Optimal control in the critical phase of movement: a functional approach to motor planning processes

Choice of end-state comfort is dependent upon the time spent at the beginning-state and the precision requirement of the end-state

Choice of posture while grasping an object typically depends upon several factors including the time spent in that posture, what postures were held prior to choosing that posture, and the precision required by the posture. The purpose of this study was to test choice of end-state thumb-up posture based on time spent at the beginning-state and the precision requirement of the end-state. To determine choice of thumb-up based on time or precision, we varied how long a subject had to hold the beginning state before moving an object to an end location. We made end-state precision either small or large and eliminated the precision needed to stand the object up at the end of the movement. A choice between "comfort" at the beginning or precision at the end-state would be demanded by the conditions with long beginning-state hold times and high precision demands. We aimed to determine which aspect of movement was of greater importance to individuals, overall "comfort" or precision. When the requirement was to hold the initial grasp longer, and the end-target was large, we predicted that we would see more thumb-up postures adopted at the beginning state. When the final placement was small and the initial posture was not constrained, we predicted we would see thumb-up postures adopted at the end state. On average, we found that, as beginning-state grasp time increased, more individuals chose beginning-state thumb-up postures. Perhaps, not surprisingly, we found distinct individual differences within our sample. Some individuals seemed to choose beginning-state thumb-up postures nearly 100% of the time, while other individuals chose end-state thumb-up postures nearly 100% of the time. Both the time spent in a posture and its precision requirements influenced planning, but not necessarily in a systematic way.

Dual-task interference in action programming and action planning - Evidence from the end-state comfort effect

In the present study, we examined the extent of interference between a cognitive task (auditory n-back task) and different aspects of motor performance. Specifically, we wanted to find out whether such interference is more pronounced for aspects of planning as compared to programming. Here, motor planning is represented by a phenomenon called the "end-state comfort effect", the fact that we tolerate uncomfortable initial postures in favour of a more comfortable final posture. We asked participants to grasp differently sized cylindrical objects and to place them on target platforms of varying height (grasp-and-place task), So, participants were required to (1) adjust their hand opening to the object width (action programming) and (2) to plan whether to grasp the object higher or lower in order to be able to place it comfortably onto the low or high target platform. We found that participants demonstrated the end-state comfort effect by anticipating the final posture und planning the movement accordingly with a higher object-grasp for low end-target position and lower object-grasp height for high end-target position, respectively. The auditory task was negatively affected by having to perform a visuomotor task in parallel, suggesting that the two tasks share cognitive and attentional resources. No significant impact from the auditory task on the motor tasks was found. Accordingly, it was not possible to determine which of the two motor aspects (programming or planning) contributed more towards the interference observed in the auditory task. To address this question, we carried out a second experiment. For this second experiment we focussed on the interference effects found in the auditory task and contrasted two versions of the grasp-and-place task. In the first version of the task, the height of the target-shelf varied from trial-to-trial but the width of the target object remained the same. We assumed that this version had high planning demands and low programming demands. In the second version the width of the target object varied and the target-shelf height remained constant. Presumably this increased programming demands but reduced planning demands. Significant interference with the auditory task was only found for the first version, supporting the hypothesis that motor planning requires more cognitive resources and thus creates higher multitasking costs.

Anticipating future actions: Motor planning improves with age in wild bearded capuchin monkeys (Sapajus libidinosus)

Self-directed object manipulation tasks illuminate development of motor planning. Grasping strategies that lead to good object control to perform the following action(s) reveal second-order motor planning. Motor planning for efficient grips develops slowly in children. Age-related differences in other primates have been poorly investigated. Here, we investigated anticipatory motor planning of infant, juvenile and adult wild capuchin monkeys grasping a horizontally positioned stick baited to the left or right side (a version of the elevated spoon task). We recorded the grasps capuchins used to bring the baited end of the stick to the mouth. The percentage of efficient radial grips positively correlated with age and adults used efficient grips significantly more frequently than infants. Adult wild capuchins' use of radial grips was higher than that reported for adult captive capuchins in similar tasks, suggesting that experience throughout life may influence motor anticipation. Self-directed object manipulation tasks will be useful to compare this aspect of cognition across primates. A video abstract of this article can be viewed at https://youtu.be/a1Zbr_AQkb8.

