The preparation of aiming movements

Hand differences in aiming task: A complementary spatial approach and analysis of dynamic brain networks with EEG

Left and right-hand exhibit differences in the execution of movements. Particularly, it has been shown that manual goal-directed aiming is more accurate with the right hand than with the left, which has been explained through the shorter time spent by the right hand in the feedback phase (FB). This explanation makes sense for the temporal aspects of the task; however, there is a lack of explanations for the spatial aspects. The present study hypothesizes that the right hand is more associated with the FB, while the left hand is more strongly associated with the pre-programming phase (PP). In addition, the present study aims to investigate differences between hands in functional brain connectivity (FBC). We hypothesize an increase in FBC of the right hand compared to the left hand. Twenty-two participants performed 20 trials of the goal-directed aiming task with both hands. Overall, the results confirm the study's hypotheses. Although the right hand stopped far from the target at the PP, it exhibited a lower final position error than the left hand. These findings imply that during the FB, the right hand compensates for the higher error observed in the PP, using the visual feedback to approach the target more closely than the left hand. Conversely, the left hand displayed a lower error at the PP than the right. Also, the right hand displayed greater FBC within and between brain hemispheres. This heightened connectivity in the right hand might be associated with inhibitory mechanisms between hemispheres.

Handedness Does Not Impact Inhibitory Control, but Movement Execution and Reactive Inhibition Are More under a Left-Hemisphere Control

The relationship between handedness, laterality, and inhibitory control is a valuable benchmark for testing the hypothesis of the right-hemispheric specialization of inhibition. According to this theory, and given that to stop a limb movement, it is sufficient to alter the activity of the contralateral hemisphere, then suppressing a left arm movement should be faster than suppressing a right-arm movement. This is because, in the latter case, inhibitory commands produced in the right hemisphere should be sent to the other hemisphere. Further, as lateralization of cognitive functions in left-handers is less pronounced than in right-handers, in the former, the inhibitory control should rely on both hemispheres. We tested these predictions on a medium-large sample of left- and right-handers (n = 52). Each participant completed two sessions of the reaching versions of the stop-signal task, one using the right arm and one using the left arm. We found that reactive and proactive inhibition do not differ according to handedness. However, we found a significant advantage of the right versus the left arm in canceling movements outright. By contrast, there were no differences in proactive inhibition. As we also found that participants performed movements faster with the right than with the left arm, we interpret our results in light of the dominant role of the left hemisphere in some aspects of motor control.

Hand preference for the visual and auditory modalities in humans

The sensory dominance effect refers to the phenomenon that one sensory modality more frequently receives preferential processing (and eventually dominates consciousness and behavior) over and above other modalities. On the other hand, hand dominance is an innate aspect of the human motor system. To investigate how the sensory dominance effect interacts with hand dominance, we applied the adapted Colavita paradigm and recruited a large cohort of healthy right-handed participants (n = 119). While the visual dominance effect in bimodal trials was observed for the whole group (n = 119), about half of the right-handers (48%) showed a visual preference, i.e., their dominant hand effect manifested in responding to the visual stimuli. By contrast, 39% of the right-handers exhibited an auditory preference, i.e., the dominant hand effect occurred for the auditory responses. The remaining participants (13%) did not show any dominant hand preference for either visual or auditory responses. For the first time, the current behavioral data revealed that human beings possess a characteristic and persistent preferential link between different sensory modalities and the dominant vs. non-dominant hand. Whenever this preferential link between the sensory and the motor system was adopted, one dominance effect peaks upon the other dominance effect's best performance.

Performance of discrete, reciprocal, and cyclic movements of the ipsilesional upper limb in individuals after stroke

Aiming movements of the upper limbs can be classified either as discrete, or reciprocal, or cyclic. The control of these movements after a stroke can be affected. The aim of this experimental, cross-sectional study was to characterize the performance of these movements after the right and left hemisphere chronic stroke. Thirty-six individuals aged between 40 and 70 years, right-handed, were allocated into three groups (control, right stroke, and left stroke). Participants were asked to perform aiming movements on a tablet. Individuals after stroke performed the tasks only with their ipsilesional limb, while the control group performed movements with both limbs. The reaction and movement times, peak velocity, and the variability and error of the endpoint were analyzed. Individuals after stroke presented a worse performance in all movement classes as expected, but differently depending on the damaged hemisphere. Participants with right hemisphere damage showed larger endpoint errors, while those with left hemisphere damage had longer reaction and movement times. Both differences were seen consistently in discrete and reciprocal, but not in cyclic movements. Cyclic movements presented shorter latencies, were faster, and showed greater endpoint errors when compared to discrete and reciprocal movements. These results suggest that stroke affects differently the performance of discrete and reciprocal movements according to the hemisphere lesion side, but not in cyclic movements. Different levels of motor control among the three classes of movements by the nervous system may justify these results.

Shifts of the point-of-change can be attributed to a lower mechanical cost of motor execution

In a previous study on hand selection in a sequential reaching task, the authors showed a shift of the point-of-change (POC) to the left of the midline. This implies that participants conducted a number of contralateral reaches with their dominant, right hand. Contralateral movements have longer planning and execution times and a lower precision. In the current study, we asked whether lower mechanical costs of motor execution or lower cognitive costs of motor planning compensated for these disadvantages. Theories on hemispheric differences postulate lower mechanical costs in the dominant hemisphere and lower cognitive costs in the left hemisphere (independent of handedness). In right-handed participants, both factors act agonistically to reduce the total cost of right-handed reaches. To distinguish between the cost factors, we had left- and right-hand-dominant participants execute a sequential, unimanual reaching task. Results showed a left-shift of the POC in the right-handed and a right-shift in the left-handed group. Both shifts were similar in magnitude. These findings indicate that only the mechanical cost of motor execution compensates for the disadvantages of the contralateral reaches, while the cognitive cost of motor planning is irrelevant for the POC shift.

