Biology subjects

Scheidt, R. A.

Publications and source records attributed to Scheidt, R. A..

8 recordsLinked to original sources

bioRxiv2024

Effects of Concussion on the Relative Contributions of Sensorimotor Memories during Adaptation to Unpredictable Spring-Like Loads

We examined the extent to which concussion impacts how implicit sensorimotor memories are used to compensate for changes in hand-held loads during goal-directed reaching. Recently concussed individuals performed computerized cognition tests and a robotic test of sensorimotor adaptation on three occasions: as soon as possible after injury; after clearance to return to activity; three months after injury. Non-concussed individuals (controls) were tested at inter-session intervals mimicking concussed group intervals. During robotic testing, subjects grasped the handle of a horizontal planar robot while reaching repeatedly to a target. The robot exerted spring-like forces that changed unpredictably between trials; this allowed us to estimate contributions of implicit sensorimotor memories to trial-by-trial performance by fitting a computational model to the time series of reach errors and robot forces. Symptom severity varied considerably within the concussed group at the first session. Computerized cognition tests revealed longer reaction times in the concussed group relative to control group in Session 1 only. Concussed subjects likewise had slower reaction time in the reaching task during the first but not later sessions. Computational modeling found abnormally high values of effective limb compliance in concussed individuals relative to the control group in the first session only, but did not find group differences in how sensorimotor memories contribute to reach adaptation in any session. Analysis of control group models identified a practice effect affecting the memory coefficients that may have masked initial effects of concussion on how implicit memories contribute to sensorimotor adaptation. Although a practice effect and a heterogeneous concussed cohort preclude strong conclusions, our findings suggest procedural improvements that may decrease the robotic tests sensitivity to practice effects and increase its sensitivity to concussion-related changes in how implicit memories contribute to sensorimotor adaptation to unpredictable hand-held loads during reaching.

neuroscience↗

bioRxiv2023

Contribution of Implicit Memory to Adaptation of Movement Extent During Reaching Against Unpredictable Spring-Like Loads: Insensitivity to Intentional Suppression of Kinematic Performance

We examined the extent to which intentionally underperforming a goal-directed reaching task impacts how memories of recent performance contribute to sensorimotor adaptation. Healthy human subjects performed computerized cognition testing and an assessment of sensorimotor adaptation wherein they grasped the handle of a horizontal planar robot while making goal-directed out-and-back reaching movements. The robot exerted forces that resisted hand motion with a spring-like load that changed unpredictably between movements. The robotic test assessed how implicit and explicit memories of sensorimotor performance contribute to the compensation for the unpredictable changes in the hand-held load. After each movement, subjects were to recall and report peak movement extent from the previous trial. Subjects performed the tests under two counterbalanced conditions: one where they performed with their best effort, and one where they intentionally suppressed performance. Results from the computerized cognition tests confirmed that subjects understood and complied with task instructions. When suppressing performance during the robotic assessment, subjects demonstrated marked changes in reach precision, time to capture the target, and reaction time. We fit a set of limited memory models to the data to identify how subjects used implicit and explicit memories of recent performance to compensate for the changing loads. In both sessions, subjects used implicit, but not explicit, memories from the most recent trial to adapt reaches to unpredictable spring-like loads. Subjects did not "give up" on large errors, nor did they discount small errors deemed "good enough." Although subjects clearly suppressed kinematic performance (response timing, movement variability, and self-reporting of reach error), the relative contributions of sensorimotor memories to trial-by-trial variations in task performance did not differ significantly between the two testing conditions. We conclude that intentional performance suppression had minimal impact on how implicit sensorimotor memories contribute to adaptation of unpredictable mechanical loads applied to the hand.

neuroscience↗

bioRxiv2022

Independence and interaction between the control of moving and holding still in post-stroke arm paresis

