Unimanual Movements Research Articles

Physically coupling two objects in a bimanual task alters kinematics but not end-state comfort

People often grasp objects with an awkward grip to ensure a comfortable hand posture at the end of the movement. This end-state comfort effect is a predominant constraint during unimanual movements. However, during bimanual movements the tendency for both hands to satisfy end-state comfort is affected by factors such as end-orientation congruency and task context. Although bimanual end-state comfort has been examined when the hands manipulate two independent objects, no research has examined end-state comfort when the hands are required to manipulate two physically-coupled objects. In the present experiment, kinematics and grasp behavior during a unimanual and bimanual reaching and placing tasks were examined, when the hands manipulate two physically-connected objects. Forty-five participants were assigned to one of three groups; unimanual, bimanual no-spring (the objects were not physically connected), and bimanual spring (the objects were connected by a spring), and instructed to grasp and place objects in various end-orientations, depending on condition. Physically connecting the objects did not affect end-state comfort prevalence. However, it resulted in decreased interlimb coupling. This finding supports the notion of a flexible constraint hierarchy, in which action goals guide the selection of lower level action features (i.e., hand grip used for grasping), and the particular movements used to accomplish that goal (i.e., interlimb coupling) are controlled throughout the movement.

Physical activity and neural correlates of aging: A combined TMS/fMRI study

Aerobic exercise has been suggested to ameliorate aging-related decline in humans. Recently, evidence has indicated chronological aging is associated with decreases in measures of interhemispheric inhibition during unimanual movements, but that such decreases may be mitigated by long-term physical fitness. The present study investigated measures of ipsilateral (right) primary motor cortex activity during right-hand movements using functional magnetic resonance imaging and transcranial magnetic stimulation (TMS). Healthy, right-handed participant groups were comprised of 12 sedentary older adults, 12 physically active older adults, and 12 young adults. Active older adults and younger adults evidenced longer ipsilateral silent periods (iSP) and less positive BOLD of ipsilateral motor cortex (iM1) as compared to sedentary older adults. Across groups, duration of iSP from TMS was inversely correlated with BOLD activity in iM1 during unimanual movement. These findings suggest that increased physical activity may have a role in decreasing aging-related losses of interhemispheric inhibition.

Motor preparation and the effects of practice: Evidence from startle.

To examine sequential movement preparation, participants practiced unimanual movements that differed in amplitude and number of elements for 4 days in either a simple (Experiment 1) or choice (Experiment 2) reaction time (RT) paradigm. On Day 1 and 4, a startling stimulus was used to probe the preparation process. For simple RT, we found increased premotor RT for the two component movement during control trials on Day 1, which was minimized with practice. During startle trials, all movements were triggered at a short latency with similar consistency to control trials, suggesting full advance preparation of all movements. For choice RT, we also found increased premotor RT for control trials for the two component movement. As advance preparation could not occur, the startling stimulus did not trigger any of the movements. We hypothesized that complexity may relate to the neural commands needed to produce the movement, rather than a sequencing requirement.

Cortical and behavioral adaptations in response to short-term inphase versus antiphase bimanual movement training

Bimanual movement training (BMT) may be an effective rehabilitative protocol for movement-related deficits following a stroke; however, it is unclear how varying types of BMT induce cortical adaptations in the healthy population. Moreover, we lack a methodology to measure cortical adaptations in response to modes of movement training. Therefore, the present study measured the cued movement-related potential (MRP) to investigate cortical adaptations during cued inphase versus antiphase BMT that transferred to a unimanual task and how cortical modulations related to behavior. Three specific hypotheses were investigated: (1) cued inphase BMT would induce cortical adaptations within regions subserving motor preparation and movement execution, (2) repetitive cued unimanual training would induce cortical activity modulations associated with motor execution, and (3) increased cortical activity would be associated with enhanced performance. On three separate days, EEG was recorded from 22 electrodes during three types of cued movement training: inphase BMT, antiphase BMT and repetitive unimanual movement, in addition to pre- and post-training unimanual movement trials involving cued right wrist flexion. The MRP was measured for each repetition during each trial. Results showed a significant training-related increase in preparatory activation correlated with a behavioral enhancement following cued inphase BMT. This effect was not attributable to a change in arousal. No significant training-related modulation occurred in response to cued antiphase BMT or repetitive unimanual movement training. These results suggest that cortical adaptations in relation to the preparation of a cued movement enhance in response to cued inphase BMT, and the MRP is an effective measurement tool to assess training-related adaptations in response to inphase BMT specifically.