Hemispheric lateralization does not affect the cognitive and mechanical cost of a sequential motor task

In sequential, repetitive tasks, we often partially reuse former motor plans. This causes a persistence of an earlier adopted posture (termed motor hysteresis). The cost-optimization hypothesis states that a partial reuse reduces the cognitive cost of a movement, while the persistence in a former posture increases its mechanical cost. An optimal fraction of reuse, which depends on the relative cognitive and mechanical cost, minimizes the total movement cost. Several studies postulate differences in mechanical or cognitive cost as a result of hemispheric lateralization. In the current study, we asked whether these differences would result in different fractions of motor plan reuse. To this end, left- and right-handed dominant participants executed a sequential motor task (opening a column of drawers) with their dominant and non-dominant hand. The size of the motor hysteresis effect was measured as a proxy for the fraction of plan reuse. Participants used similar postures and exhibited a similar hysteresis effect, irrespective of hand and handedness. This finding indicates that either the cognitive and mechanical costs of a motor task are unaffected by hemispheric differences or that their effect on motor planning is negligible.

Association Between the Catechol-O-Methyltransferase (COMT) Val158Met Polymorphism and Manual Performance Asymmetries

Within the cognitive domain, neuroscience and cognitive psychology researchers have investigated the relationship between handedness and cognitive skills. However, there have been few studies of the three-way association between manual asymmetry, its genetic components, and cognition even though this line of research could further an understanding of asymmetry. One enzyme involved in cognitive functions related to the dopaminergic system and to the prefrontal cortex is the catechol-O-methyltransferase (COMT), and it has a trimodal activity distribution in the human population due to its functional polymorphism known as Val158Met. This study investigated whether this COMT polymorphism is associated with asymmetries in the performance of a manual dexterity task. Forty-two right-handed undergraduate students ( M_age = 25.12, SD = 5.84; 15 women, 27 men) performed two trials each of place and remove conditions of the Grooved Pegboard Test with each hand (right and left), counterbalancing the order of the initial or starting hand. We calculated the mean time to perform the task for both hands on both trials and found, as hypothesized, that the Met/Met group gave a more asymmetrical performance than the Val/Met group under the place condition because dopamine levels reduced flexible behavior for the Val/Met group. We suspect that the place condition requires greater interhemispheric connectivity, as it requires a greater cognitive flexibility, and highly asymmetrical individuals are said to be less flexible. The findings of this study suggest a significant association between the COMT polymorphism and manual asymmetry in healthy populations.

Cognitive-perceptual load modulates hand selection in left-handers to a greater extent than in right-handers

Previous studies have proposed that selecting which hand to use for a reaching task appears to be modulated by a factor described as "task difficulty," defined by either the requirement for spatial precision or movement sequences. However, we previously reported that analysis of the movement costs associated with even simple movements plays a major role in hand selection. We further demonstrated, in right-handers, that cognitive-perceptual loading modulates hand selection by interfering with the analysis of such costs. It has been reported that left-handers tend to show less dominant hand bias in selecting which hand to use during reaching. We, therefore, hypothesized that hand selection would be less affected by cognitive-perceptual loading in left-handers than in right-handers. We employed a visual search task that presented different levels of difficulty (cognitive-perceptual load), as established in previous studies. Our findings indicate that left-handed participants tend to show greater modulation of hand selection by cognitive-perceptual loading than right-handers. Left-handers showed lower dominant hand reaction times than right-handers, and greater high-cost movements that reached to extremes of the contralateral workspace under the most difficult task conditions. We previously showed in this task that midline crossing has high-energy and time costs and that they occur more frequently under cognitively demanding conditions. The current study revealed that midline crossing was associated with the lowest reaction times, in both handedness groups. The fact that left-handers showed lower dominant hand reaction times, and a greater number of high-cost cross-midline reaches under the most cognitively demanding conditions suggests that these actions were erroneous.

Asymmetries in initiation of aiming movements in schizophrenia

Several studies have reported motor symptoms in schizophrenia (SCZ), in some cases describing asymmetries in their manifestation. To date, biases were mainly reported for sequential movements, and the hypothesis was raised of a dopamine-related hemispheric imbalance. Aim of this research is to better characterize asymmetries in movement initiation in SCZ by exploring single actions. Fourteen SCZ patients and fourteen healthy subjects were recruited. On a trial-by-trial basis, participants were instructed to reach for one of eight possible targets. Measures of movement initiation and execution were collected. Starting point, target and moving limb were systematically varied to check for asymmetric responses. Results showed that SCZ patients, besides being overall slower than controls, additionally presented with a bias affecting both the moving hand and the side from which movements were initiated. This finding is discussed in relation to hemispheric lateralization in motor control.

Do left hand reaction time advantages depend on localising unpredictable targets?