The influence of time constraints on posture choices during an end-state comfort task

People adopt comfortable postures for the end states of motor actions (end-state comfort; Rosenbaum & Jorgensen, 1992). The choice to end comfortably often elicits adoption of uncomfortable beginning states, demonstrating that a sequence of movement is planned in advance of movement onset. Many factors influence the choice of comfortable end-state postures including the greater precision and speed afforded by postures at joint angle mid-ranges (Short & Cauraugh, 1999). To date, there has been little evaluation of the hypothesis that postures are chosen based on minimizing the time spent in uncomfortable postures. The aim of this experiment was to examine how the relative time required to hold beginning and end-state postures influenced the choice of posture. Participants moved a two-toned wooden dowel from one location to another with the requirement to grasp the object and place a specified color down. Participants completed four conditions where no postures were held, only one posture was held, or both postures were held. We predicted more thumb-up postures for positions held longer regardless of whether these postures were at the end or beginning state. Results verified that the constraint of holding the initial posture led to decreased end-state comfort supporting the hypothesis that estimation of time spent in postures is an important constraint in planning. We also note marked individual differences in posture choices, particularly when the object was moved to the left.

Intensive Unimanual Training Leads to Better Reaching and Head Control than Bimanual Training in Children with Unilateral Cerebral Palsy

AIMS

To quantify the changes in joint movement control and motor planning of the more-affected upper extremity (UE) during a reach-grasp-eat task in children with Unilateral Spastic Cerebral Palsy (USCP) after either constraint-induced movement therapy (CIMT) or hand-arm bimanual intensive therapy (HABIT).

METHODS

Twenty children with USCP (average age 7.7; MACS levels I-II) were randomized into either a CIMT or HABIT group. Both groups received intensive training 6 h a day for 15 days. Children performed a reach-grasp-eat task before and after training with their more-affected hand using 3D kinematic analysis.

RESULTS

Both groups illustrated shorter movement time during reaching, grasping, and eating phases after training (p < 0.05). Additionally, both intensive training approaches improved joint control with decreased trunk involvement, greater elbow, and wrist excursions during the reaching phase, and greater elbow excursion during the eating phase (p < 0.05). However, only the CIMT group decreased hand curvature during reaching, lowered hand position at grasp, and decreased head rotation during the eating phase (p < 0.05).

CONCLUSIONS

The current findings showed that both CIMT and HABIT improved UE joint control, but there were greater effects of CIMT on the more-affected UE motor planning and head control for children with USCP.

Preschool children adapt grasping movements to upcoming object manipulations: Evidence from a dial rotation task

In adults, the motor plans for object-directed grasping movements reflects the anticipated requirements of intended future object manipulations. This prospective mode of planning has been termed second-order planning. Surprisingly, second-order planning is thought to be fully developed only by 10 years of age, when children master seemingly more complex motor skills. In this study, we tested the hypothesis that already 5- and 6-year-old children consistently use second-order planning but that this ability does not become apparent in tasks that are traditionally used to probe it. We asked 5- and 6-year-olds and adults to grasp and rotate a circular dial in a clockwise or counterclockwise direction. Although children's grasp selections were less consistent on an intra- and inter-individual level than adults' grasp selections, all children adjusted their grasps to the upcoming dial rotations. By contrast, in an also administered bar rotation task, only a subset of children adjusted their grasps to different bar rotations, thereby replicating previous results. The results indicate that 5- and 6-year-olds consistently use second-order planning in a dial rotation task, although this ability does not become apparent in bar rotation tasks.

