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Matthew Heath, David A. Westwood and Gordon Binsted

The goal of the present investigation was to explore the putative contributions of feedforward- and feedback-based processes in the control of memory-guided reaching movements. Participants (N = 4) completed an extensive number of reaching movements (2700) to 3 midline targets (20, 30, 40 cm) in 6 visual conditions: full-vision, open-loop, and four memory-guided conditions (0, 200, 400, and 600 ms of delay). To infer limb control, we used a regression technique to examine the within-trial correspondence between the spatial position of the limb at peak acceleration, peak velocity, peak deceleration, and the ultimate movement endpoint. A high degree of within-trial correspondence would suggest that the final position of the limb was largely specified prior to movement onset and not adjusted during the action (i.e., feedforward control); conversely, a low degree of within-trial correspondence would suggest that movements were modified during the reaching trajectory (i.e., feedback control). Full-vision reaches were found to be more accurate and less variable than open-loop and memory-guided reaches. Moreover, full-vision reaches demonstrated only modest within-trial correspondence between the spatial position of the limb at each kinematic marker and the ultimate movement endpoint, suggesting that reaching accuracy was achieved by adjusting the limb trajectory throughout the course of the action. Open-loop and memory-guided movements exhibited strong within-trial correspondence between final limb position and the position of the limb at peak velocity and peak deceleration. This strong correspondence indicates that the final position of the limb was largely determined by processes that occurred before the reach was initiated; errors in the planning process were not corrected during the course of the action. Thus, and contrary to our previous findings in a video-based aiming task, it appears that stored target information is not extensively (if at all) used to modify the trajectory of reaching movements to remembered targets in peripersonal space.

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Slobodan Jaric, Charli Tortoza, Ismael F.C. Fatarelli and Gil L. Almeida

A number of studies have analyzed various indices of the final position variability in order to provide insight into different levels of neuromotor processing during reaching movements. Yet the possible effects of movement kinematics on variability have often been neglected. The present study was designed to test the effects of movement direction and curvature on the pattern of movement variable errors. Subjects performed series of reaching movements over the same distance and into the same target. However, due either to changes in starting position or to applied obstacles, the movements were performed in different directions or along the trajectories of different curvatures. The pattern of movement variable errors was assessed by means of the principal component analysis applied on the 2-D scatter of movement final positions. The orientation of these ellipses demonstrated changes associated with changes in both movement direction and curvature. However, neither movement direction nor movement curvature affected movement variable errors assessed by area of the ellipses. Therefore it was concluded that the end-point variability depends partly, but not exclusively, on movement kinematics.

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Matthew Heath

This investigation tested the proposal that a “highly accurate” and temporally unstable stored target representation is available to the motor system for the online control of memory-guided reaches. Participants reached to a target that was: (a) visible during the response, (b) extinguished at movement onset, and (c) occluded for 0, 500, 1500 and 2,500 ms in advance of response cueing. Additionally, trials were performed with (i.e., limb visible) and without (i.e., limb occluded) vision of the reaching limb. Results showed that limb occluded trials undershot the target location in each target condition, and were characterized by a primarily offline mode of control. In contrast, limb visible trials showed a consistent level of endpoint accuracy for each target condition and elicited more online reaching corrections than limb occluded trials. It is therefore proposed that a reasonably accurate and temporally stable stored target representation can be combined with vision of the moving limb for the online control of memory-guided reaches.

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Jonathan Vaughan, David A. Rosenbaum and Ruud G. J. Meulenbroek

In this article, we review a model of the movement-planning processes that people use for direct reaching, reaching around obstacles, and grasping, and we present observations of subjects' repeated movements of the hand to touch 2 target locations, circumventing an intervening obstacle. The model defines an obstacle as a posture that, if adopted, would intersect with any part of the environment (including the actor himself or herself). The model finds a trajectory that is likely to bring the end-effector to me target by means of a one- or two-stage planning process. Each stage exploits the principles of instance retrieval and instance generation. In the first stage, a goal posture is identified, and the trajectory of a direct transition to that posture is tested for collision. If that direct movement has no collision, the movement to the target is immediately executed in joint space. If. however, the direct movement is foreseen to result in a collision, a second planning stage is invoked. The second planning stage identifies a via posture, movement through which will probably avoid the collision. Movement to and from the via posture is then superimposed on the main movement to the target so that the combined movement reaches the target without colliding with intervening obstacles. We describe the details of instance retrieval and instance generation for each of these planning stages and compare the model's performance with the observed kinematics of direct movements as well as movements around an obstacle. Then we suggest how the model might contribute to the study of movements in people with motor disorders such as spastic hemiparesis.

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Matthew Heath, Kristina Neely and Olav Krigolson

The authors manipulated the availability of monocular and binocular vision during the constituent planning and control stages of a goal-directed reaching task. Furthermore, trials were completed with or without online limb vision to determine whether monocular- or binocular-derived ego-motion cues influence the integration of visual feedback for online limb corrections. Results showed that the manipulation of visual cues during movement planning did not influence planning times or overall kinematics. During movement execution, however, binocular reaches—and particularly those completed with online limb vision—demonstrated heightened endpoint accuracy and stability, a finding directly linked to the adoption of a feedback-based mode of reaching control (i.e., online control). In contrast, reaches performed with online monocular vision produced increased endpoint error and instability and demonstrated reduced evidence of feedback-based corrections (i.e., offline control). Based on these results, the authors propose that the combination of static (i.e., target location) and dynamic (i.e., the moving limb) binocular cues serve to specifically optimize online reaching control. Moreover, results provide new evidence that differences in the kinematic and endpoint parameters of binocular and monocular reaches reflect differences in the extent to which the aforementioned engage in online and offline modes of movement control.

