Functional coupling between the limbs during bimanual reach-to-grasp movements

While it is frequently advantageous to be able to use our hands independently, many actions demand that we use our hands co-operatively. In this paper we present two experiments that examine functional binding between the limbs during the execution of bimanual reach-to-grasp movements. The first experiment examines the effect of gaze direction on unimanual and bimanual reaches. Even when subjects' eye movements are restricted during bimanual reaches so that they may only foveate one target object, the limbs remain tightly synchronized to a common movement duration. In contrast, grip aperture is independently scaled to the size of the target for each hand. The second experiment demonstrates however, that the independent scaling of grip aperture is task dependent. If the two target objects are unified so that they appear to be part of a single object, grip apertures become more similar across the hands (i.e., grip aperture to the large target object is reduced in size while peak aperture to the small target item is increased in size). These results suggest that the coupling of the limbs can operate at a functional level.     

Unilateral vs. bilateral coordination of circle-drawing tasks

The number of joint motions available in the upper extremity provides for multiple solutions to the coordination of a motor task. Making use of these abundant joint motions provides for task flexibility. Controlling bimanual movements poses another level of complexity because of possible tradeoffs between coordination within a limb and coordination between the limbs. We examined how flexible patterns of joint coordination were used to stabilize the hand's path when drawing a circle independently compared to a bimanual pattern. Across-trial variance of joint motions was partitioned into two components: goal-equivalent variance (GEV), representing variance of joint motions consistent with a stable hand path and non-goal-equivalent variance (NGEV) representing variance of joint motions that led to deviations of the hand's path. GEV was higher than NGEV in both unimanual and bimanual drawing, with one exception. Both GEV and NGEV, related to control of the individual hands' motion, decreased when engaged in the bimanual compared to unimanual drawing. Moreover, NGEV, leading to variability in the vectorial distance between the hands, was higher when the two hands drew circles in a bimanually asymmetric vs. symmetric pattern, consistent with reported differences in the relative phasing of the two hands. Our results suggest that the nervous system controls the individual hands' motions by separate intra-limb synergies during both unimanual and bimanual drawing, and superimposes an additional synergy to achieve stable relative motion of the two hands during bimanual drawing.     

Motor planning and execution in left- and right-handed individuals during a bimanual grasping and placing task

The issue of handedness has been the topic of great interest for researchers in a number of scientific domains. It is typically observed that the dominant hand yields numerous behavioral advantages over the non-dominant hand during unimanual tasks, which provides evidence of hemispheric specialization. In contrast to advantages for the dominant hand during motor execution, recent research has demonstrated that the right hand has advantages during motor planning (regardless of handedness), indicating that motor planning is a specialized function of the left hemisphere. In the present study we explored hemispheric advantages in motor planning and execution in left- and right-handed individuals during a bimanual grasping and placing task. Replicating previous findings, both motor planning and execution was influenced by object end-orientation congruency. In addition, although motor planning (i.e., end-state comfort) was not influenced by hand or handedness, motor execution differed between left and right hand, with shorter object transport times observed for the left hand, regardless of handedness. These results demonstrate that the hemispheric advantages often observed in unimanual tasks do not extend to discrete bimanual tasks. We propose that the differences in object transport time between the two hands arise from overt shifting visual fixation between the two hands/objects.     

Performance of unimanual and bimanual multiphased prehensile movements

By manipulating task action demands in 2 experiments, the author investigated whether the context-dependent effects seen in unimanual multiphase movements are also present in bimanual movements. Participants (N = 14) in Experiment 1 either placed or tossed objects into targets. The results indicated that the intention to perform a subsequent action with an object could influence the performance of an earlier movement in a sequence in both unimanual and bimanual tasks. Furthermore, assimilation effects were found when the subsequent tasks being performed by the 2 hands were incongruent. In Experiment 2, the author investigated in 12 participants whether planning in a multiphase movement includes some representation of the accuracy demands of the subsequent task. The accuracy demands of a subsequent task did not appear to influence initial movement planning. Instead, the present results support the notion that it is the action requirements of the subsequent movement that lead to context-dependent effects.     

Age Changes in Interlimb Coupling and the Development of Bimanual Coordination

Changes in interlimb coupling, and their role in the development of bimanual coordination, were studied longitudinally in 6- to 12-month-old infants (N = 6). Infants were observed while they were reaching for simple objects of 2 different sizes. Their use of a uni-versus bimanual strategy for reaching as well as the coupling of their bimanual movements were compared; progress in bimanual coordination of complementary movements was evaluated on 8 different bimanual tasks. The bimanual tasks involved an asymmetrical cooperation between the 2 hands. Although spatiotemporal coupling of bimanual reaching movements did not decrease during the age period studied, infants around 7 months of age used their 2 hands infrequently for reaching. Occurrences of bimanual reaching were particularly low at the session preceding the first bimanual success at a bimanual task. This suggests that the temporal coincidence between greater independence of the 2 hands and progress in bimanual coordination of complementary movements acts in 2 directions: Infants may be more at ease when using their 2 hands in differentiated patterns as the hands move less in synchrony, but, in turn, they may be less likely to move their hands in synchrony as the anticipate mirror manipulations of the object less. The frequency of bimanual reaches increased toward the end of the 1st year. This might have been caused by an increase in the repertoire of bimanual asymmetrical object manipulations and by the fact that the development of bimanual coordination allows infants to manipulate objects with complementary movements even after a bimanual approach toward the object.     