Perceptuo-motor planning during functional reaching after stroke

In healthy young adults, reaching movements are planned such that the initial grasp position on the object is modulated based on the final task goal. This perceptuo-motor coupling has been described as the end-state comfort effect. This study aimed to determine the extent to which visuo-perceptual and motor deficits, but not neglect, due to stroke impact end-state comfort measured as the grasp-height effect. Thirty-four older adults (17 controls, 17 chronic stroke) performed a functional goal-directed two-sequence task with each arm, consisting of reaching and moving a cylindrical object (drain plunger) from an initial to four target platform heights, standardized to body height, in a block randomized sequence. Arm motor impairment (Fugl-Meyer Assessment) and visual–perceptual deficits (Motor-Free Visual Perception Test) were assessed in stroke subjects, and arm and trunk kinematics were assessed in all subjects. The primary outcome measure of the grasp-height effect was the relationship between the grasp heights used at the home position and the final target platform heights. Mixed model analysis was used for data analysis. The grasp-height effect was present in all participants, but decreased in stroke subjects with visuo-perceptual impairments compared to controls. In stroke subjects with sensorimotor impairments alone, indicated by altered kinematics, the grasp-height effect was comparable to controls. This first study examining the grasp-height effect in individuals with stroke provides new knowledge of the impact of visuo-perceptual deficits on movement planning and execution, which may assist clinicians in selecting more effective treatment strategies to improve perceptuo-motor skills and enhance motor recovery.

Inverting the planning gradient: adjustment of grasps to late segments of multi-step object manipulations

When someone grasps an object, the grasp depends on the intended object manipulation and usually facilitates it. If several object manipulation steps are planned, the first step has been reported to primarily determine the grasp selection. We address whether the grasp can be aligned to the second step, if the second step's requirements exceed those of the first step. Participants grasped and rotated a dial first by a small extent and then by various extents in the opposite direction, without releasing the dial. On average, when the requirements of the first and the second step were similar, participants mostly aligned the grasp to the first step. When the requirements of the second step were considerably higher, participants aligned the grasp to the second step, even though the first step still had a considerable impact. Participants employed two different strategies. One subgroup initially aligned the grasp to the first step and then ceased adjusting the grasp to either step. Another group also initially aligned the grasp to the first step and then switched to aligning it primarily to the second step. The data suggest that participants are more likely to switch to the latter strategy when they experienced more awkward arm postures. In summary, grasp selections for multi-step object manipulations can be aligned to the second object manipulation step, if the requirements of this step clearly exceed those of the first step and if participants have some experience with the task.

Neurophysiology of Grasping Actions: Evidence from ERPs

We use our hands very frequently to interact with our environment. Neuropsychology together with lesion models and intracranial recordings and imaging work yielded important insights into the functional neuroanatomical correlates of grasping, one important function of our hands, pointing toward a functional parietofrontal brain network. Event-related potentials (ERPs) register directly electrical brain activity and are endowed with high temporal resolution but have long been assumed to be susceptible to movement artifacts. Recent work has shown that reliable ERPs can be obtained during movement execution. Here, we review the available ERP work on (uni) manual grasping actions. We discuss various ERP components and how they may be related to functional components of grasping according to traditional distinctions of manual actions such as planning and control phases. The ERP results are largely in line with the assumption of a parietofrontal network. But other questions remain, in particular regarding the temporal succession of frontal and parietal ERP effects. With the low number of ERP studies on grasping, not all ERP effects appear to be coherent with one another. Understanding the control of our hands may help to develop further neurocognitive theories of grasping and to make progress in prosthetics, rehabilitation or development of technical systems for support of human actions.