Resource-demanding versus cost-effective bimanual interaction in the brain

When two hands require different information in bimanual asymmetric movements, interference can occur via callosal connections and ipsilateral corticospinal pathways. This interference could potentially work as a cost-effective measure in symmetric movements, allowing the same information to be commonly available to both hands at once. Using functional magnetic resonance imaging, we investigated supra-additive and sub-additive neural interactions in bimanual movements during the initiation and continuation phases of movement. We compared activity during bimanual asymmetric and symmetric movements with the sum of activity during unimanual right and left finger-tapping. Supra-additive continuation-related activation was found in the right dorsal premotor cortex and left cerebellum (lobule V) during asymmetric movements. In addition, for unimanual movements, the right dorsal premotor cortex and left cerebellum (lobule V) showed significant activation only for left-hand (non-dominant) movements, but not for right-hand movements. These results suggest that resource-demanding interactions in bimanual asymmetric movements are involved in a non-dominant hand motor network that functions to keep non-dominant hand movements stable. We found sub-additive continuation-related activation in the supplementary motor area (SMA), bilateral cerebellum (lobule VI) in symmetric movements, and the SMA in asymmetric movements. This suggests that no extra demands were placed on these areas in bimanual movements despite the conventional notion that they play crucial roles in bimanual coordination. Sub-additive initiation-related activation in the left anterior putamen suggests that symmetric movements place lower demands on motor programming. These findings indicate that, depending on coordination patterns, the neural substrates of bimanual movements either exhibit greater effort to keep non-dominant hand movements stable, or save neural cost by sharing information commonly to both hands.

Two Agents in the Brain: Motor Control of Unimanual and Bimanual Reaching Movements

Previous studies have suggested that the left and right hands have different specialties for motor control that can be represented as two agents in the brain. This study examined how coordinated movements are performed during bimanual reaching tasks to highlight differences in the characteristics of the hands. We examined motor movement accuracy, reaction time, and movement time in right-handed subjects performing a three-dimensional motor control task (visually guided reaching). In the no-visual-feedback condition, right-hand movement had lower accuracy and a shorter reaction time than did left-hand movement, whereas bimanual movement had the longest reaction time, but the best accuracy. This suggests that the two hands have different internal models and specialties: closed-loop control for the right hand and open-loop control for the left hand. Consequently, during bimanual movements, both models might be used, creating better control and planning (or prediction), but requiring more computation time compared to the use of one hand only.

Response preparation with static and moving hands: Differential effects of unimanual and bimanual movements

This study investigated the effects of uni- and bimanual hand movements on the efficiency of within- and between-hands response preparation in a spatial cuing task. Predictions were derived from the Grouping Model of finger preparation, inspired by insights from neurophysiology (i.e., the concepts of transcollosal facilitation and cognitive overruling of basic neural coordination patterns). Sixteen participants performed the finger cuing task with one, two, or no hand(s) moving. Reaction time results revealed that unimanual and bimanual hand movements had similar effects on within-hand preparation but differential effects on between-hands preparation. This finding demonstrates a strong dissociation between within- and across-hands finger preparation, suggesting distinct underlying mechanisms as hypothesized by the Grouping Model.

