Mark Carpenter


Relevant Degree Programs


Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Nov 2020)
A novel approach to studying postural instability in Parkinson's disease (2020)

Individuals with Parkinson’s disease (PD) often experience postural instability, a debilitating and largely treatment-resistant symptom. A better understanding of the neural substrates contributing to postural instability could lead to more effective treatments. However, investigating these neural substrates is made difficult by constraints of current functional neuroimaging techniques, such as the horizontal orientation of most MRI scanners. To address this constraint, we proposed to use a novel balance simulator that allows participants, while supine, to perform tasks that mimic free-standing balance. Overall, the general purpose of this thesis was to investigate the specific nature of balance deficits in PD, as well as the neural substrates contributing to postural instability in individuals with PD.First, a narrative review of the literature was conducted to summarize the current evidence for the effect of PD, and the effect of antiparkinson treatment interventions, on static balance control. When focusing on studies that recorded quiet stance for at least 60 s, some consistent findings emerged that indicated individuals with PD display larger and faster sway compared to elderly controls, and that levodopa provides little improvement. Second, the MRI compatible balance simulator was validated in individuals with PD and elderly controls. Results indicated that the simulator was easy to use for all participants, balance behaviour during the simulated balance tasks was similar to that seen during upright standing balance, and both static and dynamic balance deficits could be detected in the individuals with PD using the simulator. Finally, the simulator was used in the MRI scanner to investigate the neural substrates of static and dynamic balance deficits in PD using both brain connectivity and brain activation amplitude analyses. The connectivity analysis suggested elderly controls show a preference of subcortical over motor cortical control networks during dynamic balancing, while dynamic balance control in individuals with PD relies more on networks involving cortical (motor) areas. A similar pattern of results was seen for static balance during the brain activation amplitude analysis.Overall, this thesis furthers our understanding of the specific nature of static balance deficits in individuals with PD, as well as the neural substrates underlying postural instability in PD.

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Postural threat modulates human perceptions of balance-related movement (2018)

Height-induced postural threat affects emotional state and upright feet-in-place standing balance behaviour during static, voluntary and dynamic tasks. Facing a threat to balance also affects sensory and cortical processes during balance tasks. As sensory and cognitive functions are crucial in forming perceptions of movement, balance-related changes during threatening conditions might be associated with changes in conscious perceptions. Therefore, the purpose of this thesis was to examine the changes and potential mechanisms underlying conscious perceptions of balance-relevant information during height-induced postural threat.A combination of five experimental procedures utilized height-induced postural threat to manipulate emotional state, balance behaviour, and/or conscious perceptions of sensory stimuli involved in balance. The first three studies assessed conscious perception of body position during static stance, voluntary leaning, and dynamic stance, respectively. During quasi-static balance, height-induced threat increased gain between actual and perceived movement as postural sway decreased in amplitude while perceived movement amplitude remained the same in the HIGH (3.2 m, at the edge) compared to LOW (1.1 m, away from edge) height condition. During voluntary leaning, perceived whole-body position in a voluntary leaning task was larger at height across ten different leaning positions (within the limits of stability). During continuous mediolateral pseudorandom support surface rotations, perceived movement of the trunk was larger while actual lateral movement of the upper trunk did not change in the HIGH compared to LOW height conditions. The continuity of results across these three studies illustrate height-induced postural threat increases the amplitude of perceived movement during balance tasks independent of behavioural changes. The final study included two experiments to determine how changes in somatosensory perceptual thresholds change with increased threat. Perceptual thresholds for ankle rotations were elevated while foot sole vibrations thresholds remained unchanged in the HIGH compared to LOW condition.These studies further our understanding of the relationship between emotional state, perceived risk, and balance performance. Taken together, postural threat can affect emotional state and conscious perceptions of balance-related movement. These results highlight the effect of postural threat influences on neurophysiological and cognitive components of balance control, and provide insight into clinical balance assessment and intervention.

