A scoping review on examination approaches for identifying tactile deficits at the upper extremity in individuals with stroke.

PURPOSE: Accurate perception of tactile stimuli is essential for performing and learning activities of daily living. Through this scoping review, we sought to summarize existing examination approaches for identifying tactile deficits at the upper extremity in individuals with stroke. The goal was to identify current limitations and future research needs for designing more comprehensive examination tools. METHODS: A scoping review was conducted in accordance with the Joanna Briggs Institute methodological framework and the PRISMA for Scoping Reviews (PRISMA-ScR) guidelines. A database search for tactile examination approaches at the upper extremity of individuals with stroke was conducted using Medline (Ovid), The Cochrane Library (Wiley), CINAHL Plus with Full Text (Ebsco), Scopus (Elsevier), PsycInfo (Ebsco), and Proquest Dissertations and Theses Global. Original research and review articles that involved adults (18 years or older) with stroke, and performed tactile examinations at the upper extremity were eligible for inclusion. Data items extracted from the selected articles included: if the examination was behavioral in nature and involved neuroimaging, the extent to which the arm participated during the examination, the number of possible outcomes of the examination, the type(s) of tactile stimulation equipment used, the location(s) along the arm examined, the peripheral nerves targeted for examination, and if any comparison was made with the non-paretic arm or with the arms of individuals who are neurotypical. RESULTS: Twenty-two articles met the inclusion criteria and were accepted in this review. Most examination approaches were behavioral in nature and involved self-reporting of whether a tactile stimulus was felt while the arm remained passive (i.e., no volitional muscle activity). Typically, the number of possible outcomes with these behavioral approaches were limited (2-3), whereas the neuroimaging approaches had many more possible outcomes ( > 15 ). Tactile examinations were conducted mostly at the distal locations along the arm (finger or hand) without targeting any specific peripheral nerve. Although a majority of articles compared paretic and non-paretic arms, most did not compare outcomes to a control group of individuals who are neurotypical. DISCUSSION: Our findings noted that most upper extremity tactile examinations are behavioral approaches, which are subjective in nature, lack adequate resolution, and are insufficient to identify the underlying neural mechanisms of tactile deficits. Also, most examinations are administered at distal locations of the upper extremity when the examinee's arm is relaxed (passive). Further research is needed to develop better tactile examination tools that combine behavioral responses and neurophysiological outcomes, and allow volitional tactile exploration. Approaches that include testing of multiple body locations/nerves along the upper extremity, provide higher resolution of outcomes, and consider normative comparisons with individuals who are neurotypical may provide a more comprehensive understanding of the tactile deficits occurring following a stroke.

MR-Compatible Tactile Stimulator System: Application for Individuals with Brain Injuries.

Accurate perception of tactile information is essential for performing activities of daily living and learning new sensorimotor skills like writing. Deficits in perceiving tactile stimuli are associated with severity in physical disability. The mechanisms contributing to tactile deficits in individuals with brain injuries remain poorly understood in part due to insufficient assessment methods. Here, we provide a tactile stimulator system for studying the neural mechanisms contributing to tactile deficits in individuals with brain injuries during functional magnetic resonance imaging (fMRI). This tactile stimulator system consists of a pneumatically-controlled inflatable and deflatable balloon that interfaces with a digit of the hand to provide small forces. The magnitude of the applied force is delivered and controlled by modifying the air pressure in the balloon. The tactile simulator provides an 8 mm diameter tactile stimulus. The device's interface at the finger is compact, allowing it to be used with individuals who have a closed-fist posture following brain injury such as stroke or cerebral palsy. The tactile stimulator contains no metallic components and can be used in MRI research. The tactile stimulator system can repeatedly apply a force between 1 N and 2.4 N. This tactile stimulator system addresses limitations in past fMRI methodologies for assessing tactile perception by providing precise and repeatable force stimuli to a small area of the finger. Custom software automates the application of the force stimuli and permits synchronization with acquired fMRI data. This system can be used in subsequent testing to investigate deficits in sensory functioning in those with brain injuries.

Individuals with hemiparetic stroke abnormally perceive their elbow torques when abducting their paretic shoulder.