Asymmetries in hand movements have routinely been attributed to properties of the two cerebral hemispheres. In right-handed participants, the non-dominant left hand tends to have shorter reaction times, with the dominant right hand achieving shorter movement durations as well as higher peak velocities. The root cause of the surprising left hand RT effect has been debated, largely in the context of right hemisphere specialisation in attention, visuospatial abilities, or "premotor" processes. Mieschke et al. (Brain Cognit 45:1, 2001) and Barthélémy and Boulinguez ( Behav Brain Res 133:1, 2002) both tried to dissociate "premotor" processes explaining the left hand RT advantage, using reaching paradigms where at least one condition required target detection, but no visually guided aiming movement. Unfortunately, the studies obtained conflicting results and conclusions. In the present study, we attempted to re-examine this kind of paradigm with methodological improvements, such as using a task with higher visuospatial demands. Our results demonstrate that whilst RTs are longer as movement complexity increases across three conditions, the left hand RT advantage is present across all conditions-and no significant interaction between hand and condition was found. No significant hand differences were found in peak velocity or duration. These results suggest that the left hand RT advantage cannot be due to movement planning advantages of the right hemisphere, and instead should be attributed to sustained attention/vigilance lateralisation to the right cerebral hemisphere.

Motor Preparation of Manual Aiming at a Visual Target Manipulated in Size, Luminance Contrast, and Location

We conducted two experiments to investigate whether the motor preparation of manual aiming to a visual target is affected by either the physical characteristics (size or luminance contrast) or spatial characteristics (location) of the target. Reaction time (RT) of both finger lifting (ie stimulus-detection time) and manual aiming (ie movement-triggering time) to the onset of the target was measured. The difference of RT (DRT) between two tasks (ie the difference of task complexity) was examined to clarify the temporal characteristics of manual aiming per se during visuomotor integration. Results show classical characteristics: RT decreased as either the target size or luminance contrast increased. Furthermore, the task-complexity and target-location factors significantly interacted with each other, where the aiming RT was longer than the finger-lifting RT and the effects of target location on RT differed for each task. However, the task factor did not interact with either the size or luminance-contrast factor, implying that the motor preparation of manual aiming is associated with the spatial characteristics rather than the physical characteristics of the target. Inspection of DRT revealed that the time needed for motor preparation for an ipsilateral target was significantly shorter than that for a contralateral target. This was the case both for the left and for the right hand. Foveal targets required longer processing time, implying a disadvantageous function of motor preparation for the gazed target. The left-hand superiority for the target appearing in the left visual field was also observed. Such lateralised effect and left-hand advantage to the left visual field in manual aiming suggest that visuospatial information processing is activated during the preparation of aiming action, with faster processing in the right hemisphere.

The influence of M. P. Bryden's work on lateralization of motor skill: Is the preferred hand selected for and better at tasks requiring a high degree of skill?

One question of interest to Dr M. P. Bryden was how skilled motor behaviour, task complexity, and the degree of handedness are linked. His research suggested that there would be dissociation between hand preference and performance for skilled versus unskilled actions. The thought was that "simple" or unskilled tasks such reaching or grasping could be performed equally well by either hand, and thus either could be selected. Skilled tasks such as fine manipulation would be performed best by the preferred hand and thus it would be selected more frequently. Over the past two decades, researchers have attempted to determine whether such a definition of skilled action best encapsulates the specialization of the preferred hand. The current paper will review M. P. Bryden's research on skilled motor behaviour, as well as more recent work, and explore the question of whether the preferred hand is actually superior and preferred for actions requiring manipulation.

Are there right hemisphere contributions to visually-guided movement? Manipulating left hand reaction time advantages in dextrals

Many studies have argued for distinct but complementary contributions from each hemisphere in the control of movements to visual targets. Investigators have attempted to extend observations from patients with unilateral left- and right-hemisphere damage, to those using neurologically-intact participants, by assuming that each hand has privileged access to the contralateral hemisphere. Previous attempts to illustrate right hemispheric contributions to the control of aiming have focussed on increasing the spatial demands of an aiming task, to attenuate the typical right hand advantages, to try to enhance a left hand reaction time advantage in right-handed participants. These early attempts have not been successful. The present study circumnavigates some of the theoretical and methodological difficulties of some of the earlier experiments, by using three different tasks linked directly to specialized functions of the right hemisphere: bisecting, the gap effect, and visuospatial localization. None of these tasks were effective in reducing the magnitude of left hand reaction time advantages in right handers. Results are discussed in terms of alternatives to right hemispheric functional explanations of the effect, the one-dimensional nature of our target arrays, power and precision given the size of the left hand RT effect, and the utility of examining the proportions of participants who show these effects, rather than exclusive reliance on measures of central tendency and their associated null hypothesis significance tests.

Motor asymmetry in elite fencers

The authors previously reported that asymmetrical patterns of hand preference are updated and modified by present sensorimotor conditions. They examined whether participation in long-term training in the upper extremity sport fencing might modify arm selection and performance asymmetries. Eight fencers and eight nonfencers performed reaching movements under 3 experimental conditions: (a) nonchoice right, (b) nonchoice left, and (c) choice, either right or left arm as selected by subject. The nonchoice conditions allowed assessment of potential interlimb differences in movement performance, while the choice condition allowed assessment of the frequency and pattern of arm selection across subject groups. Our findings showed that the athlete group showed substantially greater symmetry in the performance and selection measures. These findings suggest that arm selection and performance asymmetries can be altered by intense long-term practice.