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Vestibular, proprioceptive, and cutaneous reflex modulation explored through a height-induced postural threat (2016)

Context-dependent threats to standing balance have long been known to affect fear and anxiety about falling, as well as static, dynamic reactive, and anticipatory control of upright standing balance. While the neural mechanisms underlying changes in balance behaviours are not yet understood, changes in balance-relevant sensory-motor interactions have been suggested as a possible means to alter balance behaviours with threat. The purpose of this thesis was to understand how different balance-relevant sensory systems are affected by threats to standing balance. Four studies are presented in this thesis which each address the effects of postural threat on a different balance-relevant sensory reflex. Height-induced postural threat was employed in all studies included in this thesis to manipulate balance threat or challenge; participants stood at, or away from the edge of a hydraulic lift which was elevated to different heights, to create LOW and HIGH threat conditions. The first study revealed that the gain and coupling of balance responses to electrical vestibular stimulation was increased in the HIGH, compared to LOW threat conditions. The second study demonstrated increases in muscle spindle stretch reflexes, and steeper dynamic gain relationships between stretch velocities and short-latency reflex amplitudes with HIGH postural threat. The third study validated a novel technique for probing Golgi tendon organ Ib reflexes in standing, and used this technique to demonstrate reduced Ib inhibition in an ankle plantar flexor muscle with increased threat. The fourth study examined lower-limb muscle reflexes, as well as cortical potentials to cutaneous nerve electrical stimulation. While cutaneous reflexes were not observed to change independently from background muscle activity, cortical potentials were affected by threat at stages which may represent altered primary and/or secondary somatosensory, as well as posterior parietal processing. Combined, these studies suggest people respond to height-induced postural threat with a multi-sensory adaptation process where balance-relevant muscular and vestibular senses are tuned to facilitate reactive responses to balance disturbances and/or sensory monitoring of postural state. These novel results provide important insight into neural mechanisms underlying the effects of fear and anxiety on human balance control, and have important implications for clinical balance and neurophysiological testing.

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Exploring the nature of postural sway (2013)

Humans are unable to stand still, but rather experience continuous oscillations of the body known as postural sway. While the origins of postural sway are largely unknown, theories suggest that postural sway originates from the interaction between movements of the body (centre of mass, COM) and forces beneath the feet (centre of pressure, COP). The COP is commonly assumed to control or correct for deviations of the body from equilibrium, and delays or errors in control result in postural sway. In a sequence of 5 studies, this thesis used a novel experimental paradigm to investigate how postural sway is controlled or used by the central nervous system.The first of five experiments tested whether COP displacements would be reduced when the body was externally stabilized, as traditional theories would predict. Contrary to our hypothesis, COP displacements actually increased, suggesting an exploratory role for postural sway. Using the same experimental protocol, Study 2 provided participants with visual feedback of the COM or COP to determine if increases in COP displacements could be the result of sensory illusions or motor drift. Study 3 provided participants with an explicit verbal cue indicating how and when COM stabilization would occur to determine whether increases in COP displacements reflect an attempt to adapt the internal model of the body during stance. Study 4 examined whether increases in COP displacements could be the result of increases in oscillatory cortical drive. Using an upper limb postural task, the fifth and final study extended the findings from Studies 1-4 to determine whether exploratory behaviour may be a more global phenomenon and observed in other postural tasks that do not involve whole body stability.Individually, the results of Studies 1-4 provide evidence which challenges traditional theories of postural control. In addition, they provide evidence against alternative explanations for increases in COP displacements and suggest that this behaviour may be a more global phenomenon and observed in any postural task (Study 5). Collectively, they provide evidence supporting a potential exploratory role of postural sway and question the basis of current clinical practices designed to deal with balance control deficits due to age or disease.

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Insights into human dynamic balance control: postural response initiation explored through classical conditioning and startle (2013)

As a scientific discipline, dynamic posturography aims to understand the neurological and biomechanical mechanisms that contribute to postural stability and corrective postural responses (PRs). The main focus of this thesis was to better understand the neurophysiology of corrective PRs that prevent falls that emerge from external forces applied to the body by balance perturbations. In a sequence of 4 studies, this thesis utilized novel applications of established techniques (classical conditioning and startle paradigms) to address questions regarding the role of sensory feedback in PRs initiation and the nature of PRs that are evoked by balance perturbations. The first of 4 experiments tested the link between sensory feedback derived by balance perturbations and PR initiation by attempting to trigger PRs using auditory cues that, prior to classical conditioning provided no relevant information pertaining to balance perturbations or postural stability. The second study examined the extent to which conditioned PRs may exist as prepared motor behaviours that could be initiated by startling acoustic stimuli in the absence of balance perturbations. The third study attempted to extend the previous findings of PR motor preparation into a more ecologically valid scenario involving unexpected balance perturbations. The fourth and final study in this thesis examined whether startle responses could contribute to first-trial effects observed on PRs evoked by the first in a repeated sequence of balance perturbations.Individually, each study provided highly novel contributions to the field of dynamic posturography. However, when taken together, they provide novel insight into both the mechanisms involved in PR initiation and the understanding of reactions evoked by balance perturbations.

