Electrophysiological Correlates of Changes in Reaction Time Based on Stimulus Intensity

Background: Although reaction time is commonly used as an indicator of central nervous system integrity, little is currently understood about the mechanisms that determine processing time. In the current study, we are interested in determining the differences in electrophysiological events associated with significant changes in reaction time that could be elicited by changes in stimulus intensity. The primary objective is to assess the effect of increasing stimulus intensity on the latency and amplitude of afferent inputs to the somatosensory cortex, and their relation to reaction time. Methods: Median nerve stimulation was applied to the non-dominant hand of 12 healthy young adults at two different stimulus intensities (HIGH & LOW). Participants were asked to either press a button as fast as possible with their dominant hand or remain quiet following the stimulus. Electroencephalography was used to measure somatosensory evoked potentials (SEPs) and event related potentials (ERPs). Electromyography from the flexor digitorum superficialis of the button-pressing hand was used to assess reaction time. Response time was the time of button press. Results: Reaction time and response time were significantly shorter following the HIGH intensity stimulus compared to the LOW intensity stimulus. There were no differences in SEP (N20 & P24) peak latencies and peak-to-peak amplitude for the two stimulus intensities. ERPs, locked to response time, demonstrated a significantly larger pre-movement negativity to positivity following the HIGH intensity stimulus over the Cz electrode

Brain Vital Signs in Elite Ice Hockey: Towards Characterizing Objective and Specific Neurophysiological Reference Values for Concussion Management.

Background: Prior concussion studies have shown that objective neurophysiological measures are sensitive to detecting concussive and subconcussive impairments in youth ice-hockey. These studies monitored brain vital signs at rink-side using a within-subjects design to demonstrate significant changes from pre-season baseline scans. However, practical clinical implementation must overcome inherent challenges related to any dependence on a baseline. This requires establishing the start of normative reference data sets. Methods: The current study collected specific reference data for N = 58 elite, youth, male ice-hockey players and compared these with a general reference dataset from N = 135 of males and females across the lifespan. The elite hockey players were recruited to a select training camp through CAA Hockey, a management agency for players drafted to leagues such as the National Hockey League (NHL). The statistical analysis included a test-retest comparison to establish reliability, and a multivariate analysis of covariance to evaluate differences in brain vital signs between groups with age as a covariate. Findings: Test-retest assessments for brain vital signs evoked potentials showed moderate-to-good reliability (Cronbach's Alpha > 0.7, Intraclass correlation coefficient > 0.5) in five out of six measures. The multivariate analysis of covariance showed no overall effect for group (p = 0.105), and a significant effect of age as a covariate was observed (p < 0.001). Adjusting for the effect of age, a significant difference was observed in the measure of N100 latency (p = 0.022) between elite hockey players and the heterogeneous control group. Interpretation: The findings support the concept that normative physiological data can be used in brain vital signs evaluation in athletes, and should additionally be stratified for age, skill level, and experience. These can be combined with general norms and/or individual baseline assessments where appropriate and/or possible. The current results allow for brain vital sign evaluation independent of baseline assessment, therefore enabling objective neurophysiological evaluation of concussion management and cognitive performance optimization in ice-hockey

Perturbation Evoked Balance Control Reactions in Individuals with Stroke

Individuals with stroke suffer from impaired balance that increases their risk of falling. Controlling reactive balance is essential to maintaining stability. The objective of the first study was to identify the role of pre-perturbation stance asymmetry on limb preference for reactive stepping in healthy young adults. This study demonstrated that steps taken with a pre-loaded limb are short, directed laterally and have a rapid swing time. The objective of the second study was to investigate the challenges of reactive stepping among individuals with stroke. This study demonstrated that participants primarily execute reactive stepping with their non-paretic limb, although those steps are highlighted by delays in timing and increased incidence of multiple stepping compared to healthy controls, even though all participants had very good clinical balance scores. Outcomes from this thesis present the need for improved clinical assessment of reactive balance control to help reduce the incidence of falling following stroke.MAS

