Bimanual Grasping Adheres to Weber's Law

Weber's law states that our ability to detect changes in stimulus attributes decreases linearly with their magnitude. This principle holds true for many attributes across sensory modalities but appears to be violated in grasping. One explanation for the failure to observe Weber's law in grasping is that its effect is masked by biomechanical constraints of the hand. We tested this hypothesis using a bimanual task that eliminates biomechanical constraints. Participants either grasped differently sized boxes that were comfortably within their arm span (action task) or estimated their width (perceptual task). Within each task, there were two conditions: One where the hands’ start positions remained fixed for all object sizes (meaning the distance between the initial and final hand-positions varied with object size), and one in which the hands’ start positions adapted with object size (such that the distance between the initial and final hand-position remained constant). We observed adherence to Weber's law in bimanual estimation and grasping across both conditions. Our results conflict with a previous study that reported the absence of Weber's law in bimanual grasping. We discuss potential explanations for these divergent findings and encourage further research on whether Weber's law persists when biomechanical constraints are reduced.

Handlebar Robotic System for Bimanual Motor Control and Learning Research

Robotic devices can be used for motor control and learning research. In this work, we present the construction, modeling and experimental validation of a bimanual robotic device. We tested some hypotheses that may help to better understand the motor learning processes involved in the interlimb coordination function. The system emulates a bicycle handlebar with rotational motion, thus requiring bilateral upper limb control and a coordinated sequence of joint sub-movements. The robotic handlebar is compact and portable and can register in a fast rate both position and forces independently from arms, including prehension forces. An impedance control system was implemented in order to promote a safer environment for human interaction and the system is able to generate force fields, suitable for implementing motor learning paradigms. The novelty of the system is the decoupling of prehension and manipulation forces of each hand, thus paving the way for the investigation of hand dominance function in a bimanual task. Experiments were conducted with ten healthy subjects, kinematic and dynamic variables were measured during a rotational set of movements. Statistical analyses showed that movement velocity decreased with practice along with an increase in reaction time. This suggests an increase of the task planning time. Prehension force decreased with practice. However, an unexpected result was that the dominant hand did not lead the bimanual task, but helped to correct the movement, suggesting different roles for each hand during a cooperative bimanual task.

Corpus callosal microstructure predicts bimanual motor performance in chronic stroke survivors

Much of the research using diffusion tensor imaging (DTI) in stroke focuses on characterizing the microstructural status of corticospinal tracts and its utility as a prognostic biomarker. However, the ischemic event in the lesioned cortex also triggers structural and functional alterations in its contralateral homolog through the corpus callosum (CC), known as transcallosal diaschisis. The few studies that have characterized the microstructural status of the CC using DTI only examine its relationship with paretic limb performance. Given the well-established role of the CC for bimanual coordination, especially fibers connecting the larger sensorimotor networks such as prefrontal, premotor and supplementary motor regions, we examine the relationship between the microstructural status of the CC and bimanual performance in chronic stroke survivors (n = 41). We used movement times for two self-initiated and self-paced bimanual tasks to capture bimanual performance. Using publicly available control datasets (n = 52), matched closely for acquisition parameters, including sequence, diffusion gradient strength and number of directions, we also explored the effect of age and stroke on callosal microstructure. We found that callosal microstructure was significantly associated with bimanual performance in chronic stroke survivors such that those with lower callosal FA were slower at completing the bimanual task. Notably, while the primary sensorimotor regions (CC3) showed the strongest relationship with bimanual performance, this was closely followed by the premotor/supplementary motor (CC2) and the prefrontal (CC1) regions. We used multiple mixed regression to systematically account for loss of callosal axons (i.e., normalized callosal volume) as well as differences in lesion size and other metrics of structural damage. Chronic stroke survivors presented with significantly greater loss of callosal fiber orientation (lower mean FA) compared to neurologically intact, age-similar controls, who in turn presented with lower callosal FA compared to younger controls. The effect of age and stroke were observed for all regions of the CC except the splenium. These preliminary findings suggest that in chronic stroke survivors with relatively localized lesions, callosal microstructure can be expected to change beyond the primary sensorimotor regions and might impact coordinated performance of self-initiated and cooperative bimanual tasks.

The dominant limb preferentially stabilizes posture in a bimanual task with physical coupling

We found that subjects prefer to stabilize a bimanually held object by cocontracting their dominant limb, contradicting the established view that the nondominant limb is specialized toward stabilization.

