Mike D. Rinderknecht

Learning and generalization in an isometric visuomotor task.

Adaptation is a prominent feature of the human motor system and has been studied extensively in reaching movements. This study characterizes adaptation and generalization during isometric reaching in which the arm remains stationary and the participant controls a virtual cursor via force applied by the hand. We measured how learning of a visual cursor rotation generalizes across workspace 1) to determine the coordinate system that predominates visual rotation learning, and 2) to ascertain whether mapping type, namely position or velocity control, influences transfer. Participants performed virtual reaches to one of two orthogonal training targets with the applied rotation. In a new workspace, participants reached to a single target, similar to the training target in either hand or joint space. Furthermore, a control experiment measured within-workspace generalization to an orthogonal target. Across position and velocity mappings, learning transferred predominantly in intrinsic (joint) space, although the transfer was incomplete. The velocity mapping resulted in significantly larger aftereffects and broader within-workspace generalization than the position mapping, potentially due to slower peak speeds, longer trial times, greater target overshoot, or other factors. Although we cannot rule out a mixed reference frame in our task, the predominance of intrinsic coding of cursor kinematics in the isometric environment opposes the extrinsic coding of arm kinematics in real reaching but matches the intrinsic coding of dynamics found in prior work. These findings have implications for the design of isometric control systems in human-machine interaction or in rehabilitation when coordinated multi-degree-of-freedom movement is difficult to achieve.

Combined tendon vibration and virtual reality for post-stroke hand rehabilitation

Sensory function is essential for functional post-stroke recovery and control of basic hand movements like grasping. Despite this fact, therapy focuses strongly on motor aspects of rehabilitation, requiring active participation and thus excluding stroke patients with severe paresis. The aim of our novel therapeutic approach combining virtual reality, based on clinically proven mirror therapy, and tendon vibration of hand and wrist muscles is to induce neuroplastic changes leading to improved hand function. This paper presents the further development and evaluation of a robotic device, which can apply vibrations at precise locations on the finger flexor tendons to create illusions of extension movements and visualize the movements with a virtual hand. A preliminary study including 16 healthy subjects investigated the influence of the virtual reality on the perception of proprioceptive illusory movements. The experimental results provided evidence that the addition of the virtual reality enhanced the perception of the illusory movement generated by tendon vibration, by inducing movements with significantly higher extension (+4.5%, p <; 0.05). Furthermore, the virtual reality allowed a better controlled temporal elicitation of the illusion. These findings indicate the potential of this novel strategy for a more effective therapy, especially for severely impaired patients.

Assistance using adaptive oscillators: Robustness to errors in the identification of the limb parameters

This paper provides a robustness analysis of the method we recently developed for rhythmic movement assistance using adaptive oscillators. An adaptive oscillator is a mathematical tool capable of extracting high-level features (i.e. amplitude, frequency, offset) of a quasi-sinusoidal measured movement, a rhythmic flexion-extension of the elbow in this case. By the use of a simple inverse dynamical model, the system can predict the torque produced by a human participant, such that a fraction of this estimated torque is fed back through a series elastic actuator to provide movement assistance. This paper objectives are twofold. First, we introduce a new 1 DOF assistive device developed in our lab. Second, we derive model-based predictions and conduct experimental validations to measure the variations in movement frequency as a function of the open parameters of the inverse dynamical model. As such, the paper provides an estimation of the robustness of our method due to model approximations. As main result, the paper reveals that the movement frequency is particularly robust to errors in the estimation of the damping coefficient. This is of high interest for the applicability of our approach, this parameter being in general the most difficult to identify.

Reliable and Rapid Robotic Assessment of Wrist Proprioception Using a Gauge Position Matching Paradigm

Quantitative assessments of position sense are essential for the investigation of proprioception, as well as for diagnosis, prognosis and treatment planning for patients with somatosensory deficits. Despite the development and use of various paradigms and robotic tools, their clinimetric properties are often poorly evaluated and reported. A proper evaluation of the latter is essential to compare results between different studies and to identify the influence of possible confounds on outcome measures. The aim of the present study was to perform a comprehensive evaluation of a rapid robotic assessment of wrist proprioception using a passive gauge position matching task. Thirty-two healthy subjects undertook six test-retests of proprioception of the right wrist on two different days. The constant error (CE) was 0.87°, the absolute error (AE) was 5.87°, the variable error (VE) was 4.59° and the total variability (E) was 6.83° in average for the angles presented in the range from 10° to 30°. The intraclass correlation analysis provided an excellent reliability for CE (0.75), good reliability for AE (0.68) and E (0.68), and fair reliability for VE (0.54). Tripling the assessment length had negligible effects on the reliabilities. Additional analysis revealed significant trends of larger overestimation (constant errors), as well as larger absolute and variable errors with increased flexion angles. No proprioceptive learning occurred, despite increased familiarity with the task, which was reflected in significantly decreased assessment duration by 30%. In conclusion, the proposed automated assessment can provide sensitive and reliable information on proprioceptive function of the wrist with an administration time of around 2.5 min, demonstrating the potential for its application in research or clinical settings. Moreover, this study highlights the importance of reporting the complete set of errors (CE, AE, VE, and E) in a matching experiment for the identification of trends and subsequent interpretation of results.