Hemispheric Asymmetries of the Premotor Cortex are Task Specific as Revealed by Disruptive TMS During Bimanual Versus Unimanual Movements

The premotor cortex (PMC) is functionally lateralized, such that the left PMC is activated for unimanual movements of either hand, whereas the right PMC is particularly active during complex bimanual movements. Here we ask the question whether the high activation of right PMC in the bimanual context reflects either hemispheric specialization or handedness. Left- and right-handed subjects performed a bimanual antiphase tapping task at different frequencies while transcranial magnetic stimulation (TMS) was used to temporarily disrupt left versus right PMC during complex bimanual movements. For both handedness groups, more disruptions were induced when TMS was applied over the motor nondominant PMC than over the motor dominant PMC or when sham-TMS was used. In a second experiment, right-handers performed complex unimanual tapping with either hand, while TMS was applied to the PMC in both hemispheres. The novel result was that the high susceptibility of the motor nondominant PMC was specific to the bimanual context, indicating that hemispheric asymmetries of the PMC depend on the bimanual versus unimanual nature of the motor task. We hypothesize that asymmetries of PMC involvement in bimanual control reflect interhemispheric interactions, whereby the motor nondominant PMC appears to prevent motor cross talk arising from the dominant hemisphere.

Limitations on Coupling of Bimanual Movements Caused by Arm Dominance: When the Muscle Homology Principle Fails

Studies of bimanual movements typically report interference between motions of the two arms and preference to perform mirror-symmetrical patterns. However, recent studies have demonstrated that the two arms differ in the ability to control interaction torque (INT). This predicts limitations in the capability to perform mirror-symmetrical movements. Here, two experiments were performed to test this prediction. The first experiment included bimanual symmetrical and asymmetrical circle drawing at two frequency levels. Unimanual circle drawing was also recorded. The increases in cycling frequency caused differences between the two arms in movement trajectories in both bimanual modes, although the differences were more pronounced in the asymmetrical compared with the symmetrical mode. Based on torque analysis, the differences were attributed to the nondominant arm's decreased capability to control INT. The intraarm differences during the symmetrical pattern of bimanual movements were similar (although more pronounced) to those during unimanual movements. This finding was verified in the second experiment for symmetrical bimanual oval drawing. Four oval orientations were used to provide variations in INT. Similar to the first experiment, increases in cycling frequency caused spontaneous deviations from perfect bimanual symmetry associated with inefficient INT control in the nondominant arm. This finding supports the limitations in performing mirror-symmetrical bimanual movements due to differences in joint control between the arms. Based on our results and previous research, we argue that bimanual interference occurs during specification of characteristics of required motion, whereas lower-level generation of muscle forces is independent between the arms. A hierarchical model of bimanual control is proposed.

Moving the Arm at Different Rates: Slow Movements are Avoided

ABSTRACT Qualitative and quantitative changes characterize locomotion and rhythmic interlimb coordination at different speeds. Legs and hands do not move more or less quickly; they also adopt different relative coordination patterns. In the present article, the authors asked whether similar transitions occur for unimanual hand movements when speed is slowed below the preferred speed. Participants moved a handheld dowel back and forth between 2 large circular targets in time with a metronome at periods between 370 ms and 1667 ms. The authors analyzed the kinematics of participants’ movements at each period and found that proportional dwell time and number of peaks in the velocity profile increased as driving periods increased. Path lengths and peak velocities remained relatively constant for driving periods exceeding 800 ms. Participants made only gradual changes to their movement parameters, so that they went from a continuous mode to a more discrete mode of behavior for longer driving periods. Thus, unlike for rhythmic bimanual movements or locomotory patterns, there are quantitative but no clear qualitative changes for unimanual movements. The results suggest that participants tried to move close to their preferred tempo at different rates, and that they avoided moving slowly.

