Movement trajectory smoothness is not associated with the endpoint accuracy of rapid multi-joint arm movements in young and older adults

The minimum variance theory proposes that motor commands are corrupted by signal-dependent noise and smooth trajectories with low noise levels are selected to minimize endpoint error and endpoint variability. The purpose of the study was to determine the contribution of trajectory smoothness to the endpoint accuracy and endpoint variability of rapid multi-joint arm movements. Young and older adults performed arm movements (4 blocks of 25 trials) as fast and as accurately as possible to a target with the right (dominant) arm. Endpoint accuracy and endpoint variability along with trajectory smoothness and error were quantified for each block of trials. Endpoint error and endpoint variance were greater in older adults compared with young adults, but decreased at a similar rate with practice for the two age groups. The greater endpoint error and endpoint variance exhibited by older adults were primarily due to impairments in movement extent control and not movement direction control. The normalized jerk was similar for the two age groups, but was not strongly associated with endpoint error or endpoint variance for either group. However, endpoint variance was strongly associated with endpoint error for both the young and older adults. Finally, trajectory error was similar for both groups and was weakly associated with endpoint error for the older adults. The findings are not consistent with the predictions of the minimum variance theory, but support and extend previous observations that movement trajectories and endpoints are planned independently.     

Changes in predictive motor control in drop-jumps based on uncertainties in task execution

Drop-jumps are controlled by predictive and reactive motor strategies which differ with respect to the utilization of sensory feedback. With reaction, sensory feedback is integrated while performing the task. With prediction, sensory information may be used prior to movement onset. Certainty about upcoming events is important for prediction. The present study aimed at investigating how uncertainties in the task execution affect predictive motor control in drop-jumps. Ten healthy subjects (22±1 years, M±SD) participated. The subjects performed either (i) drop-jumps by knowing that they might had to switch to a landing movement upon an auditory cue, which was sometimes elicited prior to touch-down (uncertainty). In (ii), subjects performed drop-jumps by knowing that there would be no auditory cue and consequently no switch of the movement (certainty). The m. soleus EMG prior to touch-down was higher when subjects knew there would be no auditory cue compared to when subjects performed the same task but switching from drop-jump to landing was possible (uncertainty). The EMG was reversed in the late concentric phase, meaning that it was higher in the high uncertainty task. The results of the present study showed that the muscular activity was predictively adjusted according to uncertainties in task execution. It is argued that tendomuscular stiffness was the variable responsible for the adjustment of muscular activity. The required tendomuscular stiffness was higher in drop-jumps than in landings. Consequently, when it was not certain whether to jump or to land, muscular activity and therefore tendomuscular stiffness was reduced.     

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

Grasping movements are often planned in a way that they end in a position where joints are in an anatomically medial position. This behaviour is termed the “end-state comfort” (ESC) effect (Rosenbaum et al., 1990). We suggest that the anatomically medial position is favoured to control the most difficult part of the movement. In most experiments investigating ESC, objects have to be placed onto a target location, and the highest precision demand occurs at the end of the movement. Thus, ESC is confounded with movement difficulty. In this study, we dissociate movement difficulty and ESC. In our experiments, participants had to execute a task where the critical part of the movement was either at the end or at the beginning of the movement. Participants' grasping behaviour confirmed the hypothesis that movement planning is constrained by a goal for optimal control during the part of the movement that demands the highest precision, rather than by a goal to end in a comfortable state (Rosenbaum, Chapman, Weigelt, Weiss, & van der Wel, 2012). We identified recall and movement plan generating processes of motor planning (Cohen & Rosenbaum, 2004), that ensure the optimal control in the critical part of movement. Our results indicate that recall processes depend on motor experience which is acquired in different time scales. We suggest that motor planning processes are triggered only if the costs for executing movements controlled by recall processes exceed the costs for generating a motor plan.     

Predicting aperture crossing behavior from within-trial metrics of motor control reliability

Actors utilize intrinsically scaled information about their geometric and dynamic properties when perceiving their ability to pass through openings. Research about dynamic factors of affordance perception have shown that the reliability of a given movement, or the precision of one's motor control for that movement, increase the buffer space used when interacting with the environment. While previous work has assessed motor control reliability as a person-level variable (i.e., behavior is aggregated across many trials), the current study assessed how characteristics of motor control and movement reliability within a single trial impact real-time action strategies for passing through apertures. Participants walked 5 m and then passed through apertures of various widths while their motions were tracked. For each trial, we collected walking time-series data, then calculated the magnitude and complexity of the lateral sway. Assessing two behavioral measures of the buffer, we found that trial-level metrics of motor control reliability, in addition to the person-level metrics previously studied, significantly predicted the buffer on each trial. This study supports previous claims that actors pick up real-time information about their dynamic capabilities in order to perceive and act within their environment. Further, the study recommends that future affordance research consider trial-level movement data, including nonlinear analyses that inform the pattern and structure of motor control reliability.     

