Biomedical subjects

Gregor Schöner

Publications and source records attributed to Gregor Schöner.

14 recordsLinked to original sources

PubMed2003

Effect of accuracy constraint on joint coordination during pointing movements.

Given the number of muscles and joints of the arm, more ways are available to produce an identical hand movement when pointing to a target than are strictly necessary. How the nervous system manages these abundant degrees of freedom was the focus of this study of pointing to targets of low and high indices of difficulty (ID). Two essential features of movement synergies were examined. The first reflects the preferred relations among the outputs of each movement element and was studied through principal component analysis. The second feature of synergy reflects the flexibility of those relationships evidenced by the use of multiple, goal-equivalent solutions to joint coordination. This second feature, which is the main focus of this report, was studied using the uncontrolled manifold approach. Motor abundance was defined operationally as the component of variance of joint combinations that left unchanged the value of important performance variables (goal-equivalent variability, GEV). This variance component was contrasted with the component of variance leading to a change in the value of these variables (non-goal-equivalent variability, NGEV). The difference between GEV and NGEV was evaluated with respect to the performance variables movement extent, movement direction, and path of the arm's center of mass. More than 90% of the variance of joint motions across the pointing trial were accounted for by one principal component, indicating a consistent temporal coupling among most joint motions in a single functional synergy. The flexible nature of this synergy was revealed by the variability analysis. All subjects had significantly higher GEV than NGEV for most of the movement path. Thus, variable patterns of joint coordination did not represent noise but the use of equivalent coordinative solutions related to stabilizing important performance variables. Higher GEV than NGEV was present regardless of the task's ID. One exception was at the time of peak velocity, leading to poorer control of movement extent than movement direction. Increasing the task's ID led to an overall reduction of joint configuraion variance, particularly GEV. These results support earlier work indicating that the use of goal-equivalent solutions to joint coordination is a common feature of the control of this and many other motor tasks. Functionally important performance variables appear to be controlled through flexible but task-specific coordination among the motor elements.

Adult↗

PubMed2003

A mode hypothesis for finger interaction during multi-finger force-production tasks.

Finger forces are known to change involuntarily during multi-finger force-production tasks, even when a finger's involvement in a task is not consciously changed (the enslaving effect). Furthermore, during maximal force-production (MVC) tests, the force produced by a given finger in a multi-finger task is smaller than the force generated by this finger in its single-finger MVC test (the force-deficit effect). A set of hypothetical control variables - modes - is introduced. Modes can be estimated based on individual finger forces during single-finger MVC tests. We show that a simple formal model based on modes with only one free parameter accounts for finger forces during a variety of multi-finger MVC tests. The free parameter accounts for the force-deficit effect, and its value depends only on the number of explicitly involved fingers. This approach offers a simple framework for the analysis of finger interaction during multi-finger actions.

Biomechanical Phenomena↗

PubMed2003

Approaches to analysis of handwriting as a task of coordinating a redundant motor system.

We consider problems of motor redundancy associated with handwriting using the framework of the uncontrolled manifold (UCM) hypothesis. Recent studies of finger coordination during force production tasks have demonstrated that the UCM-hypothesis provides a fruitful framework for analysis of multi-finger actions. In particular, it has been shown that during relatively fast force changes, finger force variance across trials is structured such that a time pattern of total moment produced by the fingers with respect to a point between the two most lateral fingers involved in the task is stabilized while the time pattern of total force may be destabilized. The findings of selective moment stabilization have been interpreted as being conditioned by the experience with everyday motor tasks that commonly pose more strict requirements to stabilization of total moment than to stabilization of total force. We discuss implications of these findings for certain features of handwriting seen in elderly, children, patients with neurological disorders, and forgers.

Biomechanical Phenomena↗

PubMed2003

The dynamical foundations of motion pattern formation: stability, selective adaptation, and perceptual continuity.

A dynamical model is used to show that global motion pattern formation for several different apparent motion stimuli can be embodied in the stable distribution of activation over a population of concurrently activated, directionally selective motion detectors. The model, which is based on motion detectors being interactive, noisy, and self-stabilizing, accounts for such phenomena as bistability, spontaneous switching, hysteresis, and selective adaptation. Simulations show that dynamical solutions to the motion correspondence problem for a bistable stimulus (two qualitatively different patterns are formed) apply as well to the solution for a monostable stimulus (only one pattern is formed) and highlight the role of interactions among sequentially stimulated detectors in establishing the state dependence and, thereby, the temporal persistence of percepts.

Adaptation, Physiological↗

PubMed2002

Finger coordination during discrete and oscillatory force production tasks.

We used the framework of the uncontrolled manifold (UCM) hypothesis to analyze the structure of finger force variability in discrete (ramp) and oscillatory force production tasks performed by the index and middle fingers of the right hand acting in parallel. Subjects performed the tasks at fast and slow rates, with and without a visual template presented on the screen. The variance of finger forces was partitioned into two components, compensated variance (V(COMP)), which did not affect total force, and uncompensated variance (V(UN)), which affected total force. Only minor effects of task (discrete or oscillatory) and of template (with or without) were seen on the variance profiles, leading us to conclude that the basic principles of synergy organization are common across discrete and oscillatory tasks. In contrast, the rate of force production had major effects on the structure of force variance. A modification of Goodman's model of motor variability was used to analyze the dependences V(UN) and V(COMP) on the magnitude of force and on the rate of force production. V(UN) showed a strong relation to the rate of force production and only weak dependence on the magnitude of force. In contrast, V(COMP) showed minimal effects of the rate of force production and strong effects of the force magnitude. The findings are interpreted as demonstrations of a limitation in the ability of the central nervous system to organize a two-finger synergy such that errors in the timing of individual finger force profiles are canceling each other's effects on the total force. In contrast, the synergy is efficiently intercompensating errors related to imprecise setting of force magnitudes of the two fingers.

