Control of Trajectory Modifications in Target-Directed Reaching

Human reaching movements to fixed and displaced visual targets were recorded and compared with simulated movements generated by using a two-joint arm model based on the equilibrium-point (EP) hypothesis (lambda model) of motor control (Feldman, 1986). The aim was to investigate the form of central control signals underlying these movements. According to this hypothesis, movements result from changes in control variables that shift the equilibrium position (EP) of the arm. At any time, muscle activations and forces will depend on the difference between the arm's EP and its actual position and on the limb's velocity. In this article, we suggest that the direction of EP shift in reaching is specified at the hand level, whereas the rate of EP shift may be specified at the hand or joint level. A common mechanism underlying reaching to fixed and displaced targets is proposed whereby the EP of the hand shifts in a straight line toward the present target. After the target is displaced, the direction of the hand EP shift is modified toward the second target. The results suggest that the rate of shift of the hand EP may be modified for movements in different parts of the work space. The model, with control signals that vary in a simple fashion over time, is able to generate the kinematic patterns observed empirically.     

Control of Integrated Task Sequences Shapes Components of Reaching

Reaching toward an object usually consists of a sequence of elemental actions. Using a reaching task sequence, the authors investigated how task elements of that sequence affected feedforward and feedback components of the reaching phase of the movement. Nine right-handed adults performed, with their dominant and nondominant hands, 4 tasks of different complexities: a simple reaching task; a reach-to-grasp task; a reach-to-grasp and lift object task; and a reach-to-grasp, lift, and place object task. Results showed that in the reach-to-grasp and lift object task more time was allocated to the feedforward component of the reach phase, while latency between the task elements decreased. We also found between-hand differences, supporting previous findings of increased efficiency of processing planning-related information in the preferred hand. The presence of task-related modifications supports the concept of contextual effects when planning a movement.     

Visual Feedback and Positioning Movements

Subjects (n = 60) performed both the reproduction and learning of a linear positioning movement under one of five visual feedback conditions. Results from two experiments indicated that visual cues from the task display augmented information available from visual feedback of the movement per se. Extraneous cues from the task display have clearly confounded the manipulation of visual feedback in previous positioning studies. When these cues are eliminated, visual distance information seems more useful than visual location information.     

A Test of the Schema Theory of Discrete Motor Learning

The recall schema in a response class of discrete movements was investigated, with variability of practice being manipulated before transfer to a new instance in the response class. After the transfer, the varied-practice group performance was superior to the continuous practice group. The interaction of the low-and high-variability groups from training to recall performance supported the prediction that a group with more variable training would exhibit an increased rate of learning when introduced to a new instance of the response class. After two days, the superiority of the high-variability group over the low disappeared, suggesting the memory representations of the two groups were no different under withdrawal of knowledge of results.     

The Effect of Posture on Early Reaching Movements

Infants of about 5 months of age who have just mastered the ability to reach succeed more frequently in contacting an object when they are seated upright than when they are supine or reclined. That effect of posture disappears in the subsequent months. Whether that effect can be attributed either to insufficient muscular strength or to insufficient control over the mechanically unstable arm was the subject of the present investigation. Kinematics and electromyography (EMG) of reaching movements of 8 sitting and supine infants at 12, 16, and 20 weeks of age were recorded. Maximum levels of shoulder torque as well as kinematic stability measures were similar in both postures. Coactivation levels and the frequency of on-off switching of muscles turned out to be higher in the sitting than in the supine posture. The authors suggest that the difference in reaching behavior resulted from the degree of error in the feedforward control signal that was allowed by the different postures rather than either insufficient muscular strength or insufficient control over the mechanically unstable arm.     

Determinants of Offline Processing of Visual Information for the Control of Reaching Movements

The authors investigated the use of visual feedback as a form of knowledge of results (KR) for the control of rapid (200-250 ms) reaching movements in 40 participants. They compared endpoint accuracy and intraindividual variability of a full-vision group (FV) with those of no-vision groups provided with KR regarding (a) the endpoint in numerical form, (b) the endpoint in visual form, or (c) the endpoint and the trajectory in visual form (DEL). The FV group was more accurate and less variable than were the no-vision groups, and the analysis of limb trajectory variability indicated that their superior performance resulted primarily from better movement planning rather than from online visual processes. The FV group outperformed the DEL group even though both groups were obtaining the same amount of spatial visual information from every movement. That finding suggests that the effectiveness with which visual feedback is processed offline is not a simple function of the amount of visual information available, but depends on how that information is presented.     