Experimental Validation of a Rapid, Adaptive Robotic Assessment of the MCP Joint Angle Difference Threshold

This paper presents an experimental evaluation of a rapid, adaptive assessment of the difference threshold (DL) of passive metacarpophalangeal index finger joint flexion using a robotic device. Parameter Estimation by Sequential Testing (PEST) is compared to the method of constant stimuli (MOCS) using a two-alternative forced-choice paradigm. The pilot study with \(13\) healthy subjects provided DLs within similar ranges for MOCS and PEST, averaging at \(2.15^{\circ }\pm 0.77^{\circ }\) and \(1.73^{\circ }\pm 0.78^{\circ }\), respectively, in accordance with the literature. However, no significant correlation was found between the two methods (\(r(11) = 0.09\), \(p = 0.762\)). The average number of trials required for PEST to converge was \(58.7\pm 17.6\), and significantly lower compared to \(120\) trials for MOCS (\(p < 0.001\)), leading to an assessment time of under \(15\) min. These results suggest that rapid, adaptive methods, such as PEST, could be successfully implemented in novel robotic tools for clinical assessment of sensory deficits.

Age-based model for metacarpophalangeal joint proprioception in elderly

Neurological injuries such as stroke can lead to proprioceptive impairment. For an informed diagnosis, prognosis, and treatment planning, it is essential to be able to distinguish between healthy performance and deficits following the neurological injury. Since there is some evidence that proprioception declines with age and stroke occurs predominantly in the elderly population, it is important to create a healthy reference model in this specific age group. However, most studies investigate age effects by comparing young and elderly subjects and do not provide a model within a target age range. Moreover, despite the functional relevance of the hand in activities of daily living, age-based models of distal proprioception are scarce. Here, we present a proprioception model based on the assessment of the metacarpophalangeal joint angle difference threshold in 30 healthy elderly subjects, aged 55–80 years (median: 63, interquartile range: 58–66), using a robotic tool to apply passive flexion–extension movements to the index finger. A two-alternative forced-choice paradigm combined with an adaptive algorithm to define stimulus magnitude was used. The mixed-effects model analysis revealed that aging has a significant, increasing effect on the difference threshold at the metacarpophalangeal joint, whereas other predictors (eg, tested hand or sex) did not show a significant effect. The adaptive algorithm allowed reaching an average assessment duration <15 minutes, making its clinical applicability realistic. This study provides further evidence for an age-related decline in proprioception at the level of the hand. The established age-based model of proprioception in elderly may serve as a reference model for the proprioceptive performance of stroke patients, or of any other patient group with central or peripheral proprioceptive impairments. Furthermore, it demonstrates the potential of such automated robotic tools as a rapid and quantitative assessment to be used in research and clinical settings.

Design and Evaluation of a Fiber-Optic Grip Force Sensor with Compliant 3D-Printable Structure for (f)MRI Applications

Grip force sensors compatible with magnetic resonance imaging (MRI) are used in human motor control and decision-making research, providing objective and sensitive behavioral outcome measures. Commercial sensors are expensive, cover limited force ranges, rely on pneumatic force transmission that cannot detect fast force changes, or are electrically active, which increases the risk of electromagnetic interference. We present the design and evaluation of a low-cost, 3D-printed, inherently MRI-compatible grip force sensor based on a commercial intensity-based fiber-optic sensor. A compliant monobloc structure with flexible hinges transduces grip force to a linear displacement captured by the fiber-optic sensor. The structure can easily be adapted for different force ranges by changing the hinge thickness. A prototype designed for forces up to 800 N was manufactured and showed a highly linear behavior (nonlinearity of 2.37%) and an accuracy of 1.57% in a range between zero and 500 N. It can be printed and assembled within one day and for less than

Algorithm for improving psychophysical threshold estimates by detecting sustained inattention in experiments using PEST

300. Accurate performance was confirmed, both inside and outside a 3 T MRI scanner within a pilot study. Given its simple design allowing for customization of sensing properties and ergonomics for different applications and requirements, the proposed grip force handle offers researchers a valuable scientific tool.

Performance Metrics for an Application-driven Selection and Optimization of Psychophysical Sampling Procedures

Psychophysical procedures are applied in various fields to assess sensory thresholds. During experiments, sampled psychometric functions are usually assumed to be stationary. However, perception can be altered, for example by loss of attention to the presentation of stimuli, leading to biased data, which results in poor threshold estimates. The few existing approaches attempting to identify non-stationarities either detect only whether there was a change in perception, or are not suitable for experiments with a relatively small number of trials (e.g., <

Method for Improving Psychophysical Threshold Estimates by Detecting Sustained Inattention

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