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Fear of Falling, Proprioception and Spinal Reflex Modulation (2011)

No abstract available.

Master's Student Supervision (2010 - 2018)
Modulation of vestibular-evoked reflexes and oculomotor function when standing under height-induced states of fear, anxiety and arousal (2015)

As an important source of information for postural control, the Vestibular System may contribute to anxiety-related effects on balance control during stance and gait, particularly through increases in the vestibulospinal reflex (VSR) gain. While vestibulo-ocular reflex (VOR) gain has been associated with chronic anxiety, it is unclear if VOR and VSR gains are also sensitive to acute threat-related changes in fear, anxiety and arousal. Vestibular Evoked Myogenic Potentials (VEMPs) and Head Impulse Tests (HIT) can be used to test the gain of VSR and VOR pathways. Having subjects stand at the edge of an elevated platform can be used to threaten standing balance and induce arousal, anxiety and fear related to falling; known as a height-induced postural threat. The first aim of this thesis was to investigate how postural threat-related changes in arousal, anxiety and fear influence VEMP and HIT outcomes. Since the VOR depends also on visual pathways receiving signals relating to visual field motion and eye movements, a second study was designed to examine the independent effect of postural threat on oculomotor function using eye saccades, smooth pursuit and optokinetic nystagmus. For the first time, VEMPs were simultaneously recorded while standing from different muscles representing the three distinct vestibular reflexes. Likewise, this thesis is the first to investigate functional VOR and oculomotor outcomes with changes in state anxiety.The results from both studies provide robust evidence for increased VSR and VOR gain with acute negative emotional states. Furthermore, the observed increased gain of oculomotor function suggests that part of the VOR modulation occurs in neural centres not related to the vestibular system. These observations not only shed a light on how the VOR and gaze control are affected by state anxiety, fear and arousal, confirming previous reports on the VSR, but have also shown how emotions could alter the outcomes of clinical tests commonly used for assessing the vestibular and oculomotor function.

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Examination of Preserved Motor Pathways in Persons with Motor-Complete Spinal Cord Injury (2014)

Previous work has demonstrated that mobility is consistently one of the most, if not the most important function persons with a spinal cord injury (SCI) desire following their injury. By increasing functional mobility, even slightly, there may be improved independence, leading to improved quality of life. While the current clinical examination for determining the level and severity of an SCI has proven to be very reliable and useful for standardizing SCI classification, it still has significant limitations that may limit a patient’s future mobility. For example, the measures used to assess motor function in the limb following a SCI may not be sensitive enough to detect minimal levels of preserved motor function, as they are limited to manual palpation and/or visual inspection. Furthermore, the extent of preservation of trunk musculature and the vestibulospinal pathway following an SCI remains unclear. Therefore, there is a need for more sensitive measures of remaining motor activity and a need to examine the integrity of individual motor pathways. Using transcranial magnetic stimulation (TMS) and vestibular evoked myogenic potentials (VEMPs), this thesis examined the integrity of the cortico- and vestibulospinal pathways in 16 persons with a motor-complete SCI and 16 able-bodied (AB) matched controls. Despite being clinically classified as motor-complete, persons with an SCI showed some observable muscle activity to cortico- and vestibulospinal stimulation, as well as in response to voluntary contractions. In general, the corticospinal responses in the SCI group were delayed compared to their AB matched controls. The muscle activity detected using TMS related to voluntary activation; however, TMS appears to detect preserved muscle activity below that which can be voluntarily activated. Overall, the results from this thesis provide evidence for the use of TMS and VEMPs to assist in determining the neurophysiological integrity of various motor pathways in persons with a motor-complete SCI. Using these techniques may provide clinicians with more accurate information about the state of various motor pathways and may offer a method to more accurately target rehabilitation.

