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

Bimanual coordination during reach-to-grasp actions is sensitive to task goal with distinctions between left- and right-hemispheric stroke

The perceptual feature of a task such as how a task goal is perceived influences performance and coordination of bimanual actions in neurotypical adults. To assess how bimanual task goal modifies paretic and non-paretic arm performance and bimanual coordination in individuals with stroke affecting left and right hemispheres, 30 participants with hemispheric stroke (15 right-hemisphere damage-RHD); 15 left-hemisphere damage-LHD) and 10 age-matched controls performed reach-to-grasp and pick-up actions under bimanual common-goal (i.e., two physically coupled dowels), bimanual independent-goal (two physically uncoupled dowels), and unimanual conditions. Reach-to-grasp time and peak grasp aperture indexed motor performance, while time lags between peak reach velocities, peak grasp apertures, and peak pick-up velocities of the two hands characterized reach, grasp, and pick-up coordination, respectively. Compared to unimanual actions, bimanual actions significantly slowed non-paretic arm speed to match paretic arm speed, thus affording no benefit to paretic arm performance. Detriments in non-paretic arm performance during bimanual actions was more pronounced in the RHD group. Under common-goal conditions, movements were faster with smaller peak grasp apertures compared to independent-goal conditions for all groups. Compared to controls, individuals with stroke demonstrated poor grasp and pick-up coordination. Of the patient groups, patients with LHD showed more pronounced deficits in grasp coordination between hands. Finally, grasp coordination deficits related to paretic arm motor deficits (upper extremity Fugl-Meyer score) for LHD group, and to Trail-Making Test performance for RHD group. Findings suggest that task goal and distinct clinical deficits influence bimanual performance and coordination in patients with left- and right-hemispheric stroke.

Anticipating different grips reduces bimanual end-state comfort: A tradeoff between goal-related and means-related planning processes

The present study explored the sensitivity towards bimanual end-state comfort in a task that required anticipating different final grips. Participants simultaneously reached and grasped two objects with either a whole-hand grip (WHG) or a precision grip (PG), and placed them at two target locations by transporting them either over or under an obstacle. The transport path was varied such that it could be either congruent (i.e., both objects over or under) or incongruent (i.e., one object over and the other object under). In the congruent conditions, participants satisfied bimanual end-state comfort (and identical initial grips) on the majority of trials. That is, participants adopted a PG for either hand when the objects were transported over the obstacle and a WHG for either hand when the objects were transported under the obstacle. In contrast, in the incongruent conditions, bimanual end-state comfort was significantly reduced, indicating the presence of intermanual inference. The results indicate that goal-related planning constraints (i.e., bimanual end-state comfort) do not strictly take precedence over means-related constraints (i.e., identical initial grips) if this requires anticipating different final grips. Thus, bimanual end-state comfort per se does not provide a predominant constraint in action selection, by which sensorimotor interference can be reduced. In line with the proposal that bimanual grip planning relies on a flexible constraint hierarchy, a simple formal model that considers bimanual grip posture planning as a tradeoff between goal-related and means-related planning processes can explain our results reasonably well.

Cognitive framing in action

Cognitive framing effects have been widely reported in higher-level decision-making and have been ascribed to rules of thumb for quick thinking. No such demonstrations have been reported for physical action, as far as we know, but they would be expected if cognition for physical action is fundamentally similar to cognition for higher-level decision-making. To test for such effects, we asked participants to reach for a horizontally-oriented pipe to move it from one height to another while turning the pipe 180° to bring one end (the "business end") to a target on the left or right. From a physical perspective, participants could have always rotated the pipe in the same angular direction no matter which end was the business end; a given participant could have always turned the pipe clockwise or counter-clockwise. Instead, our participants turned the business end counter-clockwise for left targets and clockwise for right targets. Thus, the way the identical physical task was framed altered the way it was performed. This finding is consistent with the hypothesis that cognition for physical action is fundamentally similar to cognition for higher-level decision-making. A tantalizing possibility is that higher-level decision heuristics have roots in the control of physical action, a hypothesis that accords with embodied views of cognition.

