMS based assessment of corticospinal excitability and intracortical circuits. On a fourth visit participants completed their first training session using the LA arm, then were given the same wrist extension strength-training device to take home. Participants completed three 25-minute training sessions, weekly; one in the laboratory and the remaining two at home. After 5 weeks of training, participants returned to the laboratory for post-intervention retention tests. Cross-education increased strength in the LA wrist extensors (p = 0.026) and the untrained, more-affected (MA) wrist extensors (p = 0.05) in participants with chronic stroke, at the 1-week retention test. Further, LA arm strength remained increased at 5-week retention test (p = 0.023) despite there being no further training. There were strength improvements in the majority of participants in both their trained (17 of 24) and untrained (12 of 24) wrist extensors. There was a decrease in corticospinal inhibition in the LA hemisphere, and a release of interhemispheric inhibition (IHI) bilaterally. A significant increases in motor function and a decrease in motor impairment was seen, respectively. Results indicate that cross-education could be a valuable tool for increasing strength in chronic stroke. Cross-education training of the LA upper limb may allow individuals who do not have adequate function in their MA limb prior to training engage in rehabilitative interventions post-training.

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Effects of an acute bout of moderate-intensity aerobic exercise on motor learning and neuroplasticity. (2015)

Aerobic exercise has been promoted as a possible adjunct therapy to neurorehabilitation practice, given its positive effects on brain health. In healthy young adults, acute high-intensity cycling can enhance motor performance and learning of a complex motor task, and promote neuroplasticity in the motor system. However, clinical populations may not be able to participate in high-intensity exercise. To date there is inconsistent evidence for the efficacy of moderate-intensity aerobic exercise to alter motor learning and neuroplasticity in healthy young adults. Using two experiments, we aimed to determine how acute moderate-intensity cycling affects motor behavior and neuroplasticity in healthy young individuals. First, 16 participants practiced a complex motor skill after 30 minutes of moderate-intensity cycling or seated rest, on separate occasions. Motor performance was assessed at baseline, immediately after, and 5 minutes after exercise or rest. Twenty-four hours later, we assessed motor learning at a no-exercise retention test. Under the exercise condition, participants maintained performance over time, whereas, performance diminished over time under the rest condition, and became worse than post-exercise performance. Conditions did not differ at retention. Second, another group of 16 participants underwent paired associative stimulation (PAS) a transcranial magnetic stimulation (TMS) protocol known to induce neuroplasticity in the motor system. Effects of PAS were separately compared after a 30-minute bout of moderate-intensity cycling versus seated rest. At baseline, immediately after PAS, and 30 minutes post-PAS, we measured corticomotoneuronal excitability and excitability of intracortical neural circuits using TMS. We found that PAS increased corticomotoneuronal excitability when performed after exercise, but not rest. Exercise and PAS modulated activity in specific neural circuits post-intervention, without similar results under the rest condition. Moderate-intensity aerobic exercise can promote neuroplasticity in the motor system, but in this study similar effects did not transfer to behavioral measures of motor learning. In order to evaluate the clinical feasibility of this pairing moderate intensity exercise with skilled motor practice, we must first elucidate the dose-response effects of exercise on motor behavior, explore timing effects of exercise on motor learning, and examine how long-term pairing of exercise with practice impacts motor learning.

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Effects of short-term bimanual coordination training on modulation of sensorimotor cortical activity and motor performance (2015)

Chronic upper extremity hemiparesis is common after stroke (Jørgensen et al., 1995). This chronic impairment has a direct impact on functional independence and the ability to perform daily activities (Lloyd-Jones et al., 2010). Given the high levels of functional losses after stroke, investigation into treatments for chronic impairments should be considered. The purpose of the current study was to examine the effects of short-term bimanual coordination training on the modulation of sensorimotor cortical activity and motor performance. Thirty healthy participants were randomized to one of three training groups: 1) physical practice, 2) observational practice, and 3) no practice (control condition). Movement-related potentials (MRPs) and somatosensory evoked potentials (SEPs) were collected before and after training to examine the effects of training on cortical activity. Motor performance on the bimanual coordination task was also compared between groups. The results showed that: (1) there was no significant difference in MRP or SEP measures between groups, (2) the physical practice group performed significantly better (as indexed by greater accuracy following practice) than the control group on the bimanual coordination task, (3) although the observational practice group did not perform as well as the physical practice group, there was a trend for greater accuracy following observation as compared to the control group. These results suggest that both short-term physical and observational practice of a bimanual coordination task can result in improved motor performance and provide support for the use of observational practice in motor learning.