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Modulation of stochastic vestibular stimulation-induced reflexes within a dynamic balance paradigm: the effect of response phase and emotional state (2014)

The vestibular system is a complex network that plays an important role in balance control. When postural perturbations are exerted on the individual, it appears that the vestibular system plays a role in modulating the amplitude of the responses. The vestibular system is also susceptible to changes in psychosocial and autonomic states. Despite these findings, the inability to precisely record from and directly manipulate the system has hindered the field in completely understanding how the vestibular system is involved in balance. Therefore, the purposes of this thesis were 1) to investigate if there was phase-dependent modulation of the vestibular reflex during the postural responses and 2) to determine if the vestibular reflex was altered with postural threat.Stochastic vestibular stimulation (SVS) was used to electrically probe the vestibular system while participants stood on a rotating platform. The vestibular reflex was analyzed by estimating the vestibulo-muscular (SVS-EMG) relationship using time-dependent SVS-EMG coherence throughout the postural response for the first purpose while, for the second purpose, SVS-EMG coherence, cumulant density, and gain were calculated between non-threatening and threatening conditions. Results from this thesis were unable to determine if there were phase-dependent modulations of the SVS-induced vestibular reflex. However, further testing and pilot data provides a promising method for further investigation. Furthermore, an increase gain in and coupling of the vestibular reflex was observed in the most muscles while a decrease in coupling was observed for the paraspinal muscles in the threatening situation. These results suggest that the central nervous system has the ability to prepare the body for responding to an upcoming postural perturbation by optimizing the vestibular output to the muscles.

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Influence of virtual heights on dynamic postural control (2013)

Fear of falling and fall risk are strongly related. As such, falls are a leading cause of injury among older adults. Therefore, there is a need to study how balance is influenced by fear and fear related factors. Previous research has shown that when healthy young adults are exposed to a postural threat, by standing at the edge of an elevated support surface, their postural control is altered in both static and dynamic situations. While responses to support surface rotations have been studied in a high postural threat scenario [Carpenter et al., 2004b], the effects of threat on dynamic responses to support surface translations, a more ecologically valid type of perturbation, have not yet been investigated. This is due to several safety and feasibility issues that preclude translating individuals at height. Virtual reality (VR) has been established as an effective means for simulating height-related postural threat and can therefore be used to avoid the limitations associated with translating subjects at physical height. The purpose of this study was to examine the influence of fear and anxiety on postural reactions to surface translations during exposure to virtual heights. Twenty-one healthy young adults experienced support surface translations in the forward and backward directions, while immersed in a low and then high height virtual environment. Postural responses were significantly affected by height. Specifically, muscle activity in the lower leg and arm, and COP peak displacements, were earlier and larger in response to backward perturbations. No changes were observed in the responses to forward perturbations. In conclusion, virtual heights significantly altered neuromuscular responses to translational perturbations. Virtual height was capable of eliciting responses similar to real height, and thus may be used as an alternative method in investigating fear and anxiety related balance deficits in populations with a known fear of falling.

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Postural threat-induced modulation of stretch reflex pathways in static and dynamic postural control (2010)

There are clear changes to human static and dynamic postural control in situations of elevated postural threat (e.g. standing at the edge of an elevated platform). One possible explanation for these changes is that the amount of afferent information from muscle spindles in the ankle musculature is altered by postural threat. Two experiments have been conducted to explore postural threat-induced changes to soleus spinal stretch reflex function during static control of posture (Study 1), and in response to dynamic postural disturbances (Study 2). In Study 1, soleus Hoffmann (H-) and tendon stretch (T-) reflexes were used to explore changes in reflex amplitude while subjects stood quietly in conditions of low (ground level) and high (3.2m above ground) postural threat. Height-induced postural threat was associated with larger T-reflexes and higher arousal, these effects occurred without systematic changes in H-reflex amplitudes or background muscle activation. We interpret these findings as indirect evidence for arousal-mediated changes in muscle spindle sensitivity. In Study 2, emotionally-charged pictures were used to explore the effects of arousal on H- and T-reflexes, as well as whole body postural perturbations. The pictures failed to elicit significant changes in physiological arousal, H- or T-reflexes, or perturbation response parameters. However, the threat of postural perturbation caused parallel increases in T-reflexes, physiological arousal, and perceived anxiety. Therefore, we conclude that arousal-induced changes in stretch reflexes are not context specific, but rather a generalized response to postural threat. Furthermore, these results together provide substantial evidence in support of independent modulation of muscle spindle sensitivity in humans.

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