OBJECTIVE: Individuals with hemiparetic stroke exhibit an abnormal coupling between shoulder abduction and elbow flexion, or flexion synergy, due to an increased reliance on cortico-bulbospinal pathways. While this motor impairment is well documented, its impact on how movements are perceived remains unexplored. This study investigates whether individuals with hemiparetic stroke accurately perceive torques at their paretic elbow while abducting at their shoulder. METHODS: Ten individuals with hemiparetic stroke participated. We recorded the extent of their abnormal joint coupling as the torque at their elbow, with respect to the maximum voluntary torque in elbow flexion, when abducting at their shoulder. Next, we estimated the perception of their elbow torque by reporting their errors on our torque-matching task. RESULTS: When abducting at the shoulder, the participants with stroke generated a greater non-volitional torque at their paretic elbow (13.2 ± 8.7%) than their non-paretic elbow (1.2 ± 11.2%) (p = 0.003). Regarding the perception of our torque-matching task, participants overestimated their torques to a lesser extent at their paretic elbow (1.8 ± 6.6%) than at their non-paretic elbow (6.2 ± 5.4%) (p = 0.004). CONCLUSIONS: Torque perception at the paretic elbow differed from the non-paretic elbow when abducting at the shoulder. SIGNIFICANCE: This work advances our understanding of the i) somatosensory deficits occurring post hemiparetic stroke and ii) neural basis of torque perception.

Shear wave ultrasound elastography of the biceps brachii can be used as a precise proxy for passive elbow torque in individuals with hemiparetic stroke.

Muscle tissue is prone to changes in composition and architecture following stroke. Changes in muscle tissue of the extremities are thought to increase resistance to muscle elongation or joint torque under passive conditions. These effects likely compound neuromuscular impairments, exacerbating movement function. Unfortunately, conventional rehabilitation is devoid of precise measures and relies on subjective assessments of passive joint torques. Shear wave ultrasound elastography, a tool to measure muscle mechanical properties, may be readily available for use in the rehabilitation setting as a precise measure, albeit at the muscle-tissue level. To support this postulation, we evaluated the criterion validity of shear wave ultrasound elastography of the biceps brachii; we investigated its relationship with a laboratory-based criterion measure for quantifying elbow joint torque in individuals with moderate to severe chronic stroke. Additionally, we evaluated construct validity, with the specific sub-type of hypothesis testing of known groups, by testing the difference between arms. Measurements were performed under passive conditions at seven positions spanning the arc of elbow joint flexion-extension in both arms of nine individuals with hemiparetic stroke. Surface electromyography was utilized for threshold-based confirmation of muscle quiescence. A moderate relationship between the shear wave velocity and elbow joint torque was identified, and both metrics were greater in the paretic arm. Data supports the progression toward a clinical application of shear wave ultrasound elastography in evaluating altered muscle mechanical properties in stroke, while acknowledging that undetectable muscle activation or hypertonicity may contribute to the measurement. Shear wave ultrasound elastography may augment the conventional method of manually testing joint mobility by providing a high-resolution precise value. Tissue-level measurement may also assist in identifying new therapeutic targets for patient-specific impairment-based interventions.

Perception of Torque is Impacted by a Subset of Features Related to the Motor Command.

Accurate perception of one's self-generated torques is integral to sensorimotor control. Here, we examined how features of the motor control task, specifically the variability, duration, muscle activation pattern, and magnitude of torque generation, relate to one's perception of torque. Nineteen participants generated and perceived 25% of their maximum voluntary torque (MVT) in elbow flexion while simultaneously abducting at their shoulder to 10%, 30%, or 50% of their MVT in shoulder abduction (MVT[Formula: see text]). Subsequently, participants matched the elbow torque without feedback and without activating their shoulder. The shoulder abduction magnitude affected the time to stabilize the elbow torque (p 0.001), but did not significantly impact the variability of generating the elbow torque (p = 0.120) or the co-contraction between the elbow flexor and extensor muscles (p = 0.265). The shoulder abduction magnitude influenced perception (p = 0.001) in that the error in matching the elbow torque increased with an increased shoulder abduction torque. However, the torque matching errors neither correlated with the time to stabilize and variability in generating the elbow torque, nor the co-contraction of the elbow muscles. These findings suggest that the total torque generated during a multi-joint task impacts the perception of a torque about a single joint; yet, effective and efficient generation of the torque about a single joint does not impact the torque percept.

Impact of Arm Dominance and Location on Detecting Electrotactile Stimuli During Voluntary Motor Activation in Older Adults.