Age-Related Changes in Motor Control During Unimanual Movements

Event related fMRI was used to investigate age-related changes in BOLD activity during the execution of right hand finger movements in internally or externally guided tasks. All of the younger adults exhibited typical (positive) BOLD responses in supplementary motor areas (SMA) bilaterally, and in the left sensorimotor cortex. Negative BOLD responses were found, however, in the right sensorimotor cortex of the younger adults. In contrast, all but one of the older adults had positive BOLD responses in SMA and sensorimotor cortex of both hemispheres. Across both tasks, older adults showed increased activity (relative to younger adults) in right ventrolateral premotor and medial premotor areas, but more so during the internally guided task. Overall, these results suggest age-related changes in motor control. The younger adults’ hemispheric asymmetry and the lack thereof in older adults suggest a fundamental change in interhemispheric communication as part of the normal aging process.

An Assessment of Robot-Assisted Bimanual Movements on Upper Limb Motor Coordination Following Stroke

Robot-assisted training is increasingly being investigated in upper limb rehabilitation for individuals with stroke. Many studies have suggested that an appropriate synchronization of voluntary motor commands and limb movement is critical for long-term efficacy. Bimanual training is one method for enhancing this synchronization or motor coordination. The purpose of the study was to evaluate the potential efficacy of bimanual robot-assisted movements by comparing the relative timing of muscle activation and forces to those generated during unimanual robot-assisted movement. A secondary goal was to compare bimanual robot-assisted movement to bimanual voluntary movement, where both limbs moved independently without robotics. Subjects performed reaching tasks while attached to one or two robotic manipulators. A predefined movement trajectory was prescribed during unimanual robot-assisted movement; in bimanual robot-assisted movement the paretic limb trajectory mirrored the nonparetic limb. Relative to unimanual movements, during bimanual movements the timing of muscle activation and initial interface forces was more similar to the nonparetic limb. However, there were limited differences in these measures between bimanual voluntary and bimanual robot-assisted movements. Bimanual robot-assisted movements resulted in superior motor coordination compared to unimanual movements and could be beneficial for individuals with a restricted movement range. Bimanual movements without robotics were just as efficacious and may be preferred for individuals who can generate movement without assistance.

Bi-directional interhemispheric inhibition during unimanual sustained contractions

BackgroundThe interaction between homologous muscle representations in the right and left primary motor cortex was studied using a paired-pulse transcranial magnetic stimulation (TMS) protocol known to evoke interhemispheric inhibition (IHI). The timecourse and magnitude of IHI was studied in fifteen healthy right-handed adults at several interstimulus intervals between the conditioning stimulus and test stimulus (6, 8, 10, 12, 30, 40, 50 ms). IHI was studied in the motor dominant to non-dominant direction and vice versa while the right or left hand was at rest, performing isometric contraction of the first dorsal interosseous (FDI) muscle, and isometric contraction of the FDI muscle in the context of holding a pen.ResultsCompared with rest, IHI was reduced at all ISIs during contraction of either type (with or without the context of pen). IHI was reduced bi-directionally without evidence of hemispheric dominance. Further, contraction of the hand contralateral to the conditioning and test pulse yielded similar reductions in IHI.ConclusionThese data provide evidence for bi-directional reduction of IHI during unimanual contractions. During unimanual, sustained contractions of the hand, the contralateral and ipsilateral motor cortices demonstrate reduced inhibition. The data suggest that unimanual movement decreases inhibition bi-directionally across motor hemispheres and offer one explanation for the observation of ipsilateral M1 activity during hand movements.

Study of asymmetry in motor areas related to handedness using the fMRI BOLD response Gaussian convolution model

Brain asymmetry is a phenomenon well known for handedness, and has been studied in the motor cortex. However, few studies have quantitatively assessed the asymmetrical cortical activities for handedness in motor areas. In the present study, we systematically and quantitatively investigated asymmetry in the left and right primary motor cortices during sequential finger movements using the Gaussian convolution model approach based on the functional magnetic resonance imaging (fMRI) blood oxygenation level dependent (BOLD) response. Six right-handed and six left-handed subjects were recruited to perform three types of hand movement tasks. The results for the expected value of the Gaussian convolution model showed that it took the dominant hand a longer average interval of response delay regardless of the handedness and bi- or uni-manual performance. The results for the standard deviation of the Gaussian model suggested that in the mass neurons, these intervals of the dominant hand were much more variable than those of the non-dominant hand. When comparing bi-manual movement conditions with uni-manual movement conditions in the primary motor cortex (PMC), both the expected value and standard deviation in the Gaussian function were significantly smaller ( p < 0.05) in the bi-manual conditions, showing that the movement of the non-dominant hand influenced that of the dominant hand.