Bimanual Lifting: Do Fingertip Forces Work Independently or Interactively?

Bimanual coordination is a commonplace activity, but the consequences of using both hands simultaneously are not well understood. The authors examined fingertip forces across 4 experiments in which participants undertook a range of bimanual tasks. They first measured fingertip forces during simultaneous lifts of 2 identical objects, noting that individuals held the objects with more force bimanually than unimanually. They then varied the mass of the objects held by each hand, noting that when both hands lifted together performance was equivalent to unimanual lifts. The authors next measured one hand's static grip force while the other hand lifted an object. They found a gradual reduction of grip force throughout the trial, but once again no evidence of one hand influencing the other. In the final experiment the authors tested whether tapping with one hand could influence the static grip force of its counterpart. Although the authors found no changes in static grip force as a direct consequence of the other hand's actions, they found clear differences from one task to the other, suggesting an effect of task instruction. Overall, these results suggest that fingertip forces are largely independent between hands in a bimanual lifting context, but are sensitive to different task requirements.     

Hand mental rotation is not systematically altered by actual body position: Laterality judgment versus same–different comparison tasks

It is commonly believed that during mental rotation of body parts, participants tend to imagine their own body part moving toward the stimulus, thus using an egocentric strategy. Several studies have also shown that the mental rotation of hands is affected by the actual hand position, especially if the hand is kept in an awkward position. However, this hand posture effect, as well as the use of an egocentric strategy during mental rotation of body parts, is not systematic. Several experiments have demonstrated that manipulating the stimulus features or the paradigm could induce a shift to visual and allocentric strategies. Here, we studied the effects of hand posture and biomechanical constraints on one-hand mental rotation (laterality judgment task), two-hand mental rotation (same–different judgment task), and mental rotation of one or two alphanumeric symbols (control tasks). Effects of posture and biomechanical constraints were observed solely for the laterality judgment task. Response times in the same–different hand mental rotation items were influenced by the angular disparity between the stimuli. We interpreted our result as evidence of the use of different strategies for each task. Future research should focus on disentangling the exact subprocesses in which an egocentric strategy is used, in order to propose better tests for participants with motor impairments.     

Unimanual and bimanual tasks and the assessment of handedness in toddlers

The goal of this study was to examine the relations between three different measures of handedness: unimanual reaching, bimanual manipulation and unimanual manipulation. The appropriateness of the task chosen to evaluate handedness was also explored by contrasting different bimanual manipulation tasks for the more or less differentiated (passive\active) roles assigned to each hand. Forty children, between 18 and 36 months of age, were tested in the three conditions. The results show that the degree of bimanual handedness is greater on the bimanual tasks with a strong role differentiation than on the tasks with less differentiation. Bimanual tasks with a strong role differentiation elicited more right‐handedness than unimanual reaching. Among the children who showed handedness in reaching, the correlation between unimanual and bimanual handedness was high, especially for right‐handers. For some tasks, bimanual handedness appeared at the earliest age studied here (18 months), and there was little relationship between bimanual handedness and bimanual skill. In contrast with unimanual reaching, there was no age‐related change in the degree of handedness for either bimanual or unimanual manipulation. There was a bias toward the use of the right hand for unimanual manipulation. It was concluded that grasping is not the best task to employ to look for robust evidence of handedness, and that bimanual tasks offer a better way to estimate handedness in children, as long as the tasks are carefully chosen.     

Bimanual coordination dynamics in poststroke hemiparetics

Poststroke hemiparetic individuals (n = 9) and a control group (n = 9) completed a frequency-scaled circle-drawing task in unimanual and bimanual conditions. Measures of intralimb spatial and temporal task accuracy and interlimb coordination parameters were analyzed. Significant reductions in task performance were seen in both limbs of the patients and controls with the introduction of bimanual movement. Spatial performance parameters suggested that the 2 groups focused on different hands during bimanual conditions. In the controls, interlimb coordination variables indicated predictable hand dominance effects, whereas in the patient group, dominance was influenced by the side of impairment and prior handedness of the individual. Therefore, in this particular bimanual task, performance improvements in the hemiplegic side could not be elicited. Intrinsic coupling asymmetries between the hands can be altered by unilateral motor deficits.     