Roles of Visual Information for Control of Reaching Movements in Children

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

A Modeling Study of Potential Sources of Curvature in Human Reaching Movements

The authors of this article suggest that the slight but consistent posture-dependent curvature of the spatial paths in the kinematic transformation between intrinsic and extrinsic coordinates may result in a systematic curvature of movements initially planned as straight-line trajectories toward the target. A kinematic planning model is presented that takes into account the anisotropy of the intrinsic and extrinsic transformation and tends to avoid movements that require excessive joint rotations by introducing slight deviations from a straight-line trajectory. Preliminary simulations showed reasonably good agreement with experimental data, especially considering that the current model is strictly based on kinematics. A quantitative analysis showed that the strategy used in the model achieves a favorable compromise between straight-line movements and angular joint changes: By slightly increasing the spatial length of the movement (i.e., by introducing curvature), an individual can greatly reduce the total amount of joint rotation required to produce the movement.     

Visual Information and the Control of Reaching in Children: A Comparison Between Children With and Without Developmental Coordination Disorder

Three experiments were performed on reach and grasp in 9- to 10-year-old children (8 controls and 8 with developmental coordination disorder [DCD]). In normal reaching, children in the DCD group were less responsive to the accuracy demands of the task in controlling the transport component of prehension and spent less time in the deceleration phase of hand transport. When vision was removed as movement began, children in the control group spent more time decelerating and reached peak aperture earlier. Children in the DCD group did not do that, although, like the control group, they did increase grip aperture in the dark. When depth cues were reduced and only the target or only the target and hand were visible, children in the control group used target information to maintain the same grip aperture in all conditions, but DCD children behaved as if the target was not visible. Throughout the studies, the control group of 9- to 10-year-olds did not produce adult-like adaptations to reduced vision, suggesting that they had not yet attained adult-like integration of sensory input. Compared with control children, children with DCD did not exhibit increased dependence on vision but showed less recognition of accuracy demands, less adaptation to the removal of vision, and less use of minimal visual information when it was available.     

Immediate Effect of Training at the Onset of Reaching in Preterm Infants: Randomized Clinical Trial

The authors' aim was to investigate the immediate effect of a single specific training session (serial varied practice), of short duration on the kinematic parameters of reaching, in the period of the emergence of the skill in preterm and low birth weight infants. The study included 16 infants of both sexes, born at a mean gestational age of 32.13 (±1.36) weeks and mean birth weight of 1720.94 (±358.46) g. The infants were randomly divided into 2 groups: experimental and control. The experimental group was given a 5-min training session in reaching, while the control group received no training. The results showed significant differences in peak velocity in the intra (Z = –2.10, p = .036) and intergroup (U = 9.00, p = .016) evaluations, which decreased in the experimental group after training. Cohen's d test for clinical relevance suggested that the specific, short duration training proved effective in promoting slower reaches, with greater adjustment and lower number of units of movement. These results are positive for preterm infants given that these parameters more closely resemble the typical development of mature reaching behaviors in term infants, which suggests that this protocol of reaching training (serial varied practice) could be used as an evidence-based intervention strategy.     

运动轨迹和方向对视觉空间关系加工的影响

视觉空间关系分为类别与数量两种表征。研究通过控制起参照作用的运动客体消失状态与定位客体出现时间,分析客体运动信息对两种关系判断的影响。结果发现:运动信息在各间隔时间对类别任务影响呈序列排列,运动方向较轨迹信息有优势,轨迹较法线信息有优势;数量表征仅在0ms间隔受运动方向促进,在其他间隔下,三种位置上的判断均无差异。结果表明运动客体类别表征比数量表征更持久;且相对类别表征,数量判断作为精确距离表征不易受运动方向和轨迹信息影响。     