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.     

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.     

Control of rapid aimed hand movements: the one-target advantage

A series of 8 experiments examined the phenomenon that a rapid aimed hand movement is executed faster when it is performed as a single, isolated movement than when it is followed by a second movement (the 1-target advantage). Three new accounts of this effect are proposed and tested: the eye movement hypothesis, the target uncertainty hypothesis, and the movement integration hypothesis. Data are reported that corroborate the 3rd hypothesis, but not the first 2 hypotheses. According to the movement integration hypothesis, the first movement in a series is slowed because control of the second movement may overlap with execution of the first. It is shown that manipulations of target size and movement direction mediate this process and determine the presence and absence of the 1-target advantage. Possible neurophysiological mechanisms and implications for motor control theory are discussed.     

The schematic representation of effector function underlying perceptual-motor skill

Conceptual and methodological problems related to Schmidt' (1975) motor schema theory are discussed. In particular, the motor schema is interpreted as representing the dynamics of the system being controlled, which may or may not be associated with a referent movement pattern. Furthermore, it is suggested that prior familiarity with a control system's dynamics is a critical but uncontrolled factor in tests of the theory, and largely accounts for their equivocal findings. These ideas are examined by two experiments in which subjects had to bimanually control the movement of a computer-displayed cursor along a track on a CRT screen. Different track orientations required different patterns of movement not entailing a single generalized motor program. Experiment 1 shows that variable track performance with a given control system, results in better transfer to novel tracks than does fixed practice. Experiment 2 demonstrates that altering the control system disrupts performance whether or not the required movements remain the same. These results indicate the need for a fundamental modification of schema theory, such that a schematic representation of effector-environment relations (effector function) is available independently of particular movement patterns used in its acquisition.     

The Schematic Representation of Effector Function Underlying Perceptual-Motor Skill

Conceptual and methodological problems related to Schmidt’s (1975) motor schema theory are discussed. In particular, the motor schema is interpreted as representing the dynamics of the system being controlled, which may or may not be associated with a referent movement pattern. Furthermore, it is suggested that prior familiarity with a control system’s dynamics is a critical but uncontrolled factor in tests of the theory, and largely accounts for their equivocal findings. These ideas are examined by two experiments in which subjects had to bimanually control the movement of a computer-displayed cursor along a track on a CRT screen. Different track orientations required different patterns of movement not entailing a single generalized motor program. Experiment 1 shows that variable track performance with a given control system, results in better transfer to novel tracks than does fixed practice. Experiment 2 demonstrates that altering the control system disrupts performance whether or not the required movements remain the same. These results indicate the need for a fundamental modification of schema theory, such that a schematic representation of effector-environment relations (effector function) is available independently of particular movement patterns used in its acquisition.     

Implicit motor learning and complex decision making in time-constrained environments

The cost-effectiveness of the implicit (procedural) knowledge that supports motor expertise enables surprisingly efficient performance when a decision and an action must occur in close temporal proximity. The authors argue that if novices learn the motor component of performance implicitly rather than explicitly, then they will also be efficient when they make a decision and execute an action in close temporal proximity. Participants (N = 35) learned a table tennis shot implicitly or explicitly. The authors assessed participants' motor performance and movement kinematics under conditions that required a concurrent low-complexity decision or a concurrent high-complexity decision about where to direct each shot. Performance was disrupted only for participants who learned explicitly when they made high-complexity decisions but not when they made low-complexity decisions. The authors conclude that implicit motor learning encourages cognitively efficient motor control more than does explicit motor learning, which allows performance to remain stable when time constraints call for a complex decision in tandem with a motor action.     

Movement segmentation and visual perturbation increase developmental differences in motor control and learning

We examined the developmental differences in motor control and learning of a two‐segment movement. One hundred and five participants (53 female) were divided into three age groups (7–8 years, 9–10 years and 19–27 years). They performed a two‐segment movement task in four conditions (full vision, fully disturbed vision, disturbed vision in the first movement segment and disturbed vision in the second movement segment). The results for movement accuracy and overall movement time show that children, especially younger children, are more susceptible to visual perturbations than adults. The adults’ movement time in one of the movement segments could be increased by disturbing the vision of the other movement segment. The children's movement time for the second movement segment increased when their vision of the first movement segment was disturbed. Disturbing the vision of the first movement segment decreased the percentage of central control of the second movement in younger children, but not in the other two age groups. The children's normalized jerk was more easily increased by visual perturbations. The children showed greater improvement after practice in the conditions of partial vision disturbance. As the participants’ age increased, practice tended to improve their feedforward motor control rather than their feedback motor control. These results suggest that children's central movement control improves with age and practice. We discuss the theoretical implications and practical significance of the differential effects of visual perturbation and movement segmentation upon motor control and learning from a developmental viewpoint.     