One hand or the other? Effector selection biases in right and left handers

Much debate in the handedness literature has centred on the relative merits of questionnaire-based measures assessing hand preference versus simple movement tasks such as peg moving or finger tapping, assessing hand performance. A third paradigm has grown in popularity, which assesses choices by participants when either hand could be used to execute movements. These newer measures may be useful in predicting possible "reversed" asymmetries in proportions of non-right handed ("adextral") people. In the current studies we examine hand choice in large samples of dextral (right handed) and adextral participants. Unlike in some previous experiments on choice, we found that left handers were as biased towards their dominant hand as were right handers, for grasping during a puzzle-making task (study 1). In a second study, participants had to point to either of two suddenly appearing targets with one hand or the other. In study 2, left handers were not significantly less one handed than their right-handed counterparts as in study 1. In a final study, we used random effects meta analysis to summarise the possible differences in hand choice between left handers and right handers across all hand choice studies published to date. The meta analysis suggests that right handers use their dominant hand 12.5% more than left handers favour their dominant hand (with 95% confidence that the real difference lies between 7% and 18%). These last results suggest that our two experiments reported here may represent statistical Type 2 errors. This mean difference may be related to greater left hemispheric language and praxic laterality in right handers. Nevertheless, more data are needed regarding the precise proportions of left and right handers who favour their preferred hands for different tasks.

Effects of direction and index of difficulty on aiming movements after stroke

BACKGROUND

Brain hemispheres play different roles in the control of aiming movements that are impaired after unilateral stroke. It is not clear whether those roles are influenced by the direction and the difficulty of the task.

OBJECTIVE

To evaluate the influence of direction and index of difficulty (ID) of the task on performance of ipsilesional aiming movements after unilateral stroke.

METHODS

Ten individuals with right hemisphere stroke, ten with left hemisphere stroke, and ten age- and gender-matched controls performed the aiming movements on a digitizing tablet as fast as possible. Stroke individuals used their ipsilesional arm. The direction (ipsilateral or contralateral), size (0.8 or 1.6 cm), and distance (9 or 18 cm) of the targets, presented on a monitor, were manipulated and determined to be of different ID (3.5, 4.5, and 5.5). Results. Individuals with right hemisphere lesion were more sensitive to ID of the task, affecting planning and final position accuracy. Left hemisphere lesion generated slower and less smooth movements and was more influenced by target distance. Contralateral movements and higher ID increased planning demands and hindered movement execution.

CONCLUSION

Right and left hemisphere damages are differentially influenced by task constraints which suggest their complementary roles in the control of aiming movements.

Hemifield or hemispace: what accounts for the ipsilateral advantages in visually guided aiming?

Aiming movements to targets presented on the same side as the reaching limb are faster and more accurate than movements made across the body. These advantages are typically attributed to within-hemisphere sensorimotor control. However, contrary to the within- versus between-hemisphere model, we have shown that some of these advantages tend to go with the side of the movement, rather than the side of the target (Carey et al. Exp Brain Res 112:496-504, 1996; Carey and Otto-de Haart Neuropsychologia 39:894, 2001). Barthélémy and Boulinghez (Exp Brain Res 147:305-312, 2002) acknowledge that our biomechanical account fits data for post-onset movement parameters such as peak velocity and duration, yet they report evidence for some within- versus between-hemisphere contributions to reaction time (RT) advantages. To examine a possible difference between early and late movement kinematics fitting these alternative models, we have dissociated field and space in a different way, which required arm movements with differential inertial consequences, as well as unpredictability of target location in terms of visual field. The data suggest that visual field may contribute some of the variance to hemispatial effects, but only for the right hand. In a second experiment, we used an antipointing task to examine hemispatial versus visual field effects on RTs and to revisit the possible hand difference identified in experiment 1. We found that hemispace accounted for all of the ipsilateral advantages, including RT, for both right and left hands. Results are discussed in terms of the computational requirements of eye-hand coordination in relative unconstrained conditions.

Rethinking motor lateralization: specialized but complementary mechanisms for motor control of each arm

Motor lateralization in humans has primarily been characterized as “handedness”, resulting in the view that one arm-hemisphere system is specialized for all aspects of movement while the other is simply a weaker analogue. We have proposed an alternative view that motor lateralization reflects proficiency of each arm for complementary functions that arises from a specialization of each hemisphere for distinct movement control mechanisms. However, before this idea of hemispheric specialization can be accepted, it is necessary to precisely identify these distinct, lateralized mechanisms. Here we show in right-handers that dominant arm movements rely on predictive mechanisms that anticipate and account for the dynamic properties of the arm, while the non-dominant arm optimizes positional stability by specifying impedance around equilibrium positions. In a targeted-reaching paradigm, we covertly and occasionally shifted the hand starting location either orthogonal to or collinear with a particular direction of movement. On trials on which the start positions were shifted orthogonally, we did not notice any strong interlimb differences. However, on trials on which start positions were shifted orthogonally, the dominant arm largely maintained the direction and straightness of its trajectory, while the non-dominant arm deviated towards the previously learned goal position, consistent with the hypothesized control specialization of each arm-hemisphere system. These results bring together two competing theories about mechanisms of movement control, and suggest that they coexist in the brain in different hemispheres. These findings also question the traditional view of handedness, because specialized mechanisms for each arm-hemisphere system were identified within a group of right-handers. It is likely that such hemispheric specialization emerged to accommodate increasing motor complexity during evolution.

