Biology subjects

Shmuelof, L.

Publications and source records attributed to Shmuelof, L..

6 recordsLinked to original sources

bioRxiv2026

Reticulospinal Tract Hyperexcitability in the Upper Limb After Stroke is Associated with Motor Impairment and Not with Functional Compensation

BackgroundAccumulating results suggest that reticulospinal tract (RST) excitability increases after stroke. While animal studies suggest this hyperexcitability may compensate for corticospinal tract (CST) damage, its role in motor function in people with stroke (PwS) remains debated. This study aimed to: (1) replicate findings of RST hyperexcitability in PwS using the StartReact paradigm, measuring acceleration of motor response to a startling auditory stimulus; (2) examine the relationship between RST hyperexcitability and motor impairments after stroke; and (3) explore whether RST hyperexcitability provides functional benefits in severely impaired PwS. MethodsForty-six PwS completed the StartReact paradigm and motor assessments (Fugl-Meyer, ARAT, grip strength, Modified Ashworth Scale). PwS were categorized into high StartReact effect and typical StartReact effect subgroups based on comparisons with a healthy control group (n=37). Severe impairment was defined as ARAT [≤]10. ResultsPwS exhibited significantly greater StartReact effects than controls. The high StartReact effect subgroup showed worse motor function, weaker grip strength, and higher spasticity. Among severely impaired PwS, high StartReact effect was not associated with improved grip strength. ConclusionsThese findings confirm the existence of RST hyperexcitability after stroke and suggest it is associated with poorer motor outcomes, likely due to reduced cortical input to the brainstem. The absence of functional benefit in severely impaired individuals supports the interpretation that RST hyperexcitability is a maladaptive rather than a compensatory reaction to brain damage. These findings provide insight into the neurophysiological mechanisms underlying motor impairments after stroke and do no imply direct clinical or therapeutic applications.

neuroscience↗

bioRxiv2026

The changing contributions of weakness and the flexor synergy to post-stroke arm function over time: A kinematic re-examination of Twitchell

In 1951, the neurologist Thomas Twitchell published a seminal paper in Brain describing the time-course of recovery from hemiplegia after stroke in 25 participants from hospitalization to when they reached what he deemed steady state. His main emphasis was on the evolution of voluntary movements at the shoulder, elbow and hand, first within an obligatory flexor synergy, and then independently out-of-synergy. We thought that 75 years later, an update using modern motion capture technology should be attempted as it would allow for finer granularity in the characterization of the time courses of both functional recovery and of the flexor synergy, and then relate them to each other, to weakness and to well-established clinical scales. To this end, we used marker-less 3D kinematics to assess task performance and intrusion of synergies in thirty-three stroke participants longitudinally, from the early sub-acute stage (1 - 8 weeks post-stroke) to the chronic stage (24 - 64 weeks post-stroke). Participants performed an out-of-flexor synergy (shoulder flexion and elbow extension) reaching task. We assessed the time course of recovery of obligatory intrusion of pathological synergies based on measures derived from the angular velocity profiles of the shoulder and the elbow joints. Task-related kinematic measures were obtained and compared to sixteen healthy controls. Grip strength, Motor impairment (FMA), and function (ARAT) scores were also collected. Task kinematics were different from controls in the early, late sub-acute, and chronic stages, but showed gradual recovery over time. Weakness in the hand remained impaired at all time points. Flexor-synergy intrusion was maximal in the early sub-acute stage and then began to subside. Regression analysis with functional kinematic and clinical (FMA, ARAT) measures indicated that flexor-synergy intrusion was a significant predictor in the early and late sub-acute stages, but not in the chronic stage, while weakness remained a significant predictor at all stages of recovery. To better address the relationship between synergies, weakness, and function, we analyzed the more severe cases (ARAT<21) separately. In the sub-acute stage, most of them (11/13) suffered from intrusion of synergies, whereas in the chronic phase, only a minority (2/8) did. Weakness seemed to be the main contributor to poor outcome in the chronic phase. We conclude that weakness and synergy intrusion evolve separately from the subacute to the chronic phase, perhaps more so when neurorehabilitation is given at a dose higher than standard of care.

neuroscience↗

bioRxiv2023

Validation of marker-less pose estimation for 3D kinematics during upper limb reaching

Kinematic analysis of movement following brain damage is key for diagnosing motor impairments and for recovery assessment. Advances in computer vision offer novel marker-less tracking tools that could be implemented in the clinic due to their simple operation and affordability. An important question that arises is whether marker-less technologies are sufficiently accurate compared to well established marker-based technologies. This study aims to perform validation of kinematic assessment using two high-speed cameras and a 3D pose estimation model. Four participants performed reaching movements with the upper limb between fixed targets, in different velocities. Movement kinematics were simultaneously measured using the DeepBehavior model and marker-based optical motion capture (QTM), as a gold standard. The differences in corresponding joint angles, estimated from the two different methods throughout the analysis, are presented as a mean absolute error (MAE) of the elbow angle. Quantitatively, the MAE of all movements was relatively small across velocity and joints (~2{degrees}). In a condition where the movements were made towards the DeepBehavior cameras, and the view of the elbow was occluded in one of the cameras, the errors were higher. In conclusion, the results demonstrated that marker-less motion capture is a valid alternative to marker-based motion capture. Inaccuracies of the DeepBehavior system could be explained by occlusions of key-points and are not associated with failure of the pose estimation algorithm.