Activation-related sensory gating is important for sensorimotor control, filtering signals irrelevant to a task. Literature on brain lateralization suggests that motor activation patterns during sensorimotor control differ depending on arm dominance. Whether the lateralization effect extends to how sensory signals modulate during voluntary sensorimotor control remains unaddressed. We compared tactile sensory gating during voluntary motor activation between the arms of older adults. Eight right-arm dominant participants received a single-pulse, 100 µs square-wave electrotactile stimulus at their testing arm's fingertip or elbow. We identified at both arms the threshold at which the electrotactile stimulus was detected when participants were at rest (baseline) and isometrically flexing about the elbow to 25% and 50% of their maximum voluntary torque. Results reveal a difference in the detection threshold at the fingertip (p 0.001) between the arms, yet not the elbow (p = 0.264). Additionally, results demonstrate that greater isometric flexion about the elbow yields increased detection thresholds at the elbow (p = 0.005), yet not the fingertip (p = 0.069). However, the changes in detection threshold during motor activation did not significantly differ between the arms (p = 0.154). The findings regarding an impact of arm dominance and location on tactile perception are important when considering sensorimotorhaptic perception and training, including post-unilateral injury.

Impact of Voluntary Muscle Activation on Stretch Reflex Excitability in Individuals With Hemiparetic Stroke.

Objective: To characterize how, following a stretch-induced attenuation, volitional muscle activation impacts stretch reflex activity in individuals with stroke. Methods: A robotic device rotated the paretic elbow of individuals with hemiparetic stroke from 70° to 150°, and then back to 70° elbow flexion at an angular speed of 120°/s. This stretching sequence was repeated 20 times. Subsequently, participants volitionally activated their elbow musculature or rested. Finally, the stretching sequence was repeated another 20 times. The flexors' stretch reflex activity was quantified as the net torque measured at 135°. Results: Data from 15 participants indicated that the stretching sequence attenuated the flexion torque (p < 0.001) and resting sustained the attenuation (p = 1.000). Contrastingly, based on data from 14 participants, voluntary muscle activation increased the flexion torque (p < 0.001) to an initial pre-stretch torque magnitude (p = 1.000). Conclusions: Stretch reflex attenuation induced by repeated fast stretches may be nullified when individuals post-stroke volitionally activate their muscles. In contrast, resting may enable a sustained reflex attenuation if the individual remains relaxed. Significance: Stretching is commonly implemented to reduce hyperactive stretch reflexes following a stroke. These findings suggest that stretch reflex accommodation arising from repeated fast stretching may be reversed once an individual volitionally moves their paretic arm.

Investigation of how accurately individuals with hemiparetic stroke can mirror their forearm positions.

Current literature suggests that greater than 50% of survivors of a stroke cannot accurately perceive where their upper extremity is positioned. Our recent work demonstrates that the extent to which this perception is affected can depend on how the task is performed. For example, individuals with stroke who have a deficit in mirroring the position of their passively-placed paretic forearm during a between-arms task may accurately reproduce the position of their actively-controlled paretic forearm during a single-arm task. Moreover, the ability of individuals with various types of unilateral lesions to locate their thumb can depend on whether they reach for their paretic thumb or non-paretic thumb. Consequently, we investigated to what extent the accuracy of individuals post-hemiparetic stroke in mirroring forearm positions on a between-arms task is influenced by various conditions. Eighteen participants with hemiparetic stroke rotated their reference forearm to a target position, and then rotated their opposite forearm to concurrently mirror the position of their reference forearm. This task was performed when participants referenced each forearm (paretic, non-paretic) at two target positions (extension, flexion) for two modes of limb control (passive, active). We quantified for every testing scenario of each participant their position-mirroring error. The number of times for which participants were classified as having a deficit was least when mirroring forearm positions at the flexed position when referencing their non-paretic forearm. Additionally, the difference in the magnitude of errors when participants referenced each arm was greater during active than passive movements. Findings from this study provide further evidence that the accuracy with which individuals post stroke perceive the position of their limbs can depend on how a task is performed. Factors to consider include whether movements are active versus passive, which limb is referenced, and where the limb is positioned.

Impact of Abducting at the Shoulder on Perceiving Torques about the Elbow.