Sensorimotor performance asymmetries predict hand selection

Handedness is most often measured by questionnaires that assess an individual's preference for using a particular hand to perform a variety of tasks. While such assessments have proved reliable, they do not address the underlying neurobehavioral processes that give rise to the choice of which hand to use. Recent research has indicated that handedness is associated with hemispheric specializations for different aspects of sensorimotor performance. We now hypothesize that an individual's choice of which hand to use for a given task should result from an interaction between these underlying neurobehavioral asymmetries with task conditions. We test this hypothesis by manipulating two factors in targeted reaching movements: (1) region of workspace and (2) visual feedback conditions. The first manipulation modified the geometric and dynamic requirements of the task for each arm, whereas the second modified the sensorimotor performance asymmetries, an effect predicted by previous literature. We expected that arm choice would be reflected by an interaction between these factors. Our results indicated that removing visual feedback both improved the relative performance of the non-dominant arm and increased the choice to use this arm for targets near midline, an effect that was enhanced for targets requiring larger movement amplitudes. We explain these findings in the context of the dynamic dominance hypothesis of handedness and discuss their implications for the link between hemispheric asymmetries in neural control and hand preference.

Early manifestations of manual specialisation in infants: a longitudinal study from 20 to 30 weeks

This longitudinal study examined lateral differences between latency time of the two hands during the development of prehension in 12 infants from 20 to 30 weeks. Latency time (LT) is defined as the delay between the visual localisation of a reachable target and the beginning of the movement and could be considered as a phase in the preparation of action. If LT varies with the hand used and the type of movement, this would suggest differences in information processing (nature and/or quantity). Results show that the latency time is shorter for the left hand approach movements and shorter for the right hand grasping movements. These findings are in favour of a manual specialisation--clearly present from 20 weeks--and are discussed in a possible hemispheric specialisation perspective.

The cost of moving with the left hand

Precise left-hand movements take longer than right-hand movements (for right-handers). To quantify how left-hand movements are affected by task difficulty and phase of movement control, we manipulated the difficulty of repetitive speeded aiming movements while participants used the left or right hand. We observed left-hand costs in both initial impulse and current control phases of movement. While left-hand cost during the initial impulse phase was small, left-hand cost during the current control phase varied from 10 to 60 ms, in direct proportion to the movement's difficulty as quantified by Fitts' law (0.77 < R² < 0.99, across three experiments). In particular, in comparison with a difficult task for the right hand (Fitts' ID(R) = 6.6), the left hand's task would have to be made easier by 0.5 bits (ID(L) = 6.1) to be performed as quickly. The left-hand cost may reflect the time required for callosal transfer of information between the left and right hemispheres during the current control phase of precision left-hand movements or reflect movement control differences in the current control phase of movement that are inherent to the hemispheres. Overall, the present results support multiphase models of movement generation, in which separate specialized processes contribute to the launching and completion of precision hand movements.

Dynamic dominance persists during unsupported reaching

Previous studies examining lateralization of arm movements focused on supported movements in the horizontal plane, removing the effects of gravity. The authors hypothesized that interlimb differences in free reaching would be consistent with the differences shown during supported reaching. Kinematic and kinetic data were collected for the forearm and upper arm segments in a 3-direction reaching task. Results showed lateralization of coordination, reflected by initial movement direction and trajectory curvature. The nondominant arm showed increased initial direction errors, and path curvature associated with a timing deficit between elbow and shoulder peak torques. These coordination deficits did not disrupt final position accuracy. The authors conclude that nondominant arm coordination deficits are similar to those reported previously for horizontal plane movements.

Dynamic dominance varies with handedness: reduced interlimb asymmetries in left-handers

Our previous studies of interlimb asymmetries during reaching movements have given rise to the dynamic-dominance hypothesis of motor lateralization. This hypothesis proposes that dominant arm control has become optimized for efficient intersegmental coordination, which is often associated with straight and smooth hand-paths, while non-dominant arm control has become optimized for controlling steady-state posture, which has been associated with greater final position accuracy when movements are mechanically perturbed, and often during movements made in the absence of visual feedback. The basis for this model of motor lateralization was derived from studies conducted in right-handed subjects. We now ask whether left-handers show similar proficiencies in coordinating reaching movements. We recruited right- and left-handers (20 per group) to perform reaching movements to three targets, in which intersegmental coordination requirements varied systematically. Our results showed that the dominant arm of both left- and right-handers were well coordinated, as reflected by fairly straight hand-paths and low errors in initial direction. Consistent with our previous studies, the non-dominant arm of right-handers showed substantially greater curvature and large errors in initial direction, most notably to targets that elicited higher intersegmental interactions. While the right, non-dominant, hand-paths of left-handers were slightly more curved than those of the dominant arm, they were also substantially more accurate and better coordinated than the non-dominant arm of right-handers. Our results indicate a similar pattern, but reduced lateralization for intersegmental coordination in left-handers. These findings suggest that left-handers develop more coordinated control of their non-dominant arms than right-handers, possibly due to environmental pressure for right-handed manipulations.