When two limbs are weaker than one: Sensorimotor syncopation with alternating hands

This study addresses the demands of alternating bimanual syncopation, a coordination mode in which the two hands move in alternation while tapping in antiphase with a metronomic tone sequence. Musically trained participants were required to engage in alternating bimanual syncopation and five other coordination modes: unimanual syncopation where taps are made (with the left or right hand) after every tone; unimanual syncopation where taps are made after every other tone; bimanual synchronization with alternating hands; unimanual synchronized tapping with every tone; and unimanual tapping with every other tone. Variability in tap timing was greatest overall for alternating bimanual syncopation, indicating that it is the most difficult. This appears to be due to instability arising from the simultaneous presence of two levels of antiphase coordination (one between the pacing sequence and the hands, the other between the two hands) rather than factors relating to movement frequency or dexterity limits of the nonpreferred hand.     

Development of unimanual versus bimanual task performance in an isometric task

Sixty-three children between 5 and 12 years of age and 15 adults performed a unimanual and a bimanual isometric force task. The performance of the preferred hand in the unimanual task was compared to the performance of the preferred hand in the bimanual task. It was hypothesized that in the bimanual task the absolute error will be higher, there will be more irregularity and the participants will need more time due to the additional effort from the central nervous system, especially with respect to the communication between the hemispheres. Furthermore, in younger children bimanual force variability was expected to be higher due to developmental aspects concerning callosal maturation and attention. It was found that with respect to force generation the preferred hand was not affected by bilateral isometric force generation, but with respect to force regulation it was. The coefficient of variation (CV) of the force was 34% larger in the bimanual task as compared to the unimanual task. For the time to target force, the increase was 28%. With repetition of the trials the CV decreased in the bimanual task, but only in the youngest age group. During development there was no change in absolute error, yet there was a major reduction in force variability in the bimanual task. It is suggested that improvement in interhemispheric communication and in the ability to focus attention plays a role in the decrease in variability with age.     

Roles of Visual Information for Control of Reaching Movements in Children

Poststroke hemiparetic individuals (n = 9) and a control group (n = 9) completed a frequency-scaled circle-drawing task in unimanual and bimanual conditions. Measures of intralimb spatial and temporal task accuracy and interlimb coordination parameters were analyzed. Significant reductions in task performance were seen in both limbs of the patients and controls with the introduction of bimanual movement. Spatial performance parameters suggested that the 2 groups focused on different hands during bimanual conditions. In the controls, interlimb coordination variables indicated predictable hand dominance effects, whereas in the patient group, dominance was influenced by the side of impairment and prior handedness of the individual. Therefore, in this particular bimanual task, performance improvements in the hemiplegic side could not be elicited. Intrinsic coupling asymmetries between the hands can be altered by unilateral motor deficits.     

Hemispheric competition in left-handers on bimanual reaction time tasks

The authors examined possible differences in left- and right-handers on bimanual reaction times to centralized visual stimuli. Eighty participants (n = 40 in each group of left- and right-handers) were tested on unimanual and bimanual reaction time (RT) tasks. Consistently across the 2 groups, the dominant-hand RT was faster, on average, than the nondominant-hand RT, and unimanual RTs were faster than bimanual RTs. However, RT differences between hands revealed a higher percentage of dominant-hand-led trials in right-handers than in left-handers, despite similar absolute RT differences in the 2 groups. On the basis of those findings, the authors conclude that hand dominance does not generally determine which hand leads in a bimanual task and that left-handers have stronger between-hemisphere competition than right-handers do.     

Bimanual interference in children performing a dual motor task

The present study addressed the development of bimanual interference in children performing a dual motor task, in which each hand executes a different task simultaneously. Forty right-handed children (aged 4, 5-6, 7-8 and 9-11years, ten in each age group) were asked to perform a bimanual task in which they had to tap with a pen using the non-preferred hand and simultaneously trace a circle or a square with a pen using the preferred hand as quickly as possible. Tapping and tracing were also performed unimanually. Differences between unimanual and bimanual performance were assessed for number of taps, length of tap trace and mean tracing velocity. It was assumed that with increasing age, better bimanual coordination would result in better performance on the dual task showing less intermanual interference. The results showed that tapping and tracing performance increased with age, unimanually as well as bimanually, consistent with developmental advancement. However, the percentage of intermanual interference due to bimanual performance was not significantly different in the four age groups. Although performing the dual task resulted in mutual intermanual interference, all groups showed a significant effect of tracing shape. More specifically, all age groups showed a larger percentage decrease in tracing velocity when performing the circle compared to the square in the dual task. The present study reveals that children as young as four years are able to coordinate both hands when tapping and tracing bimanually.     