Moving and holding-still (holding) have been posited to be under separate control regimes for both eye and arm movements. The paretic arm after stroke exhibits different abnormalities during rest vs. movement, providing an opportunity to ask whether control of these behaviors is independently affected in stroke. Here, we quantified resting postural abnormalities in stroke patients by measuring their biases in force production as they held their hand still in various locations in a planar workspace, and then assessed the influence of these resting force biases on reaching in the same workspace. We found that patients had marked resting force biases at each location, even when the arm was supported. However, these biases did not transfer to arm-supported planar reaching movements: not during initial acceleration, not in response to mid-trajectory perturbations, and not during deceleration to a stop. Rather, the abnormal resting forces only appeared to switch on after a movement had fully stopped. These findings suggest that moving and holding are functionally separable modes of control. At the same time, we found that the resting biases mirrored characteristics of abnormal synergies during movement: they markedly decreased when arm support was provided; they were higher in more distant positions that require breaking out of flexion; and they scaled with the Fugl-Meyer score for the upper extremity (a measure of intrusion of abnormal synergies during active movement). These three shared features suggest a common mechanism for resting biases and abnormal synergies, which appears to be a contradiction given the functional separation of moving and holding observed in the same patients. To resolve this paradox, we propose a conceptual model that predicts a breakdown in the functional separation between reaching and holding when patients move in the absence of weight support. This conceptual model posits that synergies are the behavioral manifestation of a spillover of posture into movement. Mapping these functional systems onto anatomical and physiological details of lesioned substrate after stroke may provide implementation-level insight into how normal arm motor control is assembled.

neuroscience↗

bioRxiv2022

Spatial mapping of posture-dependent resistance to passive displacement of the hypertonic arm post-stroke

Muscles in the post-stroke arm commonly demonstrate abnormal reflexes that result in increased position- and velocity-dependent resistance to movement. We sought to develop a reliable way to quantify mechanical consequences of abnormal neuromuscular mechanisms throughout the reachable workspace in the hemiparetic arm post-stroke. Survivors of hemiparetic stroke (HS) and neurologically-intact (NI) control subjects were instructed to relax as a robotic device repositioned the hand of their (hemiparetic) arm between several testing locations sampling the arms passive range of motion. During transitions, the robot induced motions at either the shoulder or elbow joint at speeds ranging from very slow (6 {degrees}/s) to fast (90 {degrees}/s). The robot held the hand at the testing location for at least 20 seconds after each transition. We recorded and analyzed hand force and electromyographic activities from selected muscles spanning the shoulder and elbow during and after transitions. Hand forces and electromyographic activities were small at all speeds and sample times in NI control subjects, but varied systematically by transport speed during and shortly after movement in the HS subjects. Velocity-dependent resistance to stretch diminished within two seconds after movement ceased in the hemiparetic arms. Hand forces and EMGs changed very little from 2 seconds after the movement ended onward, exhibiting dependence on limb posture but no systematic dependence on movement speed or direction. Although each HS subject displayed a unique field of hand forces and EMG responses across the workspace after movement ceased, the magnitude of steady-state hand forces was generally greater near the outer boundaries of the workspace than in the center of the workspace for the HS group but not the NI group. In the HS group, electromyographic activities exhibited abnormalities consistent with stroke-related decreases in the stretch reflex thresholds post-stroke. These observations were consistent across repeated testing days. Implications of the findings are discussed.

bioengineering↗

bioRxiv2022

Extended practice improves the accuracy and efficiency of goal-directed reaching guided by supplemental kinesthetic vibrotactile feedback

Prior studies have shown that providing task-specific vibrotactile feedback (VTF) during reaching and stabilizing with the arm can immediately improve the accuracy and efficiency. However, such studies typically evaluate performance after less than 1 hour of practice using VTF. Here we tested the effects of extended practice using supplemental kinesthetic VTF on goal-directed reaching with the arm. Healthy young adults performed a primary reaching task and a secondary choice reaction task individually and as a dual-task. The reaching task was performed under three feedback conditions: visual feedback, proprioceptive feedback, and with supplemental kinesthetic VTF applied to the non-moving arm. We compared performances before, during, and after approximately 10 hours of practice on the VTF-guided reaching task, distributed across 20 practice sessions. Upon initial exposure to VTF-guided reaching, participants were immediately able to use the VTF to improve reaching accuracy. Performance improvements were retained from one practice session to the next. After 10 hours of practice, the accuracy and temporal efficiency of VTF-guided reaching were equivalent to or better than reaching performed without vision or VTF. However, hand paths during VTF-guided reaching exhibited a persistent strategy whereby movements were decomposed into discrete sub-movements along the cardinal axes of the VTF interface. Dual-tasking capability also improved, such that the primary and secondary tasks we performed more concurrently after extended practice. Our results demonstrate that extended practice on VTF-guided reaching can yield performance improvements that accrue in a manner increasingly resistant to dual-task interference.

neuroscience↗

bioRxiv2020

Contributions of Implicit and Explicit Memories to Sensorimotor Adaptation of Movement Extent During Goal-Directed Reaching