Habit outweighs planning in grasp selection for object manipulation

Object-directed grasping movements are adapted to intended interactions with an object. We address whether adjusting the grasp for object manipulation is controlled habitually, based on past experiences, or by goal-directed planning, based on an evaluation of the expected action outcomes. Therefore, we asked participants to grasp and rotate a dial. In such tasks, participants typically grasp the dial with an excursed, uncomfortable arm posture, which then allows to complete the dial rotation in a comfortable end-state. We extended this task by manipulating the contingency between the orientation of the grasp and the resulting end-state of the arm. A one-step (control) group rotated the dial to a single target. A two-step group rotated the dial to an initial target and then in the opposite direction. A three-step group rotated the dial to the initial target, then in the opposite direction, and then back to the initial target. During practice, the two-step and three-step groups reduced the excursion of their grasps, thus avoiding overly excursed arm postures after the second rotation. When the two-step and three-step groups were asked to execute one-step rotations, their grasps resembled those that were acquired during the two-step and three-step rotations, respectively. However, the carry-over was not complete. This suggests that adjusting grasps for forthcoming object manipulations is controlled by a mixture of habitual and goal-directed processes. In the present experiment, the former contributed approximately twice as much to grasp selection than the latter.

The development of the grasp height effect as a measure of efficient action planning in children

One effect that illustrates how people adjust aspects of their grasping according to situational constraints is the grasp height effect; when reaching for objects positioned at different heights, adults' grasp height (vertical position of the hand on the object) tends to correlate negatively with object height. This indicates that grasp positions are planned so that they facilitate later placements of the object. The current study investigated the development of the grasp height effect with 3-year-old children, 5-year-old children, and adults. This paradigm allows for studying efficient action planning in the context of a simple task with relatively low motor requirements. Other tasks used so far for studying this issue involved relatively complex adjustments of hand position that younger children might have found difficult to perform. Usually, preschoolers' performance on these tasks was relatively low. We expected that, due to the lower motor requirements of the grasp height paradigm, clearer evidence of efficient planning might be found in preschool children. A second focus of this study was to explore children's behavior in different movement phases of the grasping task. Whereas the task of placing an object at different heights involves planning, putting the object back to its original position seems to depend on recall. The results indicate a significant grasp height effect in all age groups but also significant development across the studied age range. Regarding the second movement phase, when participants were required to put the objects back on the original home shelf, 3- and 5-year-olds did not seem to act based on recall in this context.

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.

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.

Habitual vs non-habitual manual actions: an ERP study on overt movement execution

This study explored the neurophysiological mechanisms underlying the planning and execution of an overt goal-related handle rotation task. More specifically, we studied the neural basis of motor actions concerning the influence of the grasp choice. The aim of the present study was to differentiate cerebral activity between grips executed in a habitual and a non-habitual mode, and between specified and free grip choices. To our knowledge, this is the first study to differentiate cerebral activity underlying overt goal-related actions executed with a focus on the habitual mode. In a handle rotation task, participants had to use thumb-toward (habitual) or thumb-away (non-habitual) grips to rotate a handle to a given target position. Reaction and reach times were shorter for the habitual compared to the non-habitual mode indicating that the habitual mode requires less cognitive processing effort than the non-habitual mode. Neural processes for action execution (measured by event-related potentials (ERPs)) differed between habitual and non-habitual conditions. We found differential activity between habitual and non-habitual conditions in left and right frontal areas from −600 to 200 ms time-locked to reaching the target position. No differential neural activity could be traced for the specification of the grip. The results suggested that the frontal negativity reflected increased difficulty in movement precision control in the non-habitual mode compared to the habitual mode during the homing in phase of grasp and rotation actions.

Choosing actions

Actions that are chosen have properties that distinguish them from actions that are not. Of the nearly infinite possible actions that can achieve any given task, many of the unchosen actions are irrelevant, incorrect, or inappropriate. Others are relevant, correct, or appropriate but are disfavored for other reasons. Our research focuses on the question of what distinguishes actions that are chosen from actions that are possible but are not. We review studies that use simple preference methods to identify factors that contribute to action choices, especially for object-manipulation tasks. We can determine which factors are especially important through simple behavioral experiments.