Dynamics of Pushing

A standing individual can use several strategies for modulating pushing force magnitude. Using a static model, researchers have shown that the efficacy of those strategies varies considerably. In the present article, the authors propose a human motor control dynamic model for analyzing transients that occur when an individual is asked to modulate force magnitude. According to the model, the impedances of both the upper and the lower limbs influence the time course of force variations and foot placement has a profound effect on pushing force dynamics. With a feet-together posture, the center of pressure has a limited range of motion and changes in force may be preceded by initial changes in the opposite direction; that is, to decrease force, an individual must first increase force. When the feet are placed apart, individuals can move the center of pressure over a much larger range, thereby modulating pushing force magnitude, without reversing behavior, over a larger range of force magnitudes. Therefore, the best way to control pushing force at the hand may be by using the foot.     

Geometries and Dynamics of a Rod Determine How It Is Used for Reaching

Displacing an object with a hand-held rod provided a simple paradigm for studying tool use. The authors asked how reaching was affected by manipulations of rod properties. Adults held a rod (length = .10 to 1.5 m), with its tip in the air; walked toward an object on a table; chose a place to stop; and displaced the object with the rod's tip. In 3 experiments (Ns = 9, 22, and 17 participants), the authors manipulated rod length, mass, and mass distribution to determine whether and how geometric and dynamic properties affected the chosen distance and the posture. Both the chosen stopping distance and the postures were well accommodated to rod characteristics. Postural adaptations took place only in the arm, which was organized as a synergy. Predictably, rod length explained most of the variance, but small and reliable differences in both distance and posture depended on mass and mass distribution. The chosen distance anticipated not only rod length but also the upcoming posture needed to control the rod.     

Self-Sitting and Reaching in 5- to 8-Month-Old Infants: The Impact of Posture and Its Development on Early Eye-Hand Coordination

The relation between progress in the control of posture (i.e., the achievement of self-sitting posture) and the developmental transition from two-handed to one-handed engagement in infant reaching was investigated. Two groups of 5- to 8-month-old infants, who were either able or yet unable to sit on their own, were videotaped while they reached for objects in four different posture conditions that provided varying amounts of body support. Videotapes of infant reaches were microanalyzed to determine the relative engagement of both hands during reaches. Results demonstrate the interaction between postural development and the morphology of infant reaching. Nonsitting infants displayed symmetrical and synergistic engagement of both arms and hands while reaching in all but the seated posture condition. Sitting infants, by contrast, showed asymmetrical and lateralized (one-handed) reaches in all posture conditions. Results also show that, aside from posture, the perceived spatial arrangement of the object display is a determinant of infant reaching. Combined, these results are discussed as evidence for the interaction between postural and perceptual development in the control of early eye-hand coordination.     

Anticipatory reaching of seven- to eleven-month-old infants in occlusion situations

The present study examined 7- to 11-month-old infants’ anticipatory and reactive reaching for temporarily occluded objects. Infants were presented with laterally approaching objects that moved at different velocities (10, 20, and 40 cm/s) in different occlusion situations (no-, 20 cm-, and 40 cm-occlusion), resulting in occlusion durations ranging between 0 and 4 s. Results show that except for object velocity and occlusion distance, occlusion duration was a critical constraint for infants’ reaching behaviors. We found that the older infants reached more often, but that an increase in occlusion duration resulted in a decline in reaching frequency that was similar across age groups. Anticipatory reaching declined with increasing occlusion duration, but the adverse effects for longer occlusion durations diminished with age. It is concluded that with increasing age infants are able to retain and use information to guide reaching movements over longer periods of non-visibility, providing support for the graded representation hypothesis (Jonsson &; von Hofsten, 2003) and the two-visual systems model (Milner &; Goodale, 1995).     

Structuring of Early Reaching Movements: A Longitudinal Study

Reaches, performed by 5 infants, recorded at 19 weeks of age and every third week thereafter until 31 weeks of age, were studied quantitatively. Earlier findings about action units were confirmed. At all ages studied, movements were structured into phases of acceleration and deceleration. Reaching trajectories were found to be relatively straight within these units and to change direction between them. It was also found that at all ages, there was generally one dominating transport unit in each reach. The structuring of reaching movements changed in four important ways during the period studied. First, the sequential structuring became more systematic with age, with the dominating transport unit beginning the movement. Second, the duration of the transport unit became longer and covered a larger proportion of the approach. Third, the number of action units decreased with age, approaching the two-phase structure of adult reaching. Finally, reaching trajectories became straighter with age.     