The Control of temporally Urgent Movements

The ability to respond rapidly with spatial precision is required in a number of facets of everyday life, whether catching a falling object, reacting to other drivers on a busy freeway or recovering one’s balance following an unexpected perturbation. The sophisticated central nervous system (CNS) control of these reactions is often overlooked until the speed of such reactions becomes delayed, either due to ageing or brain injury, wherein the individual becomes at risk of injury. Surprisingly, little is known regarding the control of these ‘temporally urgent’ movements. Therefore, the primary objectives of this dissertation were to develop an understanding of the control of these movements by exploring the factors that may be involved in the generation of temporally urgent movements in the healthy CNS, locating the areas within the CNS that such modulation occurs and identifying the relative weighted importance of those modulators based on the initial conditions of stimulus delivery. Specific characteristics of stimulus properties, such as intensity and modality were particularly influential in the latency of motor reactions and physiological electrodermal skin responses fluctuated in accordance with input stimulus parameters. Importantly, outcomes from this dissertation identified that rapid reactions likely utilize a CNS network that includes higher cortical regions such as somatosensory cortex and primary motor cortex, which may be modulated by physiological arousal, rather than the solitary involvement of subcortical structures. The findings from this dissertation have important implications for individuals with disordered speed of processing and indicate the potential modifiability of factors that influence reaction time.Ph

Visual feedback of the centre of gravity to optimize standing balance

Force platform biofeedback training, whereby concurrent visual feedback of the centre of pressure (COP) is provided, has previously been used for balance training. Since the goal of balance is to maintain control of the centre of gravity (COG), specific feedback of the COG may be more likely than COP feedback to improve overall balance control. The purpose of this study was to compare the effect of concurrent visual feedback of the COP versus COG on postural control during a novel quiet standing task. Thirty-two young healthy adults (20-35 years old) were recruited. Participants were randomly assigned to receive concurrent visual feedback of either the COP or COG while standing on a foam pad. Training occurred over one session (20-30-second trials). Retention and transfer testing (i.e. without concurrent visual feedback) occurred after ∼24h. Variability of the COG decreased, variability of COP-COG increased, and sample entropy increased with concurrent visual feedback. With practice, variability of COP, COG and COP-COG decreased whereas sample entropy increased. The decrease in variability of COP-COG was greater for those who received COG feedback than those who received COP feedback. Training effects on COP, COG and COP-COG variability were not retained after 24h and removal of visual feedback. However, on retention and transfer testing, sample entropy was significantly higher than on baseline testing, indicating more 'automatic' postural control. These results suggest that concurrent visual feedback of neither the COP nor COG is superior for improving quiet standing balance control.We acknowledge the support of Toronto Rehabilitation Institute. Equipment and space have been funded with grants from the Canadian Foundation for Innovation, Ontario Innovation Trust, and The Ministry of Research and Innovation

Hemispheric asymmetry in myelin after stroke is related to motor impairment and function

The relationships between impairment, function, arm use and underlying brain structure following stroke remain unclear. Although diffusion weighted imaging is useful in broadly assessing white matter structure, it has limited utility in identifying specific underlying neurobiological components, such as myelin. The purpose of the present study was to explore relationships between myelination and impairment, function and activity in individuals with chronic stroke. Assessments of paretic upper-extremity impairment and function were administered, and 72-hour accelerometer based activity monitoring was conducted on 19 individuals with chronic stroke. Participants completed a magnetic resonance imaging protocol that included a high resolution T anatomical scan and a multi-component T relaxation imaging scan to quantify myelin water fraction (MWF). MWF was automatically parcellated from pre- and post-central subcortical regions of interest and quantified as an asymmetry ratio (contralesional/ipsilesional). Cluster analysis was used to group more and less impaired individuals based on Fugl-Meyer upper extremity scores. A significantly higher precentral MWF asymmetry ratio was found in the more impaired group compared to the less impaired group (p < 0.001). There were no relationships between MWF asymmetry ratio and upper-limb use. Stepwise multiple linear regression identified precentral MWF asymmetry as the only variable to significantly predict impairment and motor function in the upper extremity (UE). These results suggest that asymmetric myelination in a motor specific brain area is a significant predictor of upper-extremity impairment and function in individuals with chronic stroke. As such, myelination may be utilized as a more specific marker of the neurobiological changes that predict long term impairment and recovery from stroke