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Looking beyond the motor cortex : examining the potential of the primary sensory cortex as a target for repetitive transcranial magnetic stimulation after stroke (2014)

Stroke is the leading cause of chronic adult disability, and standard post-stroke therapiesmay not be sufficient for individuals to reach their full recovery potential. When paired withskilled motor practice, non-invasive brain stimulation techniques such as repetitive transcranialmagnetic stimulation (rTMS) may enhance motor recovery by transiently modulating corticalexcitability, effectively priming the brain to facilitate mechanisms of motor learning. Dependingon the pulse frequency, rTMS may be used to increase cortical excitability in the damagedhemisphere, or decrease it in the undamaged hemisphere, with the goal of re-establishing anormal interhemispheric balance. While theoretically promising, the majority of studiesconsidering the effects of rTMS over the primary motor cortex (M1) have shown relatively smalleffect sizes and high inter-individual variability. Improved effect sizes may be produced by 1)finding the optimal cortical target for stimulation, rather than defaulting to M1, and 2) choosingan appropriate sample that will optimally benefit from the intervention. In the following thesis,we will explore the potential of the primary sensory cortex (S1) as an alternative target for rTMSintervention, and the anatomical and physiological variables that may help to identify who maybest benefit from this intervention. First, we describe a randomized, single blind experimentcomparing the impact of active versus sham rTMS over S1 paired with practice of a skilledvisuomotor reaching task in individuals with chronic stroke. Second, we describe a retrospectiveanalysis of the participants from the first experiment, to determine whether individual differencesin morphology of the underlying sensory cortex might be predictive of rTMS responsiveness.Third, we describe an exploratory study using a paired median nerve somatosensory evokedpotential paradigm using electroencephalography in healthy individuals, to elucidate theneurophysiological mechanism of interhemispheric inhibition between S1s. We conclude that S1should be considered as a viable target for future rTMS trials as an adjunct therapy torehabilitation after stroke.

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Exploring the neural network underlying task-relevancy influences on movement-related gating (2013)

Movement-related gating is influenced by task-relevancy manipulations, such that increased sensory information ascends to the cortex when information is relevant, but does not when it is irrelevant (1). Regardless of relevancy, during movement smaller cortical somatosensory responses are produced as compared to those evoked by similar stimulation at rest (1). These task-relevancy effects have specifically been documented during movement of the lower limb (1). Task-relevancy effects have been hypothesized to be controlled by the prefrontal cortex (PFC) based on this region's known role in selective attention, as well as filtering of distracting information at later stages of somatosensory processing (2). The purpose of the current study was first to verify task-relevancy influences on movement-related gating in the upper limb, and second to test the contribution of the PFC to these relevancy effects. Eleven healthy participants received median nerve stimulation at the left wrist during three conditions: rest, task-irrelevant movement, and task-relevant movement. The cortical responses to these median nerve stimulations were measured in the form of somatosensory evoked potentials (SEPs). Each of these three conditions was collected on a baseline day and on two separate days following either continuous theta burst (cTBS), which has a net inhibitory effect on cortical excitability, over the contralateral primary somatosensory cortex (S1) or the right dorsolateral prefrontal cortex (DLPFC). Results demonstrated a significant interaction effect between the stimulation site and the condition, with post-hoc tests revealing that following cTBS over S1 or DLPFC, relevancy based modulation of SEP amplitude was abolished. These results indicate that both S1 and DLPFC are integral to individual ability to facilitate relevant sensory information in order to complete a motor task.

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Investigating the relationship between anatomic and metabolic changes in the motor cortex and upper-extremity hemiparesis in individuals with chronic subcortical stroke (2013)

Hemiparesis is one of the most prevalent chronic disabilities after stroke, particularly in subcortical stroke. Neuroimaging has provided important morphological insight in to the mechanisms associated with hemiparesis in individuals with stroke. Assessing morphological changes within the primary motor cortex may provide valuable information of the neural events that underlie upper-extremity (UE) hemiparesis in chronic stroke. The purposes of this study were to 1) evaluate anatomical and metabolic changes in the motor cortex, and 2) examine the relationship between anatomical and metabolic changes and hemiparetic arm use in individuals in the chronic stage of stroke recovery. Seventeen individuals with chronic (>6 months) subcortical ischemic stroke and eleven neurologically healthy controls were recruited. Single voxel proton magnetic resonance spectroscopy (H1MRS) was performed to measure metabolite concentrations of total N-acetylaspartate (tNAA) and glutamate+glutamine (Glx). FreeSurfer software package (http://surfer.nmr.mgh.havard.edu) was used to quantify cortical thickness of the precentral gyrus. Upper-extremity motor performance was assessed using the Wolf Motor Function Test (WMFT) and the Motor Activity Log Quality of Movement scale (MAL-QOM) and upper-extremity motor activity was assessed using activity counts from wrist-mounted accelerometers. Results demonstrated a significant decrease in tNAA and Glx concentration in the hand area of the primary motor cortex in the stroke group, particularly within the ipsilesional hemisphere. Precentral gyrus cortical thickness was also decreased in the ipsilesional hemisphere of the stroke group. Parametric correlation analysis revealed a significant positive correlation between precentral gyrus thickness and tNAA concentration bilaterally. Multivariable regression analyses revealed that, after accounting for age and post-stroke duration, the combination of ipsilesional metabolite concentration (tNAA and Glx) and ipsilesional cortical thickness was associated with hemiparetic UE motor performance, but not UE motor activity in individuals in the chronic stage of stroke recovery. The observed link between structural and neurochemical changes in the stroke-affected brain and hemiparetic UE motor performance during the chronic phase of recovery may improve the understanding of the underlying neural mechanisms that support motor impairment after stroke.