Too much anticipation? Large anticipatory adjustments of grasping movements to minimal object manipulations

When humans grasp objects, the grasps foreshadow the intended object manipulation. It has been suggested that grasps are selected that lead to medial arm postures, which facilitate movement speed and precision, during critical phases of the object manipulation. In Experiment 1, it has been tested whether grasp selections lead to medial postures during rotations of a dial. Participants twisted their arms considerably before grasping the dial, even when the upcoming dial rotation was minimal (5°). Participants neither assumed a medial posture at any point during a short rotation, nor did they assume any of the postures involved in short rotations in the opposite direction. Thus, grasp selections did not necessarily lead to specific postures at any point of the object manipulation. Experiment 2 examined the effect of various grasps on the speed of dial rotations. A medial initial grasp resulted in the fastest dial rotations for most rotation angles. Spontaneously selected grasps were more excursed than necessary to maximize dial rotation speed. This apparent overshot might be explained by participants' sensitive to the variability of their grasps and is in line with the assumption that grasps facilitate control over the grasped object.

The effect of endpoint congruency on bimanual transport and rotation tasks

The completion of many goal oriented skills requires the tight coordination of the right and left hands to achieve the task objective. Although the coordination of wrist transport and orientation of the hand before object contact has been studied in detail for discrete bimanual tasks, as yet, very few studies have examined bimanual coordination when the target is already in hand. It has been shown that congruency of the goal facilitates the production of discrete bimanual responses. The purpose of this study was to investigate the role of goal congruency on precision bimanual transport and rotate tasks. In the current investigation, participants transported two cubic objects while rotating them laterally to place them into tight-fitting targets. The magnitude of the rotation could be the same for both hands (i.e., both 45 or 90(∘)) or different (i.e., one 45 and 90(∘)) and the endpoint orientations (i.e., goal) could either be congruent or incongruent. Results indicated that when the endpoint orientation was congruent for the two hands, movement times were similar regardless of hand (left or right), rotation magnitude (45, 90(∘)) and whether the rotation magnitude for the two hands was the same or different. These results suggest that congruency of the endpoint goal facilitates the temporal synchronization of the transport component for two limbs. In contrast, a different pattern of results was obtained when considering the rotation component. Specifically, regardless of whether the hands were rotating the same magnitude or ending in congruent endpoint positions, the coordination of the rotation component between the hands was asynchronous. We hypothesize that the greater requirement to shift visual fixation from one hand/target to the other to ascertain the separate goal orientations may explain these differences. These results provide further evidence that multiple constraints act to influence the performance of skilled bimanual tasks.

Manual (a)symmetries in grasp posture planning: a short review

Many activities of daily living require that we physically interact with one or more objects. Object manipulation provides an intriguing domain in which the presence and extent of manual asymmetries can be studied on a motor planning and a motor execution level. In this literature review we present a state of the art for manual asymmetries at the level of motor planning during object manipulation. First, we introduce pioneering work on grasp posture planning. We then sketch the studies investigating the impact of future task demands during unimanual and bimanual object manipulation tasks in healthy adult populations. In sum, in contrast to motor execution, there is little evidence for hand-based performance differences in grasp posture planning. We discuss potential reasons for the lack of manual asymmetries in motor planning and outline potential avenues of future research.

The influence of reducing intermediate target constraints on grasp posture planning during a three-segment object manipulation task

The present experiment examined the influence of final target position on grasp posture planning during a three-segment object manipulation task in which the required object orientation at the first target position was unconstrained. Participants grasped a cylindrical object from a home position, placed it at an intermediate position in a freely chosen orientation, and subsequently placed it at one of four final target positions. Considerable inter-individual differences in initial grasp selection were observed which also led to differences in final grasp postures. Whereas some participants strongly adjusted their initial grasp postures to the final target orientation, and thus showed a preference for end-state comfort, other participants showed virtually no adjustment in initial grasp postures, hence satisfying initial-state comfort. Interestingly, as intermediate grasp postures were similar regardless of initial grasp adjustment, intermediate-state comfort was prioritized by all participants. These results provide further evidence for the interaction of multiple action selection constraints in grasp posture planning during multi-segment object manipulation tasks. Whereas some constraints may take strict precedence in a given task, other constraints may be more flexible and weighted differently among participants. This differentiated weighting leads to task- and subject-specific constraint hierarchies and is reflected in inter-individual differences in grasp selection.