Literature indicates that an individual's perception of their self-generated torques is largely influenced by their descending motor commands. These studies often rely on between-limbs matching protocols, which can introduce confounding factors when interpreting results from populations with unilateral impairments. Here, we demonstrate how changes in descending motor commands impact one's perception of torques using a single-arm protocol. Thirteen participants generated and perceived 25% of their maximum voluntary torque (MVT) in elbow flexion while simultaneously abducting at their shoulder to 10%, 30%, or 50% of their MVT in shoulder abduction (MVTSABD). Subsequently, the participants matched the elbow torque without feedback and without activating their shoulder. The accuracy in matching the elbow torque was influenced by the extent to which the shoulder abducted (p=0.002); the average error in matching elbow torques was greatest at 50% MVTSABD (3.9 ± 4.9 Nm), followed by 30% MVTSABD (2.1 ± 2.7 Nm), and then 10% MVTSABD (0.0 ±2.1 Nm). These results indicate that perception of a torque about the elbow is influenced by the extent of simultaneous activation about the biomechanically-coupled shoulder. Therefore, this approach can quantify, using a single arm, the impact of changes in muscle activation on torque perception.

Accuracy of older adults in judging self-generated elbow torques during multi-joint isometric tasks.

Successful execution of daily activities requires accurate perception of the torques one generates about multiple joints. Even so, previous studies are mostly limited to an individual's perception when torques are generated about a single joint. Consequently, this study investigates how accurately individuals judge torques at their arm during a multi-joint task. The accuracy of fifteen right-hand dominant participants (age: 60 ± 10 years) in matching isometric elbow torques, within the same arm, was quantified during single- and/or multi-joint tasks. Participants generated and matched elbow torques when the shoulder was: (1) not abducted (single-to-single-joint), (2) abducted (multi-to-multi-joint), and (3) abducted and then not abducted (multi-to-single-joint). The constant error for the multi-to-single-joint condition (dominant: 6.9 ± 5.9 Nm, non-dominant: 6.0 ± 5.5 Nm) was greater than that for the single-to-single-joint condition (dominant: 2.7 ± 3.1 Nm, non-dominant: 3.4 ± 2.8 Nm) (p < 0.001) and multi-to-multi-joint condition (dominant: 3.0 ± 2.8 Nm, non-dominant: 3.9 ± 2.7 Nm) (p < 0.001). The constant error for the multi-to-multi-joint condition did not significantly differ from that of the single-to-single-joint condition (p [Formula: see text] 0.780). Findings indicate that in older adults the perception of a self-generated torque during a 2-degree-of-freedom (DOF), multi-joint task is largely influenced by the motor commands associated with the 2-DOF task and is not specific to the DOF at each joint.

Quantifying Altered Neural Connectivity of the Stretch Reflex in Chronic Hemiparetic Stroke.

Post-stroke flexion synergy limits arm/hand function and is also linked to hyperactive stretch reflexes or spasticity. It is implicated in the increased role of indirect motor pathways following damage to direct corticospinal projections. We hypothesized that this maladaptive neuroplasticity also affects stretch reflexes. Specifically, multi-synaptic interactions in indirect motor pathways may increase nonlinear neural connectivity and time lag between stretch and reflex muscle response. Continuous position perturbations were applied to the elbow joint when eleven participants with stroke generated two levels of shoulder abduction (SABD) torques with their paretic arm to induce synergy-related spasticity. Likewise, the perturbations were applied to eleven control subjects while performing SABD and elbow flexion levels matching the synergy torques in stroke. We quantified linear and non-linear connectivity and the corresponding time lags between perturbations and muscle activity. Enhanced nonlinear connectivity with a prolonged time lag was found in stroke as compared to controls. Non-linear connectivity and time lag also increased with the expression of the flexion synergy, as induced by greater SABD load levels, in stroke. This study provides new evidence of changes in neural connectivity and long-latency time lag in the stretch reflex response post-stroke. The results suggest the contribution of indirect motor pathways to synergy-related spasticity.

Individuals With Hemiparetic Stroke Accurately Match Torques They Generate About Each Elbow Joint.