Schema theory: a critical appraisal and reevaluation

The authors critically review a number of the constructs and associated predictions proposed in schema theory (R. A. Schmidt, 1975). The authors propose that new control and learning theories should include a reformulated (a) notion of a generalized motor program that is not based on motor program but still accounts for the strong tendency for responses to maintain their relative characteristics; (b) mechanism or processes whereby an abstract movement structure based on proportional principles (e.g., relative timing, relative force) is developed through practice; and (c) explanation for parameter learning that accounts for the benefits of parameter variability but also considers how variability is scheduled. Furthermore, they also propose that new theories of motor learning must be able to account for the consistent findings spawned as a result of the schema theory proposal and must not be simply discounted because of some disfavor with the motor program notion, in general, or schema theory, more specifically.     

Action goal selection and motor planning can be dissociated by tool use

The preparation of eye or hand movements enhances visual perception at the upcoming movement end position. The spatial location of this influence of action on perception could be determined either by goal selection or by motor planning. We employed a tool use task to dissociate these two alternatives. The instructed goal location was a visual target to which participants pointed with the tip of a triangular hand-held tool. The motor endpoint was defined by the final fingertip position necessary to bring the tool tip onto the goal. We tested perceptual performance at both locations (tool tip endpoint, motor endpoint) with a visual discrimination task. Discrimination performance was enhanced in parallel at both spatial locations, but not at nearby and intermediate locations, suggesting that both action goal selection and motor planning contribute to visual perception. In addition, our results challenge the widely held view that tools extend the body schema and suggest instead that tool use enhances perception at those precise locations which are most relevant during tool action: the body part used to manipulate the tool, and the active tool tip.     

No evidence for accurate visuomotor memory: systematic and variable error in memory-guided reaching

The authors explored whether the motor system has access to highly accurate information about the aiming environment after visual occlusion. Participants (N = 14) reached to 1 of 3 midsagittal targets in 4 visual conditions (open-loop, brief-delay, 500-ms delay, and 2,000-ms delay). In all conditions, the aiming environment was first viewed for 2,000 ms. Movements were cued immediately after the initial viewing period in the open-loop and brief-delay conditions. Vision was not occluded until movement onset in the open-loop condition, whereas vision was occluded coincidentally with the movement cue in the brief-delay condition. In the 2 longer delay conditions, the movement was cued following a 500- or a 2,000-ms no-vision delay period. Participants overshot the target in the open-loop condition, but that tendency was significantly reduced in the 3 delay conditions. Moreover, endpoint variability was greater in the 3 delay conditions than in the open-loop condition. A speed-accuracy tradeoff account could not explain the differences between open-loop and delayed reaching. Those findings suggest that the motor system does not have access to highly accurate information about the aiming environment for any appreciable period of time following visual occlusion, consistent with the view that the visuomotor system operates in real time.     

Developmental Features of Rapid Aiming Arm Movements Across the Lifespan

Using a lifespan approach, the authors investigated developmental features of the control of ballistic aiming arm movements by manipulating movement complexity, response uncertainty, and the use of precues. Four different age groups of participants (6- and 9-year-old boys and girls and 24- and 73-year- old men and women, 20 participants in each age group) performed 7 types of rapid aiming arm movements on the surface of a digitizer. Their movement characteristics such as movement velocity, normalized jerk, relative timing, movement linearity, and intersegment intervals were profiled. Analyses of variance with repeated measures were conducted on age and task effects in varying movement complexity (Study 1), response uncertainty (Study 2), and precue use (Study 3) conditions. Young children and senior adults had slower, more variant, less smooth, and less linear arm movements than older children and young adults. Increasing the number of movement segments resulted in slower and more variant responses. Movement accuracy demands or response uncertainty interacted with age so that the 6- and 74-year-old participants had poorer performances but responded similarly to the varying treatments. Even though older children and young adults had better performances than young children and senior adults, their arm movement performance declined when response uncertainty increased. The analyses suggested that young children's and senior adults' performances are poorer because less of their movement is under central control, and they therefore use on-line adjustments. In addition, older children and young adults use a valid precue more effectively to prepare for subsequent movements than do young children and senior adults, suggesting that older children and young adults are more capable of organizing motor responses than arc young children and senior adults.     