Symmetries in action: on the interactive nature of planning constraints for bimanual object manipulation

An important functional question for understanding how people perform physical actions is to understand how they manipulate objects. Previous research suggests that people prefer to move their hands symmetrically. For bimanual object manipulation, such symmetry may take on several forms, however. Actions may be symmetrical when objects are grasped (start symmetry), when they are placed on their target locations (target symmetry), and/or relative to the objects being moved (object symmetry). We studied how these forms of symmetry influenced grasp selection when participants moved two plungers from two start locations to two target locations. We varied the heights of these locations across conditions. The grasp locations participants adopted indicated a preference for object symmetry. This preference was even stronger when initial symmetry coincided with object symmetry. These results provide a tractable illustration of how multiple planning constraints may interact to give rise to both regularity and flexibility in motor behavior.

Limits of end-state planning

The end-state comfort effect is the tendency to use an uncomfortable initial grasp posture for object manipulation if this leads to a comfortable final posture. Many studies have replicated the end-state comfort effect across a range of tasks and conditions. However, these tasks had in common that they involved relatively simple movements, such as picking up a dowel or sliding a pan from one place to another. Here we asked whether the end-state comfort effect extends to more complex tasks. We asked participants to grasp a transparent bowl and move the bowl to an instructed location, positioning it in an instructed orientation. We either found an initial-state comfort effect or equal degrees of comfort for end-grasps and start-grasps depending on task instructions. The end-state comfort effect was not consistently observed. The results suggest that the end-state comfort effect may be restricted to relatively simple grasping movements.

Problems in planning bimanually incongruent grasp postures relate to simultaneous response specification processes

The purpose of the current experiments was to examine whether the problems associated with grasp posture planning during bimanually incongruent movements are due to crosstalk at the motor programming level. Participants performed a grasping and placing task in which they grasped two objects from a table and placed them onto a board to targets that required identical (congruent) or non-identical degrees of rotation (incongruent). The interval between the presentation of the first stimulus and the second stimulus (stimulus onset asynchrony: SOA) was manipulated. Results demonstrate that the problems associated with bimanually incongruent grasp posture planning are reduced at SOA durations longer than 1000ms, indicating that the costs associated with bimanual incongruent movements arise from crosstalk at the motor programming level. In addition, reach-to-grasp times were shorter, and interlimb limb coupling was higher, for congruent, compared to incongruent, object end-orientation conditions in both Experiment 1 and 2. The bimanual interference observed during reach-to-grasp execution is postulated to arise from limitations in the visual motor system or from conceptual language representations. The present results emphasize that bimanual interference arises from constraints active at multiple levels of the neurobiological-cognitive system.

Grasp posture planning during multi-segment object manipulation tasks - interaction between cognitive and biomechanical factors

The present study examined adaptations in the planning of initial grasp postures during a multi-segment object manipulation task. Participants performed a grasping and placing task that consisted of one, two, or three movement segments. The position of the targets was manipulated such that the degree of object rotation between the home and temporally proximal positions, and between the temporally proximal and distal target positions, varied. Participants selected initial grasp postures based on the specific requirements of the temporally proximal and temporally distal action segments, and adjustments in initial grasp posture depended on the temporal order of target location. In addition, during the initial stages of the experimental session initial grasp postures were influenced to a larger extent by the demands of the temporally proximal segment. However, over time, participants overcame these cognitive limitations and adjusted their initial grasp postures more strongly to the requirements of the temporally distal segment. Taken together, these results indicate that grasp posture planning is influenced by cognitive and biomechanical factors, and that participants learn to anticipate the task demands of temporally distal task demands, which we hypothesize, reduce the burden on the central nervous system.

Perturbations in action goal influence bimanual grasp posture planning

The authors examined the effects of perturbations in action goal on bimanual grasp posture planning. Sixteen participants simultaneously reached for 2 cylinders and placed either the left or the right end of the cylinders into targets. As soon as the participants began their reaching movements, a secondary stimulus was triggered, which indicated whether the intended action goal for the left or right hand had changed. Overall, the tendency for a single hand to select end-state comfort compliant grasp postures was higher for the nonperturbed condition compared to both the perturbed left and perturbed right conditions. Furthermore, participants were more likely to plan their movements to ensure end-state comfort for both hands during nonperturbed trials, than perturbed trials, especially object end-orientation conditions that required the adoption of at least one underhand grasp posture to satisfy bimanual end-state comfort. Results indicated that when the action goal of a single object was perturbed, participants attempted to reduce the cognitive costs associated with grasp posture replanning by maintaining the original grasp posture plan, and tolerating grasp postures that result in less controllable final postures.

