The neural foundations of handedness: insights from a rare case of deafferentation

The complementary dominance hypothesis: a model for remediating the 'good' hand in stroke survivors

The complementary dominance hypothesis is a novel model of motor lateralization substantiated by decades of research examining interlimb differences in the control of upper extremity movements in neurotypical adults and hemisphere-specific motor deficits in stroke survivors. In contrast to earlier ideas that attribute handedness to the specialization of one hemisphere, our model proposes complementary motor control specializations in each hemisphere. The dominant hemisphere mediates optimal control of limb dynamics as required for smooth and efficient movements, whereas the non-dominant hemisphere mediates impedance control, important for countering unexpected mechanical conditions and achieving steady-state limb positions. Importantly, this model proposes that each hemisphere contributes its specialization to both arms (though with greater influence from either arm's contralateral hemisphere) and thus predicts that lesions to one hemisphere should produce hemisphere-specific motor deficits in not only the contralesional arm, but also the ipsilesional arm of stroke survivors - a powerful prediction now supported by a growing body of evidence. Such ipsilesional arm motor deficits vary with contralesional arm impairment, and thus individuals with little to no functional use of the contralesional arm experience both the greatest impairments in the ipsilesional arm, as well as the greatest reliance on it to serve as the main or sole manipulator for activities of daily living. Accordingly, we have proposed and tested a novel intervention that reduces hemisphere-specific ipsilesional arm deficits and thereby improves functional independence in stroke survivors with severe contralesional impairment.

Spatial mapping of posture-dependent resistance to passive displacement of the hypertonic arm post-stroke

BACKGROUND

Muscles in the post-stroke arm commonly demonstrate abnormal reflexes that result in increased position- and velocity-dependent resistance to movement. We sought to develop a reliable way to quantify mechanical consequences of abnormal neuromuscular mechanisms throughout the reachable workspace in the hemiparetic arm post-stroke.

METHODS

Survivors of hemiparetic stroke (HS) and neurologically intact (NI) control subjects were instructed to relax as a robotic device repositioned the hand of their hemiparetic arm between several testing locations that sampled the arm's passive range of motion. During transitions, the robot induced motions at either the shoulder or elbow joint at three speeds: very slow (6°/s), medium (30°/s), and fast (90°/s). The robot held the hand at the testing location for at least 20 s after each transition. We recorded and analyzed hand force and electromyographic activations from selected muscles spanning the shoulder and elbow joints during and after transitions.

RESULTS

Hand forces and electromyographic activations were invariantly small at all speeds and all sample times in NI control subjects but varied systematically by transport speed during and shortly after movement in the HS subjects. Velocity-dependent resistance to stretch diminished within 2 s after movement ceased in the hemiparetic arms. Hand forces and EMGs changed very little from 2 s after the movement ended onward, exhibiting dependence on limb posture but no systematic dependence on movement speed or direction. Although each HS subject displayed a unique field of hand forces and EMG responses across the workspace after movement ceased, the magnitude of steady-state hand forces was generally greater near the outer boundaries of the workspace than in the center of the workspace for the HS group but not the NI group.

CONCLUSIONS

In the HS group, electromyographic activations exhibited abnormalities consistent with stroke-related decreases in the stretch reflex thresholds. These observations were consistent across repeated testing days. We expect that the approach described here will enable future studies to elucidate stroke's impact on the interaction between the neural mechanisms mediating control of upper extremity posture and movement during goal-directed actions such as reaching and pointing with the arm and hand.

Proprioceptive short-term memory in passive motor learning

A physical trainer often physically guides a learner's limbs to teach an ideal movement, giving the learner proprioceptive information about the movement to be reproduced later. This instruction requires the learner to perceive kinesthetic information and store the instructed information temporarily. Therefore, (1) proprioceptive acuity to accurately perceive the taught kinesthetics and (2) short-term memory to store the perceived information are two critical functions for reproducing the taught movement. While the importance of proprioceptive acuity and short-term memory has been suggested for active motor learning, little is known about passive motor learning. Twenty-one healthy adults (mean age 25.6 years, range 19-38 years) participated in this study to investigate whether individual learning efficiency in passively guided learning is related to these two functions. Consequently, learning efficiency was significantly associated with short-term memory capacity. In particular, individuals who could recall older sensory stimuli showed better learning efficiency. However, no significant relationship was observed between learning efficiency and proprioceptive acuity. A causal graph model found a direct influence of memory on learning and an indirect effect of proprioceptive acuity on learning via memory. Our findings suggest the importance of a learner's short-term memory for effective passive motor learning.