Developmental features of rapid aiming arm movements across the lifespan

Using a lifespan approach, the authors investigated developmental features of the control of ballistic aiming arm movements by manipulating movement complexity, response uncertainty, and the use of precues. Four different age groups of participants (6- and 9-year-old boys and girls and 24- and 73-year-old men and women, 20 participants in each age group) performed 7 types of rapid aiming arm movements on the surface of a digitizer. Their movement characteristics such as movement velocity, normalized jerk, relative timing, movement linearity, and intersegment intervals were profiled. Analyses of variance with repeated measures were conducted on age and task effects in varying movement complexity (Study 1), response uncertainty (Study 2), and precue use (Study 3) conditions. Young children and senior adults had slower, more variant, less smooth, and less linear arm movements than older children and young adults. Increasing the number of movement segments resulted in slower and more variant responses. Movement accuracy demands or response uncertainty interacted with age so that the 6- and 74-year-old participants had poorer performances but responded similarly to the varying treatments. Even though older children and young adults had better performances than young children and senior adults, their arm movement performance declined when response uncertainty increased. The analyses suggested that young children's and senior adults' performances are poorer because less of their movement is under central control, and they therefore use on-line adjustments. In addition, older children and young adults use a valid precue more effectively to prepare for subsequent movements than do young children and senior adults, suggesting that older children and young adults are more capable of organizing motor responses than are young children and senior adults.     

Target location effects in tapping tasks

Previous experiments on tracking have shown that target location (measured in terms of the distance of the target from the boundary circumscribing the area of movement) affects the speed and accuracy of movement. The present experiment examined the effects of boundary distance on the speed and accuracy of tapping. Subjects performed on two-, three- and five-position tasks varying in movement amplitude and target width. Results showed that movement time increased, and constant error became more positive as boundary distance increased. These results differed from those found in respect of pursuit tracking in that constant error was affected and not variable error. They increase the generality of the finding that motor performance varies with target location, and they support theories of motor control implicating target location.     

Target-size influences on reaction time with movement time controlled

The variable that affect motor programming time may be studied by changing the nature of the response and measuring the subsequent changes in reaction time (RT). One notion of motor programming suggests that aiming responses with reduced target size and/or increased target amplitude require more "complex" motor programs that require longer RTs. In a series of five experiments which movement time (MT) was experimentally varied target size neither influences RT when the movement amplitude was 2 or 30 cm nor when the target sizes differed by as much as a factor of 16:1. Increasing the movement amplitude from 15 to 30 cm also had no influence on RT. Movement time, however, did affect RT, with 200-msec movements having longer RTs than 120-msec movements. Target size and movement amplitude did not appear to be factors that influence programming time or program complexity.     

The independence of recall and recognition in motor learning

A basic tenet of both current closed-loop theories of motor learning (Adams, 1971; Schmidt, 1975) is that the generation of response specifications during learning is required for the development of recall memory. Two experiments were performed to test this tenet by attempting to demonstrate the development of recall memory in the absence of response specification production. The task in both experiments required blindfolded subjects to learn to produce a rapid, novel criterion movement on a linear positioning device. Control subjects in both experiments actively produced movements during learning with knowledge of results (KR) while experimental subjects in Experiment 1 experienced only the endpoint locations and in Experiment 2 were passively moved to the endpoint locations. Following initial KR trials, both experimental and control groups attempted to actively produce the criterion movement in the absence of KR. The results of both experiments support closed-loop theory that active practice is required to develop recall memory. There was some suggestion, however, that passive experience with sensory feedback may also aid recall memory development, contrary to the two closed-loop theories.     

Transfer of motor learning engages specific neural substrates during motor memory consolidation dependent on the practice structure

The authors investigated how brain activity during motor-memory consolidation contributes to transfer of learning to novel versions of a motor skill following distinct practice structures. They used 1 Hz repetitive Transcranial Magnetic Stimulation (rTMS) immediately after constant or variable practice of an arm movement skill to interfere with primary motor cortex (M1) or dorsolateral prefrontal cortex (DLPFC). The effect of interference was assessed through skill performance on two transfer targets: one within and one outside the range of practiced movement parameters for the variable practice group. For the control (no rTMS) group, variable practice benefited delayed transfer performance more than constant practice. The rTMS effect on delayed transfer performance differed for the two transfer targets. For the within-range target, rTMS interference had no significant affect on the delayed transfer after either practice structure. However, for the outside-range target, rTMS interference to DLPFC but not M1 attenuated delayed transfer benefit following variable practice. Additionally, for the outside-range target, rTMS interference to M1 but not DLPFC attenuated delayed transfer following constant practice. This suggests that variable practice may promote reliance on DLPFC for memory consolidation associated with outside-range transfer of learning, whereas constant practice may promote reliance on M1 for consolidation and long-term transfer.