Interlimb coordination during a cooperative bimanual object manipulation task

This experiment examined asymmetries in the execution of an object manipulation task that requires the coordinated use of both hands. To this end, twenty right-hand-dominant participants performed a bimanual object manipulation task, which required that they reach for and grasp two objects located on a tabletop, fit the two objects through a hole in a horizontally or vertically oriented fitting board, and then rotate the objects 180° to produce a "beep" tone. Overall, the two hands were highly synchronized at the start, but not at end, of each movement phase. The decrease in interlimb coupling at later stages of the movement phase was primarily driven by the shorter movement time values for the dominant right hand. In addition, degree of left object rotation was greater than the right object, irrespective of board orientation. In sum, the results suggest that manual asymmetries and role assignment are not hardwired constraints, but depend on the overall task constraints and the manner in which the task is conceptualized.

Individual differences in motor planning during a multi-segment object manipulation task

Research has demonstrated that people will adopt initially awkward grasps if they afford more comfortable postures at the end of the movement. This end-state comfort effect provides evidence that humans represent future posture states and select appropriate grasps in anticipation of these postures. The purpose of the study was to examine to what extent the final action goal of a task influences motor planning of preceding segments, and whether grasp postures are planned to optimize end-state comfort during a three-segment action sequence in which two objects are manipulated, and participants can select from a continuous range of possible grasp postures. In the current experiment, participants opened a drawer, grasped an object from inside the drawer, and placed it on a table in one of the three target orientations (0°, 90°, or 180° object rotation required). Grasp postures during the initial movement segment (drawer opening) were not influenced by the final action goal (i.e., required target orientation). In contrast, both the intermediate (i.e., object grasping) and the final movement segment (i.e., object placing) were influenced by target orientation. In addition, participants adopted different strategies to achieve the action goal when the object required 180° rotation, with 42 % of participants prioritizing intermediate-state comfort and 58 % prioritizing end-state comfort. The results indicate that individuals optimize task performance by selecting lower level constraints that allow for successful completion of the action goal and that the selection of these constraints is dependent upon contextual, environmental, and internal influences.

Effects of stimulus cueing on bimanual grasp posture planning

The purpose of the present study was to investigate whether difficulties in bimanual grasp posture planning arise from conflicts in response selection. Forty-five participants were assigned to one of three groups (symbolic cueing, semi-symbolic cueing, and direct cueing) and instructed to reach for, grasp, and place two objects on a board in various end-orientations, depending on condition. In general, the tendency to adopt initial grasps that resulted in end-state comfort was significantly higher for the semi-symbolic, than that for the other two groups. There were, however, noticeable individual differences in grip behavior in the symbolic and direct cueing groups. Although the majority of participants performed the task in a similar fashion to the semi-symbolic group, there was a subset of participants (40% in each group) who grasped the two objects using an overhand grip in virtually all trials, regardless of condition. It is hypothesized that the observed individual differences in grasp posture strategy arise from differences in motor planning abilities, or the strategies participants employ in order to comply with task demands. A secondary finding is that the degree of interlimb coupling was larger for congruent, than incongruent, conditions irrespective of stimulus cueing. This finding indicates that the interference in the execution of bimanual grasping and placing tasks arises from interference during the specification of movement parameters specific to planning and execution of bimanual movements, or neuronal cross-talk in efferent pathways, rather than response selection conflicts.

An investigation into manual asymmetries in grasp behavior and kinematics during an object manipulation task

Manual asymmetries in the control of movements have been investigated in a variety of experimental paradigms. Initial studies demonstrated that the dominant right hand has advantages over the non-dominant left hand in many aspects of motor control. However, more recent studies have shown that the presence and extent of these asymmetries depends on the task context and accuracy demands. Typically, manual asymmetries on a motor planning and motor execution level are examined separately. However, given that recent research has demonstrated that specific task constraints do not influence both levels equally, the purpose of the present experiment was to investigate manual asymmetries in motor planning and execution. To this end, initial grasp behavior (motor planning) and kinematics (motor execution) were examined in thirteen right-handed participants during a unimanual grasping and placing task. We specifically manipulated grasping hand, target location, object end orientation, and object grasp time at the start location. There were three main findings. First, motor planning or movement execution was similar regardless of grasping hand. Second, prospectively planned actions were influenced by target location and the required end orientation of the object. Third, the amount of time spent in an initial posture did not influence initial grasp postures. However, it did alter the movement kinematics during the grasping (approach phase) and placing (transport phase) portion of the task. We posit that grasping and placing movements are comprised of an initial grasp and a transport component, which are differentially influenced by task constraints.

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.