We examined how explicit memory of kinematic performance contributes to motor adaptation during goal-directed reaching. Twenty subjects grasped the handle of a horizontal planar robot that rendered spring-like resistance to movement. Subjects made rapid "out-and-back" reaches to capture a remembered visual target at the point of maximal reach extent. The robots resistance changed unpredictably between reaches, inducing target capture errors that subjects attempted to correct from one trial to the next. The subjects performed four sets of trials. Three were performed without concurrent visual cursor feedback of hand motion. Two of these required self-assessment of performance between trials, whereby subjects reported peak reach extent on the most recent trial. This was done by either moving a cursor on a horizontal display (visual self-assessment), or by moving the robots handle back to the recalled location (proprioceptive self-assessment). Two control conditions were performed either without or with concurrent visual cursor feedback of hand motion. We analyzed movement kinematics and used regression analyses to quantify the extent to which prior reach errors and explicit memories of prior performance contribute to subsequent reach performance. Consistent with prior reports, providing concurrent visual feedback of hand motion increased reach accuracy and reduced the impact of past performance errors on future performance, relative to the no-vision control condition. By contrast, we found no impact of interposed self-assessment on reach kinematics or on how prior target capture errors influence subsequent reach performance. Self-assessments were biased toward the remembered target location, differing markedly from actual reach performances. Including self-assessments as explicit memory of performance in the regression analyses did not improve model predictive accuracy. Therefore, we conclude that explicit memories of prior kinematic performance do not contribute to implicit sensorimotor adaptation of movement extent.

neuroscience↗

bioRxiv2018

Spatial and Temporal Influences on Discrimination of Vibrotactile Stimuli on The Arm

Body-machine interfaces (BMIs) provide a non-invasive way to use and control external devices such as powered wheelchairs. Vibrotactile stimulation has been proposed as a way for BMIs to provide device performance feedback to the user, thereby reducing visual demands of closed-loop control. To advance the goal of developing a compact, multivariate vibrotactile display for BMIs, we performed two 2-alternative, forced choice experiments to determine the extent to which vibrotactile perception might vary across multiple stimulation sites. The first experiment assessed vibrotactile discrimination of sequentially presented stimuli within each of four dermatomes of the arm (C5, C7, C8, T1) and on the ulnar head. The second compared discrimination when pairs of vibrotactile stimuli were presented simultaneously vs. sequentially both within and across dermatomes. Although the first experiment found small but statistically significant differences across dermatomes C7 and T1, discrimination thresholds at the other three locations did not differ one from another or from those at either C7 or T1. These results suggest that stimuli applied to each of the sites may be able to convey approximately the same amount of information. The second experiment found that sequential delivery of vibrotactile stimuli resulted in better discrimination than simultaneous delivery, independent of whether the pairs were located within the same dermatome or across dermatomes. Taken together, our results suggest that the arm may be a viable site to transfer multivariate information via vibrotactile feedback for body-machine interfaces. However, user training may be needed to overcome the perceptual disadvantage of simultaneous vs. sequentially-presented stimuli.

neuroscience↗

bioRxiv2018

Supplemental vibrotactile feedback of real-time limb position enhances precision of goal-directed reaching

We examined vibrotactile stimulation as a form of supplemental limb state feedback to enhance on-going control goal-directed movements. Subjects wore a two-dimensional vibrotactile display on their non-dominant arm while performing horizontal planar reaching movements with their dominant arm. The vibrotactile display provided feedback of hand position such that small hand displacements were more easily discriminable using vibrotactile feedback than with intrinsic proprioceptive feedback. When subjects relied solely on proprioceptive feedback to capture visuospatial targets, target capture performance was degraded by proprioceptive drift and an expansion of task space. By contrast, reach accuracy was enhanced immediately when subjects were provided vibrotactile feedback, and further improved over two days of training. Improvements reflected a resolution of proprioceptive drift which occurred only when vibrotactile feedback was active, demonstrating that the benefits of vibrotactile feedback are due in part to its integration into the ongoing control of movement. A partial resolution of task space expansion that persisted even when the vibrotactile feedback was inactive demonstrated that training with vibrotactile feedback also induced changes in movement planning. However, the benefits of vibrotactile feedback come at a cognitive cost. All subjects adopted a stereotyped, movement decomposition strategy wherein they attempted to capture targets by moving first along one axis of the vibrotactile display and then the other. For most subjects, this inefficient movement approach did not resolve over two bouts of training performed on separate days, suggesting that additional training is needed to fully integrate vibrotactile feedback into the planning and online control of goal-directed reaching.

neuroscience↗