The impact of head direction on lateralized choices of target and hand

We examined choices made by monkeys performing a task in which two food-well targets were positioned on either side of the monkey, and LEDs provided instructions on hand use and food target availability. We have previously reported that when gaze and head direction were unrestricted, lateralized choices were biased primarily by hand preference and secondarily by a preference to retrieve a target ipsilateral to the preferred hand. Here, we used a similar behavioral paradigm, but now during trial instructions the monkeys were required to maintain head direction aimed toward a left, a center, or a right fixation LED. When a lateralized head direction was required during presentation of the instructional cues, monkeys were more likely to choose the hand and target ipsilateral to the head direction. Lateralized head direction more strongly biased the monkeys' choice of hand than their choice of target, but hand preference produced even stronger bias on target choices than did head direction. Although target cues were presented before hand cues, our data indicate that target and hand choices were made interactively. We also found that the monkeys' choices were better correlated with their success rate for particular combinations of hand and target than with movement times.

Insights into the control of arm movement during body motion as revealed by EMG analyses

Recent studies have revealed that vestibulomotor transformations contribute to maintain the hand stationary in space during trunk rotation. Here we tested whether these vestibulomotor transformations have the same latencies and whether they are subject to similar cognitive control than the visuomotor transformations during manual tracking of a visual target. We recorded hand displacement and shoulder-muscle activity in two tasks: a stabilization task in which subjects stabilized their hand during passive 30 degrees body rotations, and a tracking task in which subjects tracked with their finger a visual target as it moved 30 degrees around them. The EMG response times recorded in the stabilization task (approximately 165 ms) were twice as short as those observed for the tracking task (approximately 350 ms). Tested with the same paradigm, a deafferented subject showed EMG response times that closely matched those recorded in healthy subjects, thus, suggesting a vestibular origin of the arm movements. Providing advance information about the direction of the required arm movement reduced the response times in the tracking task (by approximately 115 ms) but had no significant effect in the stabilization task. Generally, when providing false information about movement direction in the tracking task, an EMG burst first appeared in the muscle moving the arm in the direction opposite to the actual target motion (i.e., in accord with the precueing). This behavior was rarely observed in the stabilization task. These results show that the sensorimotor transformations that move the arm relative to the trunk have shorter latencies when they originate from vestibular inputs than from visual information and that vestibulomotor transformations are more resistant to cognitive processes than visuomotor transformations.

Conceptual binding: integrated visual cues reduce processing costs in bimanual movements

In discrete reaction time (RT) tasks, it has been shown that nonsymmetric bimanual movements are initiated slower than symmetric movements in response to symbolic cues. By contrast, no such RT differences are found in response to direct cues ("direct cue effect"). Here, we report three experiments showing that the direct cue effect generalizes to rhythmical bimanual movements and that RT cost depends on different cue features: 1) symbolic versus direct or 2) integrated (i.e., action of both hands is indicated as one entity) versus dissociated (i.e., action of each hand is indicated separately). Our main finding was that dissociated symbolic cues were most likely processed serially, resulting in the longest RTs, which were substantially reduced with integrated symbolic cues. However, extra RT costs for switching to nonsymmetrical bimanual movements were overcome only when the integrated cues were direct. We conclude that computational resources might have been exceeded when the response needs to be determined for each hand separately, but not when a common response for both hands is selected. This supports the idea that bimanual control benefits from conceptual binding.

Does hand dominance affect the use of motor abundance when reaching to uncertain targets?

This study investigated hemispheric differences in utilizing motor abundance to achieve flexible patterns of joint coordination when reaching to uncertain target locations. Right-handed participants reached with each arm to the same central target when its final location was certain or when there was a 66% probability that its location could change after movement initiation. Use of greater motor abundance was observed when participants reached to the central target under target location uncertainty regardless of the arm used to reach. Joint variance associated with variability of movement direction was larger when reaching with the left, non-dominant arm. This arm also exhibited higher hand path variability compared to the dominant arm. These arm differences were not found when the final (central) target location was known in advance. The results provide preliminary evidence for a greater ability of the dominant (right) arm/left hemisphere to decouple directions in joint space. That is, to increase the use of motor abundance without simultaneously inducing unwanted hand path variability requires that joint variations be restricted to a limited subspace of joint space. Hemispheric differences in motor planning did not appear to account for arm differences related to the use of motor abundance.

Categories of manual asymmetry and their variation with advancing age

Manual asymmetries were analyzed in 18- to 63-year-old right-handers in different motor tasks. This analysis aimed at describing the asymmetry profile for each task and assessing their stability across ages. For this purpose, performance of the right and left hands were analyzed in the following aspects: simple reaction time, rate of sequential finger movements, maximum grip force, accuracy in anticipatory timing, rate of repetitive tapping, and rate of drawing movements. In addition, stability of manual preference across ages was assessed through the Edinburgh inventory (Oldfield, 1971). The results indicated different profiles of manual asymmetry, with identification of three categories across tasks: symmetric performance (asymmetry indices close to zero), inconsistent asymmetry (asymmetry indices variable in magnitude and direction), and consistent asymmetry (asymmetry indices favoring a single hand). The different profiles observed in the young adults were stable across ages with two exceptions: decreased lateral asymmetry for maximum grip force and increased asymmetry for sequential drawing in older individuals. These results indicate that manual asymmetries are task specific. Such task specificity is interpreted to be the result of different sensorimotor requirements imposed by each motor task in association with motor experiences accumulated over the lifetime. Analysis of manual preference showed that strength of preference for the right hand was greater in older individuals.

Extent of innate dexterity and ambidexterity across handedness and gender: Implications for training in laparoscopic surgery

BACKGROUND

As innate dexterity is considered one of the important predictors of eventual operative competence, an experimental human factors study was conducted to determine innate dexterity and ambidexterity across handedness and gender.