Background: Successful execution of a task as simple as drinking from a cup and as complicated as cutting food with a fork and knife requires accurate perception of the torques that one generates in each arm. Prior studies have shown that individuals with hemiparetic stroke inaccurately judge their self-generated torques during bimanual tasks; yet, it remains unclear whether these individuals inaccurately judge their self-generated torques during unimanual tasks. Objective: The goal of this work was to determine whether stroke affected how accurately individuals with stroke perceive their self-generated torques during a single-arm task. Methods: Fifteen individuals with hemiparetic stroke and fifteen individuals without neurological impairments partook in this study. Participants generated a target torque about their testing elbow while receiving visual feedback, relaxed, and then matched the target torque about the same elbow without receiving feedback. This task was performed for two target torques (5 Nm, 25% of maximum voluntary torque), two movement directions (flexion, extension), and two arms (left, right). Results: Clinical assessments indicate that eleven participants with stroke had kinaesthetic deficits and two had altered pressure sense; their motor impairments spanned from mild to severe. These participants matched torques at each elbow, for each target torque and movement direction, with a similar accuracy and precision to controls, regardless of the arm tested (p > 0.050). Conclusions: These results indicate that an individual with sensorimotor deficits post-hemiparetic stroke may accurately judge the torques that they generate within each arm. Therefore, while survivors of a hemiparetic stroke may have deficits in accurately judging the torques they generate during bimanual tasks, such deficits do not appear to occur during unimanual tasks.

Accuracy of Individuals Post-hemiparetic Stroke in Matching Torques Between Arms Depends on the Arm Referenced.

Background: Prior work indicates that 50-75% of individuals post-hemiparetic stroke have upper-extremity weakness and, in turn, inaccurately judge the relative torques that their arms generate during a bimanual task. Recent findings also reveal that these individuals judge the relative torques their arms generate differently depending on whether they reference their paretic vs. non-paretic arm. Objective: Our goal was to determine whether individuals with hemiparetic stroke inaccurately matched torques between arms, regardless of the arm that they referenced. Methods: Fifteen participants with hemiparetic stroke and 10 right-hand dominant controls matched torques between arms. Participants performed this task with their right arm referencing their left arm, and vice versa. Participants generated (1) 5 Nm and (2) 25% of their reference elbow's maximum voluntary torque (MVT) in flexion and extension using their reference arm while receiving audiovisual feedback. Then, participants matched the reference torque using their opposite arm without receiving feedback on their matching performance. Results: Participants with stroke had greater magnitudes of error in matching torques than controls when referencing their paretic arm (p < 0.050), yet not when referencing their non-paretic arm (p > 0.050). The mean magnitude of error when participants with stroke referenced their paretic and non-paretic arm and controls referenced their dominant and non-dominant arm to generate 5 Nm in flexion was 9.4, 2.6, 4.2, and 2.5 Nm, respectively, and in extension was 5.3, 2.8, 2.5, and 2.3 Nm, respectively. However, when the torques generated at each arm were normalized by the corresponding MVT, no differences were found in matching errors regardless of the arm participants referenced (p > 0.050). Conclusions: Results demonstrate the importance of the arm referenced, i.e., paretic vs. non-paretic, on how accurately individuals post-hemiparetic stroke judge their torques during a bimanual task. Results also indicate that individuals with hemiparetic stroke judge torques primarily based on their perceived effort. Finally, findings support the notion that training individuals post-hemiparetic stroke to accurately perceive their self-generated torques, with a focus of their non-paretic arm in relation to their paretic arm, may lead to an improved ability to perform bimanual activities of daily living.

Development of a Training Game to Coordinate Torques Produced Between Arms.

The ability of individuals to accurately judge the forces that they generate is integral to seamlessly controlling their movements during everyday life. Individuals with chronic hemiparetic stroke have been shown to be impaired when matching forces between arms; this impairment may make activities as simple as carrying a tray challenging. Our goal was to develop a training protocol that individuals with stroke could use to improve their accuracy in judging the torques that they generate between arms. We designed a torque coordination game for this goal and tested its feasibility in six individuals without neurological impairments. Participants interacted with an instrumented isometric device at each arm and received automated audiovisual cues in response to the torques that they generated about each elbow joint. During the game, the participant's task was to keep a launched ball on its planned course. The participant achieved this task by sequentially applying required elbow torques at the correct times to close a left flap using the left arm and a right flap using the right arm. Participants performed this task 20 times when initiating with their left arm and 20 times when initiating with their right arm. Results indicate that all participants had a success rate in the range of 60% to 80% regardless of the arm dominance of the leading arm. Additionally, all participants anecdotally reported the game to be intuitive, and they provided an average difficulty rating that indicated the task was relatively easy to learn (i.e., 3 out of 10). Based on these findings, we conclude that this game may be suitable, enjoyable, and motivational for training coordination of torques between arms in individuals with stroke.