Rapid crossed responses in an intrinsic hand muscle during perturbed bimanual movements

Mechanical perturbations in one upper limb often elicit corrective responses in both the perturbed as well as its contralateral and unperturbed counterpart. These crossed corrective responses have been shown to be sensitive to the bimanual requirements of the perturbation, but crossed responses (CRs) in hand muscles are far less well studied. Here, we investigate corrective CRs in an intrinsic hand muscle, the first dorsal interosseous (1DI), to clockwise and anticlockwise mechanical perturbations to the contralateral index finger while participants performed a bimanual finger abduction task. We found that the CRs in the unperturbed 1DI were sensitive to the direction of the perturbation of the contralateral index finger. However, the size of the CRs was not sensitive to the amplitude of the contralateral perturbation nor its context within the bimanual task. The onset latency of the CRs was too fast to be purely transcortical (<70 ms) in 12/12 participants. This confirms that during isolated bimanual finger movements, sensory feedback from one hand can influence the other, but the pathways mediating the earliest components of this interaction are likely to involve subcortical systems such as the brainstem or spinal cord, which may afford less flexibility to the task demands. NEW & NOTEWORTHY An intrinsic hand muscle shows a crossed response to a perturbation of the contralateral index finger. The crossed response is dependent on the direction of the contralateral perturbation but not on the amplitude or the bimanual requirements of the movement, suggesting a far less flexible control policy than those governing crossed responses in more proximal muscles. The crossed response is too fast to be purely mediated by transcortical pathways, suggesting subcortical contributions.

Myoelectric untethered robotic glove enhances hand function and performance on daily living tasks after stroke

Introduction Wearable robots controlled using electromyography could motivate greater use of the affected upper extremity after stroke and enable bimanual activities of daily living to be completed independently. Methods We have developed a myoelectric untethered robotic glove (My-HERO) that provides five-finger extension and grip assistance. Results The myoelectric controller detected the grip and release intents of the 9 participants after stroke with 84.7% accuracy. While using My-HERO, all 9 participants performed better on the Fugl-Meyer Assessment-Hand (8.4 point increase, scale out of 14, p < 0.01) and the Chedoke Arm and Hand Activity Inventory (8.2 point increase, scale out of 91, p < 0.01). Established criteria for clinically meaningful important differences were surpassed for both the hand function and daily living task assessments. The majority of participants provided satisfaction and usability questionnaire scores above 70%. Seven participants desired to use My-HERO in the clinic and at home during their therapy and daily routines. Conclusions People with hand impairment after stroke value that myoelectric untethered robotic gloves enhance their motion and bimanual task performance and motivate them to use their muscles during engaging activities of daily living. They desire to use these gloves daily to enable greater independence and investigate the effects on neuromuscular recovery.

Impact of mirror movements on the bimanual hand function and functional independence with comparison of the bimanual hand function in children suffering from spastic cerebral palsy, with and without mirror movements

Objective: To compare the bimanual hand function in children with and without mirror movements and to find the correlation between the bimanual hand function, functional independence and severity of mirror movements.Design: Comparative cross sectional study.Place and duration of the study: The study was conducted in special education schools of Lahore and Islamabad, Pakistan, from August 2017 to January 2018.Patients and Methods: Children suffering from spastic cerebral palsy including both males and females in age groups ranging from 5 to 18 years who were able to make a gross grip were included in the study. Children having low tone, not able to make a gross grip and with any deformity that makes the assessment difficult were excluded. The non probability sampling technique was used. The Woods and Teuber criteria were used to check the severity of the mirror movements and bimanual activities to compare the hand functions. Data was analyzed using SPSS 21.Results: The mean age of the participants was 11.17±3.69 years. Children who presented with mirror movements had a mean age of 12±3.70 years and those without mirror movements had mean age of 10.72±3.63 years. According to the results of the bimanual task scale, there was no difference in the bimanual hand function of children with mirror movements and without mirror movements (P value >0.05)Conclusion: According to the results of the bimanual task scale there is no significant difference in the bimanual hand function of children suffering from spastic cerebral palsy, either with with or without mirror movements. However, functional independence was found more in the children suffering from cerebral palsy who presented without mirror movements.

The effect of single-task training on learning transfer to a novel bimanual task

BackgroundThe contextual interference effect suggests that the random practice of multiple-tasks is more beneficial for the retention and transfer of the learning as compared to blocked practice. Therefore, the transfer of learning is usually attributed to the contextual interference effect and is studied in a multi-task setting.ObjectiveThe goals of this study were to evaluate whether the transfer of learning (i) can occur when a single bimanual task is practiced, (ii) is affected by the knowledge of results (feedback), and (iii) sustains over an extended number of trials.MethodsFifty-two healthy subjects were equally divided into four groups. Before the transfer test, two groups practiced a bimanual finger-tapping task with feedback (EF) and without feedback (ENF). The third group (IM) practiced the same task using the kinesthetic motor imagery, whereas the last group acted as a control (CTRL) and performed only a bimanual button-pressing task used for the transfer test.ResultsLinear mixed-model showed that in the transfer test, groups EF, ENF and IM had similar performance but significantly higher scores compared to the CTRL group. Compared to the CTRL, the EF and IM groups showed significantly improved performance in most of the sessions but group ENF had similar results.ConclusionThis study suggests that the single-task practice of a discrete bimanual task can facilitate the learning transfer to a novel task and knowledge of results (feedback) have no significant impact on the transfer of learning. Moreover, the transfer of learning effect does not disappear in extended trials.HighlightsSingle-task practice of a discrete bimanual task can facilitate the learning of a novel bimanual taskKnowledge of results (feedback) does not improve learning transfer in single-task settingTransfer of learning effect does not disappear in extended trials