Meghan E. Huber

Acquisition of novel and complex motor skills: stable solutions where intrinsic noise matters less.

Most experimental paradigms in motor neuroscience have used relatively focal and experimentally constrained tasks to allow precise measurement and experimental control. Therefore, practice-induced improvements and learning have been confined to relatively simple changes or adaptations to external perturbations. Here, we propose an approach to study more complex skills that are novel and require more extensive practice, leading to quantitative and qualitative changes in overt performance. Central to these skills is that they have extrinsic redundancy that allows exploration and exploitation of dynamic properties of the task. We hypothesize that in such skills, humans seek stable solutions that are robust to perturbations that make their intrinsic noise matter less. Three experimental paradigms exemplify our model-based and hypothesis-driven approach to skill acquisition: discrete throwing, rhythmic ball bouncing, and complex object manipulation. In skittles, a throwing skill, results show that actors are sensitive to the error tolerance afforded by the task. In ball bouncing, we show that subjects exploit the dynamic stability of the task, where small errors and noise self-stabilize without explicit corrections. In manipulating a “cup of coffee,” subjects learn to optimize the safety or energy margins and scale it to their intrinsic variability. This research presents new experimental paradigms that characterize the behavioral correlates of neuroplasticity in more complex skill acquisition. This fundamental work is a platform for future work to develop behavioral interventions for clinical applications.

Validity and reliability of Kinect skeleton for measuring shoulder joint angles: a feasibility study

OBJECTIVE To test the reliability and validity of shoulder joint angle measurements from the Microsoft Kinect™ for virtual rehabilitation. DESIGN Test-retest reliability and concurrent validity, feasibility study. SETTING Motion analysis laboratory. PARTICIPANTS A convenience sample of 10 healthy adults. METHODS Shoulder joint angle was assessed in four static poses, two trials for each pose, using: (1) the Kinect; (2) a three-dimensional motion analysis system; and (3) a clinical goniometer. All poses were captured with the Kinect from the frontal view. The two poses of shoulder flexion were also captured with the Kinect from the sagittal view. MAIN OUTCOME MEASURES Absolute and relative test-retest reliability of the Kinect for the measurement of shoulder angle was determined in each pose with intraclass correlation coefficients (ICCs), standard error of the measure and minimal detectable change. The 95% limits of agreement (LOA) between the Kinect and the standard methods for measuring shoulder angle were computed to determine concurrent validity. RESULTS While the Kinect provided to be highly reliable (ICC 0.76-0.98) for measuring shoulder angle from the frontal view, the 95% LOA between the Kinect and the two measurement standards were greater than ±5° in all poses for both views. CONCLUSIONS Before the Kinect is used to measure movements for virtual rehabilitation applications, it is imperative to understand its limitations in precision and accuracy for the measurement of specific joint motions.

The effect of stereotype threat on performance of a rhythmic motor skill

Many studies using cognitive tasks have found that stereotype threat, or concern about confirming a negative stereotype about ones group, debilitates performance. The few studies that documented similar effects on sensorimotor performance have used only relatively coarse measures to quantify performance. This study tested the effect of stereotype threat on a rhythmic ball bouncing task, where previous analyses of the task dynamics afforded more detailed quantification of the effect of threat on motor control. In this task, novices hit the ball with positive racket acceleration, indicative of unstable performance. With practice, they learn to stabilize error by changing their ball-racket impact from positive to negative acceleration. Results showed that for novices, stereotype threat potentiated hitting the ball with positive racket acceleration, leading to poorer performance of stigmatized females. However, when the threat manipulation was delivered after having acquired some skill, reflected by negative racket acceleration, the stigmatized females performed better. These findings are consistent with the mere effort account that argues that stereotype threat potentiates the most likely response on the given task. The study also demonstrates the value of identifying the control mechanisms through which stereotype threat has its effects on outcome measures.

Persistence of reduced neuromotor noise in long-term motor skill learning

It is well documented that variability in motor performance decreases with practice, yet the neural and computational mechanisms that underlie this decline, particularly during long-term practice, are little understood. Decreasing variability is frequently examined in terms of error corrections from one trial to the next. However, the ubiquitous noise from all levels of the sensorimotor system is also a significant contributor to overt variability. While neuromotor noise is typically assumed and modeled as immune to practice, the present study challenged this notion. We investigated the long-term practice of a novel motor skill to test whether neuromotor noise can be attenuated, specifically when aided by reward. Results showed that both reward and self-guided practice over 11 days improved behavior by decreasing noise rather than effective error corrections. When the challenge for obtaining reward increased, subjects reduced noise even further. Importantly, when task demands were relaxed again, this reduced level of noise persisted for 5 days. A stochastic learning model replicated both the attenuation and persistence of noise by scaling the noise amplitude as a function of reward. More insight into variability and intrinsic noise and its malleability has implications for training and rehabilitation interventions.