METHODS

50 medical students (right-handed males, left-handed males, and right-handed females) were recruited as participants in a study designed to assess innate dexterity and degree of ambidexterity for endoscopic manipulations in a validated virtual-reality simulator. All participants performed unilateral and bilateral tasks with both dominant and nondominant hands in random sequence. The outcome measures were execution time, extent of ambidexterity (ambidexterity index), aiming errors, and maximum tissue damage.

RESULTS

Right-handed males exhibited a greater level of ambidexterity than left-handed males (p = 0.02 for path length, p = 0.001 for angular path) and right-handed females (p = 0.01 for path length, p = 0.02 for angular path), and more-efficient task performance as measured by execution time (p = 0.001 for males and p = 0.03 across gender). The task quality when executed by the dominant hand was best in right-handed males (p = 0.001 vs. left-dominant males and p = 0.03 across gender). No significant difference was observed in terms of precision control and fine movements (aiming errors and maximum tissue damage) between the three groups.

CONCLUSIONS

These findings indicate that training surgical curricula in laparoscopic surgery should be more flexible to accommodate the innate differences across handedness and gender.

Hand dominance and multi-finger synergies

Recent studies of arm movement reaching to a target have provided support for the Dynamic Dominance model of handedness, which proposes that each hemisphere/limb system is specialized for controlling different features of performance: The dominant system for control of the trajectory, and the nondominant system for control of the steady state final position. We now examine a more general form of this hypothesis by investigating differences between the right and left hands of right-handed persons in their ability to stabilize the combined action of a set of fingers. The subjects produced very quick pulses and steps of force from a low background force level while pressing with four fingers of one hand. An index of force stabilizing synergy showed a strong multi-finger synergy during steady-state force production followed by a small anticipatory drop in the index prior to the force increase. Hand differences emerged during the force increase: The left (non-dominant) hand showed a significantly larger drop in the synergy index. While our findings support the idea that the dominant system is specialized for stabilizing quick changes in performance variables, we failed to support a nondominant specialization for stabilizing steady-state isometric force. This may be a ceiling effect due to the simplicity of the task for either hand, or it might indicate that this aspect of the dynamic dominance hypothesis does not generalize to isometric conditions.

Corticomotor excitability during a choice-hand reaction time task

Fourteen neurologically healthy, right-handed subjects performed a choice-hand reaction time (RT) task, which involved wrist flexion or extension of either the left or right hand to one of three fixed target locations corresponding to 45 degrees flexion, 20 degrees flexion, or 20 degrees extension from the starting position. In each trial, a pre-cue provided information regarding the forthcoming target location. The hand was specified by the imperative signal. Focal transcranial magnetic stimulation (TMS) was delivered over the hand motor area of either the right or left hemisphere at set times during the foreperiod, and at random intervals during the RT interval defined by electromyography onset. As expected, an increase in corticomotor excitability was observed in the agonist of the responding hand over the RT interval. When the cue appeared at a location that required flexion with either hand, an increase in excitability was observed following stimulation over the hemisphere ipsilateral to the responding hand, indicating activation of the homologous muscle. However, when the cue appeared at a location at which the response would require flexion with one hand and extension with the other, the modulation of excitability was also based on the direction of the response. This direction-specific effect was only observed for TMS delivered to the left hemisphere during the left-hand movement, and suggested goal-based preparation in the left hemisphere independent of whether the actual movement is made with the right or left hand. These results indicate that both the homologous-muscle and the directional-specific constraints affect the corticomotor excitability of the non-responding hand.

Interhemispheric Transmission of Visuomotor Information for Motor Implementation

Using transcranial magnetic stimulation (TMS), we addressed the contribution of both hemispheres to the visuomotor control of each hand. The subjects had to press one of two buttons as quickly as possible after the go-signal. A precue preceding this conveyed full, partial or no advance information (hand and/or button), such that reaction time (RT) shortened with increasing amount of information. We gave TMS over each hemisphere at various time intervals (100-350 ms) after the go-signal and before the expected onset of response, and measured its effect on RT, movement time (MT) and error rate. At short intervals (100-200 ms), left hemisphere TMS delayed RT and prolonged MT of both hands, while right hemisphere TMS delayed RT only of the right hand, without affecting error rates. At long intervals (250-350 ms), TMS produced slightly more pronounced RT delays of the contralateral hand. RT was delayed more if the precues were less informative. The results suggest the importance of interhemispheric transmission of visuomotor information for motor implementation. The right hemisphere may play a role mainly in calculating target and effector information, determining RT, while the left hemisphere may play a role in elaborating the motor program and determining MT.

Gait adjustments in response to an obstacle are faster than voluntary reactions

It has been reported that obstacle avoidance reactions during gait have very short latencies. This raises the question whether the cortex can be involved, as it is in voluntary reactions. In this study, latencies of obstacle avoidance (OA) reactions were determined and related to latencies of voluntary stride modifications and simple reaction times (SRT) of hand and foot. Twenty-five healthy young adults participated in this study. While they were walking on the treadmill, an obstacle suddenly fell in front of their left leg. The first reaction to the obstacle was the moment at which the differentiated acceleration curve of the foot deviated from the control signal. Latencies of OA reactions were 122 ms (SD 14 ms) on average. Two very different avoidance reactions (lengthening and shortening of the stride) were noticed, but there was no avoidance strategy effect on OA latencies. OA latencies were significantly shorter as compared to latencies of voluntary stride modifications and simple reaction times of hand and foot. The short OA latencies could not only be explained from the dynamic nature of the task. It is suggested that subcortical pathways might be involved in obstacle avoidance.