Control of interjoint coordination in the performance of manual circular movements can explain lateral specialization

Between-arm performance asymmetry can be seen in different arm movements requiring specific interjoint coordination to generate the desired hand trajectory. In the current investigation, we assessed between-arm asymmetry of shoulder-elbow coordination and its stability in the performance of circular movements. Participants were 16 healthy right-handed university students. The task consisted of performing cyclic circular movements with either the dominant right arm or the nondominant left arm at movement frequencies ranging from 40% of maximum to maximum frequency in steps of 15%. Kinematic analysis of shoulder and elbow motions was performed through an optoelectronic system in the three-dimensional space. Results showed that as movement frequency increased circularity of left arm movements diminished, taking an elliptical shape, becoming significantly different from the right arm at higher movement frequencies. Shoulder-elbow coordination was found to be asymmetric between the two arms across movement frequencies, with lower shoulder-elbow angle coefficients and higher relative phase for the left compared to the right arm. Results also revealed greater variability of left arm movements in all variables assessed, an outcome observed from low to high movement frequencies. From these findings, we propose that specialization of the left cerebral hemisphere for motor control resides in its higher capacity to generate appropriate and stable interjoint coordination leading to the planned hand trajectory.

Reaction time asymmetries provide insight into mechanisms underlying dominant and non-dominant hand selection

Handedness is often thought of as a hand "preference" for specific tasks or components of bimanual tasks. Nevertheless, hand selection decisions depend on many factors beyond hand dominance. While these decisions are likely influenced by which hand might show performance advantages for the particular task and conditions, there also appears to be a bias toward the dominant hand, regardless of performance advantage. This study examined the impact of hand selection decisions and workspace location on reaction time and movement quality. Twenty-six neurologically intact participants performed targeted reaching across the horizontal workspace in a 2D virtual reality environment, and we compared reaction time across two groups: those selecting which hand to use on a trial-by-trial basis (termed the choice group) and those performing the task with a preassigned hand (the no-choice group). Along with reaction time, we also compared reach performance for each group across two ipsilateral workspaces: medial and lateral. We observed a significant difference in reaction time between the hands in the choice group, regardless of workspace. In contrast, both hands showed shorter but similar reaction times and differences between the lateral and medial workspaces in the no-choice group. We conclude that the shorter reaction times of the dominant hand under choice conditions may be due to dominant hand bias in the selection process that is not dependent upon interlimb performance differences.

A Lifespan Approach to Balance in Static and Dynamic Conditions: The Effect of Age on Balance Abilities

Postural control is a complex sensorimotor skill that is fundamental to our daily life. The abilities to maintain and recover balance degrade with age. However, the time decay of balance performance with age is not well understood. In this study, we aim at quantifying the age-dependent changes in standing balance under static and dynamic conditions. We tested 272 healthy subjects with ages ranging from 20 to 90. Subjects maintained the upright posture while standing on the robotic platform hunova®. In the evaluation of static balance, subjects stood on the fixed platform both with eyes open (EO) and eyes closed (EC). In the dynamic condition, subjects stood with eyes open on the moving foot platform that provided three different perturbations: (i) an inclination proportional to the center of pressure displacements, (ii) a pre-defined predictable motion, and (iii) an unpredictable and unexpected tilt. During all these tests, hunova® measured the inclination of the platform and the displacement of the center of pressure, while the trunk movements were recorded with an accelerometer placed on the sternum. To quantify balance performance, we computed spatio-temporal parameters typically used in clinical environments from the acceleration measures: mean velocity, variability of trunk motion, and trunk sway area. All subjects successfully completed all the proposed exercises. Their motor performance in the dynamic balance tasks quadratically changed with age. Also, we found that the reliance on visual feedback is not age-dependent in static conditions. All subjects well-tolerated the proposed protocol independently of their age without experiencing fatigue as we chose the timing of the evaluations based on clinical needs and routines. Thus, this study is a starting point for the definition of robot-based assessment protocols aiming at detecting the onset of age-related standing balance deficits and allowing the planning of tailored rehabilitation protocols to prevent falls in older adults.

Deficits in Performance on a Mechanically Coupled Asymmetrical Bilateral Task in Chronic Stroke Survivors with Mild Unilateral Paresis

Typical upper limb-mediated activities of daily living involve coordination of both arms, often requiring distributed contributions to mechanically coupled tasks, such as stabilizing a loaf of bread with one hand while slicing with the other. We sought to examine whether mild paresis in one arm results in deficits in performance on a bilateral mechanically coupled task. We designed a virtual reality-based task requiring one hand to stabilize against a spring load that varies with displacement of the other arm. We recruited 15 chronic stroke survivors with mild hemiparesis and 7 age-matched neurologically intact adults. We found that stroke survivors produced less linear reaching movements and larger initial direction errors compared to controls (p < 0.05), and that contralesional hand performance was less linear than that of ipsilesional hand. We found a hand × group interaction (p < 0.05) for peak acceleration of the stabilizing hand, such that the dominant right hand of controls stabilized less effectively than the nondominant left hand while stroke survivors showed no differences between the hands. Our results indicate that chronic stroke survivors with mild hemiparesis show significant deficits in reaching aspects of bilateral coordination, but no deficits in stabilizing against a movement-dependent spring load in this task.