Determinants of limb preference for initiating compensatory stepping poststroke

Objective: To investigate the determinants of limb preference for initiating compensatory stepping post-stroke. Design: Retrospective chart review. Setting: In-patient rehabilitation. Participants: Convenience sample of 49 individuals admitted to in-patient rehabilitation with post-stroke hemiparesis. Interventions: Not applicable. Main outcome measures: Compensatory stepping responses were evoked using a lean-and-release postural perturbation. The limb used to initiate compensatory stepping was determined. The relationships between stepping with the paretic limb and pre-morbid limb dominance, weight-bearing on the paretic limb in quiet standing, ability to bear weight on the paretic limb, pre-perturbation weight bearing on the paretic limb, and lower-limb motor recovery scores were determined. Results: The majority (59.1%) of responses were steps initiated with the non-paretic limb. Increased lower-limb motor recovery scores and pre-perturbation weight-bearing on the non-paretic limb were significantly related to increased frequency of stepping with the paretic limb. When the preferred limb was physically blocked, an inappropriate response was initiated in 21% of trials (i.e. non-step responses or an attempt to step with the blocked limb). Conclusions: This study reveals the challenges that individuals with post-stroke hemiparesis face when executing compensatory stepping responses to prevent a fall following a postural perturbation. The inability or challenges to executing a compensatory step with the paretic limb may increase the risk for falls post-stroke.This work was supported by the Heart and Stroke Foundation Centre for Stroke Recovery, the Heart and Stroke Foundation of Canada, the Canadian Institutes of Health Research, and the Canadian Stroke Network. We also acknowledge the support of Toronto Rehabilitation Institute, who receives funding under the Provincial Rehabilitation Research Program from the Ministry of Health and Long-Term Care in Ontario

Motor and Visuospatial Attention and Motor Planning After Stroke: Considerations for the Rehabilitation of Standing Balance and Gait

Attention and planning can be altered by stroke, which can influence motor performance. Although the influence of these factors on recovery from stroke has been explored for the upper extremity (UE), their impact on balance and gait are unknown. This perspective article presents evidence that altered motor and visuospatial attention influence motor planning of voluntary goal-directed movements poststroke, potentially affecting balance and gait. Additionally, specific strategies for rehabilitation of balance and gait poststroke in the presence of these factors are discussed. Visuospatial attention selects relevant sensory information and supports the preparation of responses to this information. Motor attentional impairments may produce difficulty with selecting appropriate motor feedback, potentially contributing to falls. An original theoretical model is presented for a network of brain regions supporting motor and visuospatial attention, as well as motor planning of voluntary movements. Stroke may influence this functional network both locally and distally, interfering with input or output of the anatomical or functional regions involved and affecting voluntary movements. Although there is limited research directly examining leg function, evidence suggests alterations in motor and visuospatial attention influence motor planning and have a direct impact on performance of gait and balance. This model warrants testing comparing healthy adults with individuals with stroke

Timing of response differentiation in human motor cortex during a speeded Go/No-go task

We explored the brain\u27s ability to quickly prevent a pre-potent but unwanted motor response. To address this, transcranial magnetic stimulation was delivered over the motor cortex (hand representation) to probe excitability changes immediately after somatosensory cues prompted subjects to either move as fast as possible or withhold movement. Our results showed a difference in motor cortical excitability 90 ms post-stimulus contingent on cues to either promote or prevent movement. We suggest that our study design emphasizing response speed coupled with well-defined early probes allowed us to extend upon similar past investigations into the timing of response inhibition