Girls can play ball: Stereotype threat reduces variability in a motor skill

The majority of research on stereotype threat shows what is expected: threat debilitates performance. However, facilitation is also possible, although seldom reported. This study investigated how stereotype threat influences novice females when performing the sensorimotor task of bouncing a ball to a target. We tested the predictions of two prevailing accounts for debilitation and facilitation due to sterotype threat effects: working memory and mere effort. Experimental results showed that variability in performance decreased more in stigmatized females than in control females, consistent with the prediction of the mere effort account, but inconsistent with the working memory account. These findings suggest that stereotype threat effects may be predicated upon the correctness of the dominant motor behavior, rather than on a novice-expert distinction or task difficulty. Further, a comprehensive understanding should incorporate the fact that stereotype threat can facilitate, as well as debilitate, performance.

Validity and reliability of kinect for measuring shoulder joint angles

Virtual reality-based physical rehabilitation, or virtual rehabilitation, provides several advantages over conventional therapy. These include the capacity to provide patient-specific treatment that adapts with functional improvements over practice, obtain quantitative measures of progress, deliver real-time performance feedback through varying modalities, and improve adherence by heightening patient motivation and entertainment. By exploiting commercially available gaming technology, virtual rehabilitation systems can even be developed at a low cost and conveniently used in the home for outpatient therapy. The Microsoft Kinect is one such gaming technology that has gained recent popularity within the virtual rehabilitation community. This highly advanced, and yet low cost, sensor enables users to interact with the system by monitoring 3D body movements, making its clinical utility highly attractive to the rehabilitation community. Before the use of this technology can be translated into the clinical setting, ensuring its precision and accuracy of measuring joint motion is paramount. The present study aimed to test the reliability and validity of upper extremity joint angle measurements with the Kinect for shoulder rehabilitation. Results indicate that while the Kinect is reliable for measuring shoulder joint angles in the frontal view, it is only valid for non-occlusive poses compared to the gold (magnetic tracker) and clinical (goniometer) standards for the shoulder.

Accuracy of kinect for measuring shoulder joint angles in multiple planes of motion

The present study aimed to test the validity of upper extremity joint angle measurements with the Microsoft Kinect for shoulder rehabilitation. Results indicate that there are large discrepancies in measured shoulder angles from the Kinect compared to the gold standard (magnetic tracker) and the clinical standard (goniometer). Before the Kinect can be used to measure movements for clinical rehabilitation, understanding its limitations in precision and accuracy of measuring specific joint motions is imperative.

Velocity-Curvature Patterns Limit Human–Robot Physical Interaction

Physical human–robot collaboration is becoming more common, both in industrial and service robotics. Cooperative execution of a task requires intuitive and efficient interaction between both actors. For humans, this means being able to predict and adapt to robot movements. Given that natural human movement exhibits several robust features, we examined whether human–robot physical interaction is facilitated when these features are considered in robot control. The present study investigated how humans adapt to biological and nonbiological velocity patterns in robot movements. Participants held the end-effector of a robot that traced an elliptic path with either biological (two-thirds power law) or nonbiological velocity profiles. Participants were instructed to minimize the force applied on the robot end-effector. Results showed that the applied force was significantly lower when the robot moved with a biological velocity pattern. With extensive practice and enhanced feedback, participants were able to decrease their force when following a nonbiological velocity pattern, but never reached forces below those obtained with the 2/3 power law profile. These results suggest that some robust features observed in natural human movements are also a strong preference in guided movements. Therefore, such features should be considered in human–robot physical collaboration.

Exploiting the geometry of the solution space to reduce sensitivity to neuromotor noise

Throwing is a uniquely human skill that requires a high degree of coordination to successfully hit a target. Timing of ball release appears crucial as previous studies report required timing accuracies as short as 1-2ms, which however appear physiologically challenging. This study mathematically and experimentally demonstrates that humans can overcome these seemingly stringent timing requirements by shaping their hand trajectories to create extended timing windows, where ball releases achieve target hits despite temporal imprecision. Subjects practiced four task variations in a virtual environment, each with a distinct geometry of the solution space and different demands for timing. Model-based analyses of arm trajectories revealed that subjects first decreased timing error, followed by lengthening timing windows in their hand trajectories. This pattern was invariant across solution spaces, except for a control case. Hence, the exquisite skill that humans evolved for throwing is achieved by developing strategies that are less sensitive to temporal variability arising from neuromotor noise. This analysis also provides an explanation why coaches emphasize the “follow-through” in many ball sports.

Low-dimensional organization of angular momentum during walking on a narrow beam

Walking on a beam is a challenging motor skill that requires the regulation of upright balance and stability. The difficulty in beam walking results from the reduced base of support compared to that afforded by flat ground. One strategy to maintain stability and hence avoid falling off the beam is to rotate the limb segments to control the body’s angular momentum. The aim of this study was to examine the coordination of the angular momentum variations during beam walking. We recorded movement kinematics of participants walking on a narrow beam and computed the angular momentum contributions of the body segments with respect to three different axes. Results showed that, despite considerable variability in the movement kinematics, the angular momentum was characterized by a low-dimensional organization based on a small number of segmental coordination patterns. When the angular momentum was computed with respect to the beam axis, the largest fraction of its variation was accounted for by the trunk segment. This simple organization was robust and invariant across all participants. These findings support the hypothesis that control strategies for complex balancing tasks might be easier to understand by investigating angular momentum instead of the segmental kinematics.