Reaching the Champions of Social Justice

Since the formation of the World Council of Churches (WCC) in 1948, the ecumenical voice against social injustice in the church and society has been strengthening. As one expression of unity among the fellowship, the WCC embarked in 2013 on a Pilgrimage of Justice and Peace to work, pray, and walk together for life-affirming economies, climate change, nonviolent peace building, and reconciliation and human dignity. Champions of these issues exist within the ecumenical movement. Yet one also finds that champions of one theme are pushing back on another theme. Sometimes it is due to diversity of contexts and biblical and theological interpretations. At other times it is due to unconscious bias about the holistic nature of God's mission of justice for all God's people and creation. This paper grapples with this question: Why are people who are so alive to economic and ecological injustice sometimes blind to racial and gender injustice? To answer this, I explore the existence of conscious and unconscious bias despite the many powerful ecumenical statements that have been issued on racial justice.     

Constraints on Arm Selection Processes When Reaching: Degrees of Freedom and Joint Amplitudes Interact to Influence Limb Selection

With an interest in identifying the variables that constrain arm choice when reaching, the authors had 11 right-handed participants perform free-choice and assigned-limb reaches at 9 object positions. The right arm was freely selected 100% of the time when reaching to positions at 30° and 40° into right hemispace. However, the left arm was freely selected to reach to positions at ?30° and ?40° in left hemispace 85% of the time. A comparison between free- and assigned-limb reaching kinematics revealed that free limb selection when reaching to the farthest positions was constrained by joint amplitude requirements and the time devoted to limb deceleration. Differences between free- and assigned-arm reaches were not evident when reaching to the midline and positions of ±10°, even though the right arm was freely selected most often for these positions. Different factors contribute to limb selection as a function of distance into a specific hemispace.     

Visuokinesthetic Realignment in a Video-Controlled Reaching Task

The authors investigated the accuracy of horizontal pointing movements toward a visual target viewed on a vertical video monitor; the view included a directional distortion between perceptual and action spaces. Although accurate coding of the movement vector in a relative (visual) system of coordinates has been found to occur when there is a prismatic perturbation, provided that the hand and the target are continuously visible, such accurate performance has never been reported for video-controlled situations with larger deviations. To evaluate whether visual relative coding is task specific or depends on the magnitude of the induced misalignment, the authors manipulated the intensity of directional perturbation (10° or 40°) in a video-controlled task. Whatever the directional bias, participants (N = 40) were initially inaccurate but adapted quickly within a few trial rehearsals, with a concomitant recalibration of segmental proprioception. In contrast with prism studies, relative coding of the hand-to-target vector seemed not to be operative in video-controlled situations, suggesting that target location is specified in an egocentric system of reference that includes hand-related proprioceptive signals, despite the presence of a (consciously) detected misalignment between visual and kinesthetic systems.     

Lifting weights in neonates: Developing visual control of reaching

To test whether newborn babies take account of external forces in moving their limbs, spontaneous arm-waving movements were measured while the baby lay supine with its head turned to one side. Free-hanging weights, attached to each wrist by strings passing over pulleys, pulled on the arms in the direction of the toes. The results showed the babies applied compensatory forces to keep the hand they faced moving in the same region. In contrast, the (invisible) contra-lateral hand was pulled down by the weights. In a second experiment, where the arms were occluded, both arms were pulled down, suggesting that sight of the arm was necessary in compensating for the weight. In a third, conclusive experiment the babies viewed the arm they were not facing on a small video-monitor and this time the babies kept the visible contra-lateral hand up despite the weights. The results challenge the general view that spontaneous arm movements of neonates are purposeless and either reflexive or due to spontaneous patterned efference to the muscles. Instead, the findings suggest that in waving their arms, neonates are developing visual control of reaching.