Network activation during bimanual movements in humans

The coordination of movement between the upper limbs is a function highly distributed across the animal kingdom. How the central nervous system generates such bilateral, synchronous movements, and how this differs from the generation of unilateral movements, remain uncertain. Electrophysiologic and functional imaging studies support that the activity of many brain regions during bimanual and unimanual movement is quite similar. Thus, the same brain regions (and indeed the same neurons) respond similarly during unimanual and bimanual movements as measured by electrophysiological responses. How then are different motor behaviors generated?To address this question, we studied unimanual and bimanual movements using fMRI and constructed networks of activation using Structural Equation Modeling (SEM). Our results suggest that (1) the dominant hemisphere appears to initiate activity responsible for bimanual movement; (2) activation during bimanual movement does not reflect the sum of right and left unimanual activation; (3) production of unimanual movement involves a network that is distinct from, and not a mirror of, the network for contralateral unimanual movement; and (4) using SEM, it is possible to obtain robust group networks representative of a population and to identify individual networks which can be used to detect subtle differences both between subjects as well as within a single subject over time. In summary, these results highlight a differential role for the dominant and non-dominant hemispheres during bimanual movements, further elaborating the concept of handedness and dominance. This knowledge increases our understanding of cortical motor physiology in health and after neurological damage.

Dynamic intra- and interhemispheric interactions during unilateral and bilateral hand movements assessed with fMRI and DCM

Any motor action results from a dynamic interplay of various brain regions involved in different aspects of movement preparation and execution. Establishing a reliable model of how these areas interact is crucial for a better understanding of the mechanisms underlying motor function in both healthy subjects and patients. We used fMRI and dynamic causal modeling to reveal the specific excitatory and inhibitory influences within the human motor system for the generation of voluntary hand movements. We found an intrinsic balance of excitatory and inhibitory couplings among core motor regions within and across hemispheres. Neural coupling within this network was specifically modulated upon uni- and bimanual movements. During unimanual movements, connectivity towards the contralateral primary motor cortex was enhanced while neural coupling towards ipsilateral motor areas was reduced by both transcallosal inhibition and top-down modulation. Bimanual hand movements were associated with a symmetric facilitation of neural activity mediated by both increased intrahemispheric connectivity and enhanced transcallosal coupling of SMA and M1. The data suggest that especially the supplementary motor area represents a key structure promoting or suppressing activity in the cortical motor network driving uni- and bilateral hand movements. Our data demonstrate that fMRI in combination with DCM allows insights into intrinsic properties of the human motor system and task-dependent modulations thereof.

Coordination dynamics and attentional costs of continuous and discontinuous bimanual circle drawing movements

It has been suggested that the temporal control of rhythmic unimanual movements is different between tasks requiring continuous (e.g., circle drawing) and discontinuous movements (e.g., finger tapping). Specifically, for continuous movements temporal regularities are an emergent property, whereas for tasks that involve discontinuities timing is an explicit part of the action goal. The present experiment further investigated the control of continuous and discontinuous movements by comparing the coordination dynamics and attentional demands of bimanual continuous circle drawing with bimanual intermittent circle drawing. The intermittent task required participants to insert a 400ms pause between each cycle while circling. Using dual-task methodology, 15 right-handed participants performed the two circle drawing tasks, while vocally responding to randomly presented auditory probes. The circle drawing tasks were performed in symmetrical and asymmetrical coordination modes and at movement frequencies of 1Hz and 1.7Hz. Intermittent circle drawing exhibited superior spatial and temporal accuracy and stability than continuous circle drawing supporting the hypothesis that the two tasks have different underlying control processes. In terms of attentional cost, probe RT was significantly slower during the intermittent circle drawing task than the continuous circle drawing task across both coordination modes and movement frequencies. Of interest was the finding that in the intermittent circling task reaction time (RT) to probes presented during the pause between cycles did not differ from the RT to probes occurring during the circling movement. The differences in attentional demands between the intermittent and continuous circle drawing tasks may reflect the operation of explicit event timing and implicit emergent timing processes, respectively.