bioengineering↗

bioRxiv2022

Intrusion of pathological synergies does not explain impaired 3D arm movements in subacute stroke

It has long been of interest to characterize the components that generate the motor control abnormalities in the arm after stroke. One approach has been to decompose the hemiparesis phenotype into negative signs, such as weakness, and positive signs, such as intrusion of synergies. Here, we sought to identify the contributions of weakness and the flexor synergy to motor function and impairment, as defined by kinematic and clinical scales, respectively, in sub-acute stroke using two 3D arm tasks that differed in their requirement for elbow extension. Thirty-three sub-acute post-stroke participants and sixteen healthy controls performed a cup-to-mouth task, requiring shoulder and elbow flexion (within flexor synergy), and a reaching task, requiring shoulder flexion and elbow extension (out of flexor synergy). Using markerless 3D pose-estimation, we analyzed upper limb kinematics to assess overall task performance and intrusion of pathological synergies. Weakness was measured using a grip dynamometer. Performance in both tasks was impaired to a similar degree in the stroke participants compared to controls. Subsequent analysis of coordination patterns between the elbow and the shoulder joints revealed intrusion of synergies in the reaching task based on the time spent within a flexion-flexion pattern (flexor synergy proportion) and the correlation between shoulder and elbow angles when the shoulder was flexing (flexion synergy strength). Regression analysis indicated that the significant predictors of poor task performance were weakness and flexor synergy intrusion. Notably, the Fugl-Meyer Assessment was abnormal even when just weakness caused the impairment, which means that caution is required when using this scale to quantify synergies. We conclude that both weakness and synergy intrusion contribute to impaired coordination of the elbow and shoulder joints in the sub-acute post-stroke period. This study shows that careful kinematic analysis of naturalistic movements is required to better characterize the components of upper limb impairment after stroke.

neuroscience↗

bioRxiv2019

Environmental Consistency Modulation of Error Sensitivity During Motor Adaptation is Explicitly Controlled

Motor adaptation, the adjustment of sensorimotor representations in face of changes in the environment, may operate at different rates. When human participants encounter repeated or consistent perturbations, their corrections for the experienced errors are larger compared to when the perturbations are new or inconsistent. Such modulations of error sensitivity were traditionally considered to be an implicit process that does not require attentional resources. In recent years, the implicit view of motor adaptation is challenged by evidence showing a contribution of explicit strategies to learning. These findings raise a fundamental question regarding the nature of the error sensitivity modulation processes. We tested the effect of explicit control on error sensitivity in a series of experiments, in which participants controlled a screen cursor to virtual targets. We manipulated environmental consistency by presenting rotations in random (low consistency) or random walk (high consistency) sequences, and illustrated that perturbation consistency affects the rate of adaptation, corroborating previous studies. When participants were instructed to ignore the cursor and move directly to the target, thus, eliminating the contribution of explicit strategies, consistency-driven error sensitivity modulation was abolished. In addition, delaying the visual feedback, a manipulation that affects implicit learning, did not influence error sensitivity under consistent perturbations. These results suggest that increases of learning rate in consistent environments are attributable to an explicit rather than implicit process in sensorimotor adaptation. Significant Statement When experiencing an error in a motor task (e.g., missing a basketball shot in a windy day), the motor system modifies its next action based on environmental consistency (how frequent the changes in winds direction and strength are). It is unknown whether this process is driven by an implicit and automatic process, or by an explicit process that employs cognitive strategies. We examined these possibilities in a simple visuomotor task by perturbing the feedback in each trial with different consistency levels, and manipulating the use of implicit and explicit processes. We found that participants increase their sensitivity to errors in consistent environments when employing explicit strategies, and do not change their behavior when the implicit process is operating alone.

neuroscience↗

bioRxiv2017

State-Based Delay Representation and Its Transfer from a Game of Pong to Reaching and Tracking

To accurately estimate the state of the body, the nervous system needs to account for delays between signals from different sensory modalities. To investigate how such delays may be represented in the sensorimotor system, we asked human participants to play a virtual pong game in which the movement of the virtual paddle was delayed with respect to their hand movement. We tested the representation of this new mapping between the hand and the delayed paddle by examining transfer of adaptation to blind reaching and blind tracking tasks. These blind tasks enabled to capture the representation in feedforward mechanisms of movement control. A Time Representation of the delay is an estimation of the actual time lag between hand and paddle movements. A State Representation is a representation of delay using current state variables: the distance between the paddle and the ball originating from the delay may be considered as a spatial shift; the low sensitivity in the response of the paddle may be interpreted as a minifying gain; and the lag may be attributed to a mechanical resistance that influences paddles movement. We found that the effects of prolonged exposure to the delayed feedback transferred to blind reaching and tracking tasks and caused participants to exhibit hypermetric movements. These results, together with simulations of our representation models, suggest that delay is not represented based on time, but rather as a spatial gain change in visuomotor mapping.\n\nSignificance StatementIt is known that the brain copes with sensory feedback delays to control movements, but it is unclear whether it does so using a representation of the actual time lag. We addressed this question by exposing participants to a visuomotor delay during a dynamic game of pong. Following the game, participants exhibited hypermetric reaching and tracking movements that indicate that delay is represented as a visuomotor gain rather than as a temporal shift.

neuroscience↗