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Hsieh-ching Chen, Keh-chung Lin, Chia-ling Chen and Ching-yi Wu

This study evaluated the effect of context on the reaching performance of the unaffected arm and postural control while standing in patients with right cerebral vascular accidents (RCVA) and in healthy adults. Fifteen subjects with RCVA and sixteen healthy subjects performed tasks with the right hand under two conditions while standing. One condition involved moving coins forward on a table as far as possible (concrete task) and the other reaching forward without a target (abstract task). Forward reaching distance, forward displacement and lateral shift of center of pressure (CoP), and weight distribution were the dependent variables. The RCVA and control groups achieved significantly greater reaching distances in the concrete task than in the abstract one. The RCVA group showed significantly less lateral shift of the CoP and placed more weight on the affected leg in the concrete than the abstract task, whereas the control group made a greater lateral shift in the concrete task and had a similar mean ratio of weight distribution during both tasks. The results demonstrate that a functional application of task targets may favorably modulate both reaching and posture performance and exert various positive affects on postural control. Such applications may have a place in the therapeutic recovery efforts for patients afflicted with stroke.

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Matthew Heath and David A. Westwood

We investigated whether a representation of a visual target can be stored in memory and used to support the online control of reaching movements. To distinguish between the use of a stored target representation for movement planning versus online control, we employed a novel movement environment in which participants could not fully plan their action in advance of movement initiation; that is, the spatial mapping between the movement of a computer mouse and the on-screen movement of a cursor was randomly varied from trial to trial. As such, participants were required to use online control to reach the target position. Reaches were examined in full-vision and three memory-dependent conditions (0, 2, and 5 s of delay). Absolute constant error did not accumulate between full-vision and brief delay trials (i.e., the 0-s delay), suggesting a stored representation of the visual target can be used for online control of reaching given a sufficiently brief delay interval. Longer delay trials (2 and 5 s) were less accurate and more variable than brief delay trials; however, the residual accuracy of these memory-dependent actions suggests that the motor system may have access to a stored representation of the visual target for online control processes for upwards of 5 s following target occlusion.

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Annieck X.C. Ricken, Simon J. Bennett and Geert J.P. Savelsbergh

Coordination of reaching with the impaired and non-impaired arm in 10 children with spastic hemiparetic cerebral palsy (SHCP) was examined in a stationary ball and moving ball context. Kinematic data on trunk, arm, and wrist movements, and coordination patterns between joint angles of elbow, shoulder, and trunk, were analyzed to determine how reaching was influenced by impairment and object motion. Results showed longer deceleration time and movement time and greater trunk contribution following decreased elbow and shoulder excursion when reaching with the impaired arm compared to the non-impaired arm. The coordination of joint angle pairs showed little linearity for the impaired arm, indicating more segmented movements of shoulder and elbow. It was also found that coordination patterns between elbow, shoulder, and trunk displayed less similarity when reaching with the impaired arm compared to the non-impaired arm in both stationary and moving ball conditions. Regardless of the timing constraints, children with SHCP could make successful interceptions using the impaired arm, indicating that they coordinated and controlled the degrees of freedom within their own functional possibilities.

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Daniela Mattos, Joshua Kuhl, John P. Scholz and Mark L. Latash

The concept of motor equivalent combinations of arm muscles, or M-modes, was investigated during reaching to insert a pointer into a cylindrical target with and without an elbow perturbation. Five M-modes across 15 arm/scapula muscles were identified by principal component analysis with factor extraction. The relationship between small changes in the M-modes and changes in the position/orientation of the pointer were investigated by linear regression analyses. The results revealed a motor equivalent organization of the M-modes for perturbed compared with nonperturbed reaches, both with respect to hand position and orientation, especially in the first 100-ms postperturbation. Similar findings were obtained for motor equivalence computed based on changes in the joint configuration, although the kinematically defined motor equivalence was stronger for pointer orientation. The results support the hypothesis that the nervous system organizes muscles into M-modes and flexibly scales M-mode activation to preserve stable values of variables directly related to performance success.

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Geetanjali Gera, Kelsey E. McGlade, Darcy S. Reisman and John Peter Scholz

In this study, we investigated deficits in coordination of trunk muscle modes involved in the stabilization of the trunk’s trajectory for reaching upward and downward beyond functional arm length. Trunk muscle activity from 10 stroke survivors (8 men, 2 women; 64.1 ± 10.5 years old) and 9 healthy control subjects (7 men, 2 women; 59.3 ± 9.3 years old) was analyzed. Coordination of trunk muscle modes to stabilize the trunk trajectory was investigated using the uncontrolled manifold (UCM) analysis. The UCM analysis decomposes the variability of muscle modes into good and bad variability. The good variability does not affect the control of trunk motion, whereas the bad variability does. In stroke survivors, deficits in the ability to flexibly combine trunk muscle modes was associated with reduced ability to minimize those combinations of trunk muscle modes that led to an error in trunk trajectory (bad variability), and this had a greater effect on reaching upward. This reduced coordination of trunk muscle modes during reaching was correlated with a clinical measure of trunk impairment.