Ability of individuals with chronic hemiparetic stroke to locate their forearms during single-arm and between-arms tasks.

BACKGROUND: According to between-arms assessments, more than 50% of individuals with stroke have an impaired position sense. Our previous work, which employed a clinical assessment and slightly differing tasks, indicates that individuals who have a deficit on a between-forearms position-localization task do not necessarily have a deficit on a single-forearm position-localization task. OBJECTIVE: Our goal here was to, using robotics tools, determine whether individuals with stroke who have a deficit when matching forearm positions within an arm also have a deficit when mirroring forearm positions between arms, independent of the arm that leads the task. METHODS: Eighteen participants with chronic hemiparetic stroke and nine controls completed a single-arm position-matching experiment and between-arms position-mirroring experiment. For each experiment, the reference forearm (left/right) passively rotated about the elbow joint to a reference target location (flexion/extension), and then the participant actively rotated their same/opposite forearm to match/mirror the reference forearm's position. Participants with stroke were classified as having a position-matching/-mirroring deficit based on a quantitative threshold that was derived from the controls' data. RESULTS: On our single-arm task, one participant with stroke was classified as having a position-matching deficit with a mean magnitude of error greater than 10.7° when referencing their paretic arm. Position-matching ability did not significantly differ for the controls and the remaining seventeen participants with stroke. On our between-arms task, seven participants with stroke were classified as having a position-mirroring deficit with a mean magnitude of error greater than 10.1°. Position-mirroring accuracy was worse for these participants with stroke, when referencing their paretic arm, than the controls. CONCLUDING REMARK: Findings underscore the need for assessing within-arm position-matching deficits, in addition to between-arms position-mirroring deficits when referencing each arm, to comprehensively evaluate an individual's ability to locate their forearm(s).

Impact of motor task execution on an individual's ability to mirror forearm positions.

This work is motivated by our goal of determining why individuals with stroke are impaired when locating their arms in space. We assessed the ability of individuals without neurological impairments to mirror their forearms during various motor tasks so that we could identify baseline performance in an unimpaired population. Nine right-hand dominant participants without neurological impairments mirrored forearm positions bi-directionally (i.e., right forearm mirrors left forearm, vice versa) for three motor tasks (i.e., passive, passive/active, and active) and two position identification modes (i.e., mirroring to a position stored in working memory versus concurrently felt by the opposite arm). During each trial, the participant's reference forearm moved to a flexion ([Formula: see text]) or extension ([Formula: see text]) position, and then, their opposite forearm mirrored the position of their reference forearm. The main finding across all tested conditions is that participants mirrored forearm positions with an average magnitude of error [Formula: see text]. When controlling their forearms' movements (active motor task), participants mirrored forearm positions more accurately by up to, on average, [Formula: see text] at the flexion location than at the extension location. Moreover, participants mirrored forearm positions more accurately by up to, on average, [Formula: see text] when their forearms were moved for them rather than when they controlled their forearms' movements. Task directionality and position identification mode did not significantly affect participant arm mirroring accuracy. These findings are relevant for interpreting in future work the reason why impairments occur, on similar tasks, in individuals with altered motor commands, working memory, and arm impedance, e.g., post-stroke hemiparesis.

Task directionality impacts the ability of individuals with chronic hemiparetic stroke to match torques between arms: Preliminary findings.

Post hemiparetic stroke an individual may face difficulty performing bimanual tasks due to an asymmetry in their arms' strengths. Here, we determined whether participants with a strength asymmetry were impaired bi-directionally when matching torques between arms (i.e., paretic arm matches non-paretic arm, non-paretic arm matches paretic arm). Six participants with chronic hemiparetic stroke and four participants without neurological impairments partook in this study. First, we identified the maximum voluntary torque that participants could generate about each elbow joint (τmvt). Then, we determined how accurately and precisely participants could match, bidirectionally, submaximal isometric flexion torques (0.25 · τMVT:Reference) between arms. Results demonstrate that task directionality impacted the ability of our participants with stroke who had a strength asymmetry to match torques between arms; specifically, participants were unimpaired matching to a referenced non-paretic arm yet impaired in the opposite direction. Additionally, results reveal that the degree to which participants overshot the target torque when matching with their non-paretic arm could be predicted based on their strength asymmetry (R2Adjusted = 0.67). We propose that individuals with stroke may avoid torque matching impairments during bimanual tasks by matching their paretic arm to their non-paretic arm.