Coordination and concurrency in bimanual rotation tasks when moving away from and toward the body

In the present series of experiments we investigated how object transport and rotate movements are performed when they are directed away from (Experiment 1) and toward (Experiment 2) the body under both unimanual and bimanual conditions. Our results indicated that unimanual conditions are faster and more efficiently produced than bimanual movements in far peripersonal space, suggesting that there is a cost to performing bimanual movements. However, in near peripersonal space, bimanual same movements were performed in a manner similar to unimanual movements, indicating that there is no significant cost associated with similar bimanual movements that are performed using the lower visual field and in near peripersonal space. Both experiments also indicate that the two hands are tightly synchronized when the two movements being performed require the same rotation. However, when performing bimanual movements where the rotation being performed by the two hands is different, this synchronization is weaker. Finally, the combined results from the two experiments indicated that movements made toward the body are not performed in a similar manner to movements that are made away from the body. Specifically, it is clear from the current studies that movements toward the body are performed faster and possibly that the hands are less synchronized for bimanual movements requiring different rotations by the two hands.

Time-varying Brain Potentials and Interhemispheric Coherences of Anterior and Posterior Regions during Repetitive Unimanual Finger Movements

Previous brain electrophysiological research has studied the interregionalconnectivity during the tapping task and found that inter-hemispheric alpha coherence wasmore significant under bimanual task conditions than that under unilateral conditions, butthe interregional connectivity situation in the unilateral tapping condition was not exploredclearly. We have designed a unilateral repetitive finger-tapping task to delineate the anteriorand posterior cortex contributions to unilateral finger movement. Sixteen right handedcollege students participated in this study. Event related potentials (ERPs) and the strengthof event related coherence (ERCoh) were analyzed to examine the antero-posterodominance of cortical activity in the phase of early visual process (75-120ms), pre-execution(175-260ms), execution (310-420ms) and post-execution (420-620ms). Results showed thatthe occipital (Oz, O1 and O2), frontal (Fz, F3, and F4), fronto-central (Fz, Cz, F3 and C3),and parietal regions were the most pronounced in the early visual, pre-execution, execution,and post-execution phases, respectively. Moreover, among four inter-hemispheric pairs onlythe Coh (C3 and C4) was significantly correlated to reaction time (RT) of tapping in theexecution phase. In conclusion, the aforementioned variability of electrophysiological data(ERPs and coherence) and the change of antero-postero regional dominance with timereflect the relative importance of different mechanisms in different phases. The mechanismsof visual processing, motor planning, motor execution and feedback reward wereoperational, respectively.

Covert unimanual response preparation triggers attention shifts to effectors rather than goal locations

The premotor theory of attention postulates that during response preparation, attention shifts are elicited towards the goal of a prepared movement. Support for this claim comes from research demonstrating enhanced performance at the location of upcoming saccades. To investigate whether attention shifts occur towards effectors or goal locations during the covert preparation of unimanual movements, we recorded event related brain potentials (ERPs) to task-irrelevant tactile probes that were presented while participants prepared to move one hand towards the index finger of the other hand, as directed by visual response cues presented at the start of each trial. These cues specified either the effector or the goal location of an upcoming movement. The somatosensory N140 component was enhanced when probes were presented to the effector hand relative to the goal hand, regardless of cue instructions. Analogous modulations of the N80 component were only present with effector cues. These results demonstrate a close link between covert response preparation and attention shifts, and strongly suggest that attention shifts are directed to the effector, and not to the goal location of manual movements.