A rare case of deafferentation reveals an essential role of proprioception in bilateral coordination

Loss of proprioception has been shown to produce deficits in intralimb coordination and in the ability to stabilize limb posture in the absence of visual feedback. However, the role of proprioceptive signals in the feedforward and feedback control of interlimb coordination remains unclear. To address this issue, we examined bimanual coordination in a deafferented participant (DP) with large-fiber sensory neuropathy, which resulted in the loss of proprioception and touch in both arms, and in age-matched control participants. The task required participants to move a single virtual bar with both hands to a rectangular target with horizontal orientation. The participants received visual feedback of the virtual bar, but not of the hand positions along the bar-axis. Although the task required symmetrical movement between the arms, there were significant differences in the trajectories of the dominant and non-dominant hands in the deafferented participant, and thus more final errors and impaired coordination compared to controls. Deafferentation was also associated with an asymmetric deficit in stabilizing the hand at the end of motion, where the dominant arm showed more drift than the non-dominant arm. While the findings with DP may reflect a unique adaptation to deafferentation, they suggest that 1) Bilateral coordination depends on proprioceptive feedback, and 2) Postural stability at the end of motion can be specified through feedforward mechanisms, in the absence of proprioceptive feedback, but this process appears to be asymmetric, with better stability in the non-dominant arm.

Anticipation and compensation for somatosensory deficits in object handling: evidence from a patient with large fiber sensory neuropathy

The purpose of this study was to determine the contributions of feedforward and feedback processes on grip force regulation and object orientation during functional manipulation tasks. One patient with massive somatosensory loss resulting from large fiber sensory neuropathy and 10 control participants were recruited. Three experiments were conducted: 1) perturbation to static holding; 2) discrete vertical movement; and 3) functional grasp and place. The availability of visual feedback was also manipulated to assess the nature of compensatory mechanisms. Results from experiment 1 indicated that both the deafferented patient and controls used anticipatory grip force adjustments before self-induced perturbation to static holding. The patient exhibited increased grip response time, but the magnitude of grip force adjustments remained correlated with perturbation forces in the self-induced and external perturbation conditions. In experiment 2, the patient applied peak grip force substantially in advance of maximum load force. Unlike controls, the patient's ability to regulate object orientation was impaired without visual feedback. In experiment 3, the duration of unloading, transport, and release phases were longer for the patient, with increased deviation of object orientation at phase transitions. These findings show that the deafferented patient uses distinct modes of anticipatory control according to task constraints and that responses to perturbations are mediated by alternative afferent information. The loss of somatosensory feedback thus appears to impair control of object orientation, whereas variation in the temporal organization of functional tasks may reflect strategies to mitigate object instability associated with changes in movement dynamics.NEW & NOTEWORTHY This study evaluates the effects of sensory neuropathy on the scaling and timing of grip force adjustments across different object handling tasks (i.e., holding, vertical movement, grasping, and placement). In particular, these results illustrate how novel anticipatory and online control processes emerge to compensate for the loss of somatosensory feedback. In addition, we provide new evidence on the role of somatosensory feedback for regulating object orientation during functional prehensile movement.

When the non-dominant arm dominates: the effects of visual information and task experience on speed-accuracy advantages

Speed accuracy trade-off, the inverse relationship between movement speed and task accuracy, is a ubiquitous feature of skilled motor performance. Many previous studies have focused on the dominant arm, unimanual performance in both simple tasks, such as target reaching, and complex tasks, such as overarm throwing. However, while handedness is a prominent feature of human motor performance, the effect of limb dominance on speed-accuracy relationships is not well-understood. Based on previous research, we hypothesize that dominant arm skilled performance should depend on visual information and prior task experience, and that the non-dominant arm should show greater skill when no visual information nor prior task information is available. Forty right-handed young adults reached to 32 randomly presented targets across a virtual reality workspace with either the left or the right arm. Half of the participants received no visual feedback about hand position throughout each reach. Sensory information and task experience were lowest during the first cycle of exposure (32 reaches) in the no-vision condition, in which visual information about motion was not available. Under this condition, we found that the left arm group showed greater skill, measured in terms of position error normalized to speed, and by error variability. However, as task experience and sensory information increased, the right arm group showed substantial improvements in speed-accuracy relations, while the left arm group maintained, but did not improve, speed-accuracy relations throughout the task. These differences in performance between dominant and non-dominant arm groups during the separate stages of the task are consistent with complimentary models of lateralization, which propose different proficiencies of each hemisphere for different features of control. Our results are incompatible with global dominance models of handedness that propose dominant arm advantages under all performance conditions.

Somatosensory deafferentation reveals lateralized roles of proprioception in feedback and adaptive feedforward control of movement and posture

Proprioception provides crucial information necessary for determining limb position and movement, and plausibly also for updating internal models that might underlie the control of movement and posture. Seminal studies of upper-limb movements in individuals living with chronic, large fiber deafferentation have provided evidence for the role of proprioceptive information in the hypothetical formation and maintenance of internal models to produce accurate motor commands. Vision also contributes to sensorimotor functions but cannot fully compensate for proprioceptive deficits. More recent work has shown that posture and movement control processes are lateralized in the brain, and that proprioception plays a fundamental role in coordinating the contributions of these processes to the control of goal-directed actions. In fact, the behavior of each limb in a deafferented individual resembles the action of a controller in isolation. Proprioception, thus, provides state estimates necessary for the nervous system to efficiently coordinate multiple motor control processes.