Multimodal inhibition of return effects in adults with and without Down syndrome

Data from a previous study (Welsh & Elliott, 2001) has been reanalyzed to explore inhibition of return (IOR) effects in adults with and without Down syndrome (DS). Participants were required to react and move with either the left or right hand as quickly as possible to 1 of 2 target locations based on either a visual or a verbal cue. Although persons with DS demonstrated a different pattern of information processing capabilities, they demonstrated the same magnitude of IOR across all conditions of presentation as their peers without DS. This pattern of results provides further support for the multimodal and response-based nature of IOR. Moreover, the results indicate that the inhibitory processes that underlie IOR in the average population seem to be functional in persons with DS.

Interlimb differences in control of movement extent

The current study was designed to examine potential interlimb asymmetries in controlling movement extent. Subjects made repetitive single-joint elbow extension movements while the arm was supported on a horizontal, frictionless, air-jet system. Four targets of 10, 20, 35, and 45 degrees excursions were randomly presented over the course of 150 trials. For both arms, peak tangential hand velocity scaled linearly with movement distance. There was no significant difference between either peak velocities or movement accuracies for the two arms. However, the mechanisms responsible for achieving these velocities and extents were quite distinct for each arm. For the dominant arm, peak tangential finger acceleration varied systematically with movement distance. In contrast, nondominant-arm peak tangential acceleration varied little across targets and, as such, was a poor predictor of movement distance. Instead the velocities of the nondominant arm were determined primarily by variation in the duration of the initial acceleration impulse, which corresponds to the time of peak velocity. These different strategies reflect previously identified mechanisms in controlling movement distance: pulse-height control and pulse-width control. The former is characterized by a variation in peak acceleration and has been associated with preplanning mechanisms. The latter occurs after peak acceleration and has been shown to depend on peripheral sensory feedback. Our findings indicate that the dominant-arm system controls movement extent largely through planning mechanisms that specify pulse-height control, whereas the nondominant system does so largely through feedback mediated pulse-width control.

Left- versus right-hand tracking performance by right-handed boys and girls: examination of performance asymmetry

This study compared left- versus right-hand performance within healthy, right-handed, 8- or 9-yr.-old boys and girls on a finger-movement tracking task. 38 boys and 38 girls were randomly assigned to use either the left hand first and right hand second or vice versa in tracking a sine wave target with extension and flexion movements of the index finger. The data were analyzed with a three-way analysis of variance with repeated measures followed by pair-wise comparisons with a Bonferroni correction. Analysis yielded a significant hand x test interaction and a significant improvement for subjects tracking with the right hand on Test 1 and left hand on Test 2. No significant change occurred for subjects tracking with the left hand on Test 1 and right hand on Test 2. No interaction was observed with sex as a factor. This study suggests that asymmetry of performance favoring the left hand occurs in right-handed boys and girls during finger-movement tracking.

Nondominant arm advantages in load compensation during rapid elbow joint movements

This study was designed to examine interlimb asymmetries in responding to unpredictable changes in inertial loads, which have implications for our understanding of the neural mechanisms underlying handedness. Subjects made repetitive single joint speed constrained 20 degrees elbow flexion movements, while the arm was supported on a horizontal, frictionless, air-jet system. On random trials, a 2-kg mass was attached to the arm splint prior to the "go" signal. Subjects were not given explicit information about the mass prior to movement nor were they able to view their limb or the mass. Accordingly, muscle activity, recorded prior to peak tangential finger acceleration, was the same for loaded and baseline trials. After this point, substantial changes in muscle activity occurred. In both limbs, the load compensation response was associated with a reduction in extensor muscle activity, resulting in a prolonged flexion phase of motion. For the nondominant arm, this resulted in effective load compensation, such that no differences in final position accuracy occurred between loaded and baseline trials. However, the dominant arm response also included a considerable increase in flexor muscle activity. This substantially prolonged the flexor acceleration phase of motion, relative to that of the nondominant arm. As a result, the dominant arm overcompensated the effects of the load, producing a large and systematic overshoot of final position. These results indicate more effective load compensation responses for the nondominant arm; supporting a specialized role of the nondominant arm/hemisphere system in sensory feedback mediated error correction mechanisms. The results also suggest that specialization of the dominant arm system for controlling limb and task dynamics is specifically related to feedforward control mechanisms.

Hemispheric asymmetry for trajectory perception

Broadly, the right hemisphere is known to be specialized for spatial processing whereas the left hemisphere is known to be specialized for temporal processing. However, it remains unclear how both hemispheres interact when processing spatio-temporal information. This study investigates, from a behavioral point of view, whether spatio-temporal processing involved in trajectory perception generates hemispheric asymmetries. An experimental task requiring the prediction of coincidence between ballistic trajectories and a stationary target was used. Reaction times were analyzed according to various interhemispheric conditions determined by the visual hemifield on which the stimulus was presented and the hand of response. There was shorter reaction time for the left hand than the right hand, and shorter reaction times for the left visual hemifield than the right visual hemifield for both hands. From these findings, it is inferred that there is likely to be right hemisphere specialization for trajectory perception and that this hemispheric asymmetry is independent of handedness.