Perception of force and stiffness in the presence of low-frequency haptic noise.

OBJECTIVE: This work lays the foundation for future research on quantitative modeling of human stiffness perception. Our goal was to develop a method by which a human's ability to perceive suprathreshold haptic force stimuli and haptic stiffness stimuli can be affected by adding haptic noise. METHODS: Five human participants performed a same-different task with a one-degree-of-freedom force-feedback device. Participants used the right index finger to actively interact with variations of force (∼5 and ∼8 N) and stiffness (∼290 N/m) stimuli that included one of four scaled amounts of haptically rendered noise (None, Low, Medium, High). The haptic noise was zero-mean Gaussian white noise that was low-pass filtered with a 2 Hz cut-off frequency; the resulting low-frequency signal was added to the force rendered while the participant interacted with the force and stiffness stimuli. RESULTS: We found that the precision with which participants could identify the magnitude of both the force and stiffness stimuli was affected by the magnitude of the low-frequency haptically rendered noise added to the haptic stimulus, as well as the magnitude of the haptic stimulus itself. The Weber fraction strongly correlated with the standard deviation of the low-frequency haptic noise with a Pearson product-moment correlation coefficient of ρ > 0.83. The mean standard deviation of the low-frequency haptic noise in the haptic stimuli ranged from 0.184 N to 1.111 N across the four haptically rendered noise levels, and the corresponding mean Weber fractions spanned between 0.042 and 0.101. CONCLUSIONS: The human ability to perceive both suprathreshold haptic force and stiffness stimuli degrades in the presence of added low-frequency haptic noise. Future work can use the reported methods to investigate how force perception and stiffness perception may relate, with possible applications in haptic watermarking and in the assessment of the functionality of peripheral pathways in individuals with haptic impairments.

Individuals with chronic hemiparetic stroke can correctly match forearm positions within a single arm.

OBJECTIVE: Previous studies determined, using between arms position matching assessments, that at least one-half of individuals with stroke have an impaired position sense. We investigated whether individuals with chronic stroke who have impairments mirroring arm positions also have impairments identifying the location of each arm in space. METHODS: Participants with chronic hemiparetic stroke and age-matched participants without neurological impairments (controls) performed a between forearms position matching task based on a clinical assessment and a single forearm position matching task, using passive and active movements, based on a robotic assessment. RESULTS: 12 out of our 14 participants with stroke who had clinically determined between forearms position matching impairments had greater errors than the controls in both their paretic and non-paretic arm when matching positions during passive movements; yet stroke participants performed comparable to the controls during active movements. CONCLUSIONS: Many individuals with chronic stroke may have impairments matching positions in both their paretic and non-paretic arm if their arm is moved for them, yet not within either arm if these individuals control their own movements. SIGNIFICANCE: The neural mechanisms governing arm location perception in the stroke population may differ depending on whether arm movements are made passively versus actively.

Customization, control, and characterization of a commercial haptic device for high-fidelity rendering of weak forces.

BACKGROUND: The emergence of commercial haptic devices offers new research opportunities to enhance our understanding of the human sensory-motor system. Yet, commercial device capabilities have limitations which need to be addressed. This paper describes the customization of a commercial force feedback device for displaying forces with a precision that exceeds the human force perception threshold. NEW METHOD: The device was outfitted with a multi-axis force sensor and closed-loop controlled to improve its transparency. Additionally, two force sensing resistors were attached to the device to measure grip force. Force errors were modeled in the frequency- and time-domain to identify contributions from the mass, viscous friction, and Coulomb friction during open- and closed-loop control. The effect of user interaction on system stability was assessed in the context of a user study which aimed to measure force perceptual thresholds. RESULTS: Findings based on 15 participants demonstrate that the system maintains stability when rendering forces ranging from 0-0.20 N, with an average maximum absolute force error of 0.041 ± 0.013 N. Modeling the force errors revealed that Coulomb friction and inertia were the main contributors to force distortions during respectively slow and fast motions. COMPARISON WITH EXISTING METHODS: Existing commercial force feedback devices cannot render forces with the required precision for certain testing scenarios. Building on existing robotics work, this paper shows how a device can be customized to make it reliable for studying the perception of weak forces. CONCLUSIONS: The customized and closed-loop controlled device is suitable for measuring force perceptual thresholds.