Kinematic parameters of hand movement during a disparate bimanual movement task in children with unilateral Cerebral Palsy

Children with unilateral Cerebral Palsy (uCP) experience problems performing tasks requiring the coordinated use of both hands (bimanual coordination; BC). Additionally, some children with uCP display involuntary symmetrical activation of the opposing hand (mirrored movements). Measures, used to investigate therapy-related improvements focus on the functionality of the affected hand during unimanual or bimanual tasks. None however specifically address spatiotemporal integration of both hands. We explored the kinematics of hand movements during a bimanual task to identify parameters of BC. Thirty-seven children (aged 10.9 ± 2.6 years, 20 male) diagnosed with uCP participated. 3D kinematic motion analysis was performed during the task requiring opening of a box with their affected- (AH) or less-affected hand (LAH), and pressing a button inside with the opposite hand. Temporal and spatial components of data were extracted and related to measures of hand function and level of impairment. Total task duration was correlated with the Jebsen–Taylor Test of Hand Function in both conditions (either hand leading with the lid-opening). Spatial accuracy of the LAH when the box was opened with their AH was correlated with outcomes on the Children’s Hand Use Experience Questionnaire. Additionally, we found a subgroup of children displaying non-symmetrical movement interference associated with greater movement overlap when their affected hand opened the box. This subgroup also demonstrated decreased use of the affected hand during bimanual tasks. Further investigation of bimanual interference, which goes beyond small scaled symmetrical mirrored movements, is needed to consider its impact on bimanual task performance following early unilateral brain injury.     

Moving hands, moving entities

In this study we investigated with a priming paradigm whether uni and bimanual actions presented as primes differently affected language processing. Animals’ (self-moving entities) and plants’ (not self-moving entities) names were used as targets. As prime we used grasping hands, presented both as static images and videos. The results showed an interference effect with unimanual action primes (both static and moving) with plants’ names. No modulation of responses for animals’ names was found. We argue that in the present task plants elicit information on unimanual grasping actions they support, while the lack of effect for animals could be due to them being better characterized as active agents.     

Bilateral associations of low-level unilateral performance: an unremarked aspect of limb control

The experimenters examine upper limb movement discrimination performance in an arm-raising task for bilateral associations of low-level unilateral performance. On a cue from the experimenter, young adults (n = 23) with no history of shoulder injury raised either their left arm, right arm, or both together in a forward flexion movement until their hand or hands contacted an unseen, adjustable, overhead stop. The participant then judged which of the 5 possible stop positions, in the 12-20 degrees range forward of true vertical, the participant had contacted on the particular trial. Results showed that for the 16 participants whose best performance was in 1 of the unimanual conditions discrimination scores in the bilateral condition were equivalent to those of their worse-performing limb. For the 7 participants whose best discrimination performance was obtained on bimanual arm-raising, scores for the 2 unimanual conditions were equivalently low. Therefore, when a single limb that can perform well operates in conjunction with a limb performing at a lower level, the consequence is lowering the bimanual movement discrimination performance.     

Analytical Inverse Optimization in Two-Hand Prehensile Tasks

The authors explored application of analytical inverse optimization (ANIO) method to the normal finger forces in unimanual and bimanual prehensile tasks with discrete and continuously changing constraints. The subjects held an instrumented handle vertically with one or two hands. The external torque and grip force changed across trials or within a trial continuously. Principal component analysis showed similar percentages of variance accounted for by the first two principal components across tasks and conditions. Compared to unimanual tasks, bimanual tasks showed significantly more frequent inability to find a cost function leading to a stable solution. In cases of stable solutions, similar second-order polynomials were computed as cost functions across tasks and condition. The bimanual tasks, however, showed significantly worse goodness-of-fit index values. The authors show that ANIO can be used in tasks with slowly changing constraints making it an attractive tool to study optimality of performance in special populations. They also show that ANIO can fail in multifinger tasks, likely due to irreproducible behavior across trials, more likely to happen in bimanual tasks compared to unimanual tasks.     

Subcortical locus of temporal coupling in the bimanual movements of a callosotomy patient

The timing of repetitive movements was assessed in a callosotomy patient under unimanual and bimanual conditions. Similar to neurologically healthy individuals, the patient exhibited strong temporal coupling in the bimanual condition. Moreover, for both the left and right hands, within-hand temporal variability was reduced in the bimanual condition compared to the unimanual conditions. This bimanual advantage is hypothesized to reflect the temporal integration of separable timing signals, one associated with the left hand and one associated with the right hand (Helmuth, L. L., & Ivry, R. B. (1996). When two hands are better than one: Reduced timing variability during bimanual movements. Journal of Experimental Psychology: Human Perception and Performance, 2, 278–293). The fact that it persists following callosotomy is inconsistent with models that attribute bimanual coordination in these patients to the control of a single hemisphere. Rather, the results suggest that motor commands from the two hemispheres are integrated subcortically.PsychINFO Classification: 2330; 2340; 2520