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Continuous theta burst stimulation combined with skilled motor practice after stroke : effects on implicit learning and electroneurophysiology (2011)

After stroke, cortical excitability is decreased in the ipsilesional primary motor cortex andincreased in the contralesional primary motor cortex. This abnormal pattern of excitabilitydetrimentally affects performance with the hemiparetic arm. Short lasting improvements inmotor performance occur following repetitive transcranial magnetic stimulation (rTMS) over thecontralesional hemisphere after stroke; however, no work has considered the impact of pairingrTMS with skilled motor practice over multiple days on motor learning, hemiparetic armfunction, or electroneurophysiology in the brain. The aim of this thesis was to determine theimpact of 3 days of continuous theta burst stimulation (cTBS) over contralesional primary motorcortex paired with skilled motor practice on 1) learning of a novel motor task and hemipareticarm motor function and 2) levels of intracortical inhibition, intracortical facilitation, andtranscallosal inhibition following stroke. In a cross-over design, participants with chronic strokewere randomized to first receive either active or sham cTBS over the contralesional primarymotor cortex. Functional measures, motor task performance, and electroneurophysiology wereassessed at baseline. 3 days of cTBS paired with skilled motor practice were completed;functional measures, motor learning, and electroneurophysiology were re-evaluated at posttesting.After a 2-week washout period participants underwent the second half of the study withthe other form of cTBS. Participants showed larger motor learning related change followingactive cTBS than sham cTBS. The magnitude of this improvement correlated with enhancedperformance on standardized measures of arm function after stroke. Active cTBS also decreasedlevels of facilitation in the contralesional hemisphere and decreased the amount of inhibitionbeing sent from the contralesional hemisphere to the ipsilesional hemisphere. No adverse effectswere reported. Results of this thesis suggest that cTBS over the contralesional motor cortexiiipaired with skilled motor practice facilitates both improved hemiparetic arm function and motorlearning beyond that seen with skilled motor practice alone. The results of this thesis contributeto research relevant to rehabilitation of individuals with stroke and may facilitate thedevelopment of new rehabilitation strategies to improve functional recovery after stroke.

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Does primary motor cortex plasticity parallel adaptive modification to human walking? (2011)

Walking in a novel environment, such as with resistance, has been associated with changes in muscle activity (Lam, Anderschitz & Dietz, 2006), limb position (Lam et al., 2006; 2008) and cortical spinal activity (Capaday, Lavoie, Barbeau, Schneider & Bonnard, 1999; Bonnard, Camus, Coyle & Pailhous, 2002) compared to baseline walking measurements. Previous literature on locomotor adaptation, suggests that the nervous system has the ability to adapt to task demands. The location and mechanisms of these physiological and kinematic changes are still unknown. The purpose of the current study was to verify that corticospinal (CS) excitability is altered by resisted walking. Second, we explored whether CS changes are modulated by attention and lastly whether changes in excitability are muscle specific. Locomotor adaptations were induced in 40 healthy participants using a robotic gait-assisted treadmill (Lokomat). Velocity-dependent resistance was applied against hip and knee movements during walking. CS excitability was assessed by quantifying motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation immediately before and after adaptation to both resisted and nonresisted walking. Recruitment curves were collected by stimulating at increments of 5% from 105-145%AMT. To determine whether adaptation is muscle specific, MEPs were measured through random assignment of either the biceps femoris (BF) or rectus femoris (RF). To evaluate the impact of attention on adaptive walking, half the participants attended to their walking pattern via a visual feedback tracking task (post_cog). The other half watched a controlled visual stimulus (post_nocog). Results demonstrated a significant increase in MEP amplitude in the BF and not the RF following resisted walking. The post_cog condition adaptations did not reveal an increase in MEP amplitude compared the post_nocog condition. Results suggest that locomotor adaptations result in an increase in CS excitability that is muscle specific. Focused attention to motor adaptation may not be an important modulator of movement and motor learning as has been reported in past work. The current study is the first to consider both the role of the CS system in adaptations during walking and the impact of attention on CS excitability and parallels previous findings on muscle specific locomotor adaptations.

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