Adult↗

PubMed2002

Behavioral and electrocortical evidence of an interaction between probability and task metrics in movement preparation.

Recent neurophysiological evidence suggests that cognitive factors shape neural activity in cortical areas such as parietal (area 5), premotor, and primary motor cortex. The implication of these findings is that behavioral signatures of cognitive factors and movement-specific factors should likewise be interdependent. The present study provides evidence of this interdependence in both behavioral (reaction time) and electrophysiological (P300) measures. Subjects performed a two-choice pointing task, in which the angular distance between the two required movement directions (task metrics) and the probability of the two responses was varied. In a control condition, a single reaction was required in response to both stimuli to test for the influence of stimulus metrics. Results from the pointing task showed a clear interaction between the metrics and effects of probability. When the potential targets were widely separated (120 degrees), stimulus probability influences reaction time and P300 amplitude in the classic fashion (longer reaction times and larger P300 amplitudes to less probable responses). When pointing to targets that were narrowly separated (20 degrees), probability had no effect: both rare and frequent targets were "functionally frequent." The same interaction was not observed in the control condition, indicating that metrics were primarily influencing movement preparation rather than stimulus processing. The results are consistent with the theoretical framework of dynamic field theory and demonstrate that metrics are an important factor that must be taken into account when assessing the processes associated with movement preparation.

Adolescent↗

PubMed2002

Timing, clocks, and dynamical systems.

Theoretical and experimental issues for our understanding of the timing of motor acts are reviewed, contrasting stochastic and dynamic timing models. It is argued that the theory of dynamical systems and, in particular, of limit cycle attractors, provides a unified framework within which these issues can be appreciated. The strength of stochastic timing models in the domain of absolute timing is contrasted with the strength of dynamic timing models in the domain of relative timing, the unification of the two domains being currently under way. It is further argued that accounts of timing must examine the interrelation between timing and other levels of processing involved in movement generation, in particular, the representation of spatial aspects of movement and the control of movement. The emergence of discrete event structure in timing skills is discussed from a dynamical systems perspective. Finally, the understanding of the timing structure of discrete movement is raised as a further challenge for future work.

Humans↗

PubMed2002

Understanding finger coordination through analysis of the structure of force variability.

Most common motor acts involve highly redundant effector systems. Understanding how such systems are controlled by the nervous system is a long-standing scientific challenge. Most proposals for solving this problem are based on the assumption that a particular solution, which optimizes additional constraints, is selected by the nervous system out of the many possible solutions. This study attempts to address this question in the context of coordinating individual finger forces to produce a controlled total force oscillation between 5% and 35% of each subject's maximum force of voluntary contraction, under two different combinations of four fingers. The structure of variability of individual finger forces was evaluated with respect to hypotheses that, at each instance in time, subjects attempt to: (1) stabilize the value of total force and (2) stabilize the total moment created by the fingers about the long axis passing through the forearm and midline of the hand. The results provide evidence that a range of goal-equivalent finger force combinations is generated to stabilize the values of total force and the total moment. The control of total force was specified explicitly by the task. However, it was stabilized only near the time of peak force. In contrast, the total moment was stabilized throughout most of the force cycle. The results lead to the suggestion that successful task performance is achieved, not by selecting a single optimal solution, but by discovering an appropriate control law that selectively stabilizes certain combinations of degrees of freedom relevant to the task while releasing from control other combinations.

Adult↗

PubMed2002

Self-organized pattern formation: experimental dissection of motion detection and motion integration by variation of attentional spread.

The formation of global motion patterns depends on the stimulus activation of local motion detectors as well as integrative excitatory and/or inhibitory interactions among the activated detectors. The counterphase row-of-elements [Vis. Res. 34 (1994) 1843] is an ideal stimulus for examining the relationship between the activational/energizing effect of the stimulus and interaction among the activated detectors. This is because the formation of the alternative unidirectional and oscillatory motion patterns for this stimulus requires the stimulation of local motion detectors, but there is no information in the stimulus that specifies either of the patterns. Their formation depends instead on the relative contributions of excitatory and inhibitory interactions to detector activation; the temporal patterns are self-organized. Broadly spread attention affects motion integration by changing the balance of excitatory versus inhibitory interactions, increasing the perception of unidirectional compared with oscillatory motion. (It likewise increases the perception of group compared with element motion for the Ternus stimulus.) There is, however, little if any effect of attentional spread on the luminance contrast required for the perception of single-element motion. The results indicate that the balance of integrative excitatory and/or inhibitory detector interactions can be modified by the perceiver's spread of attention, and further, that such changes need not be mediated by changes in the local, stimulus activation of the detectors.

Adult↗

PubMed2002

Coordination underlying the control of whole body momentum during sit-to-stand.

The stability of linear and angular momentum of the center of mass (CM) and the underlying coordination of body segments was investigated for a sit-to-stand task to better understand how the nervous system organizes the redundant degrees of freedom available to accomplish this task. From the effector geometry, we derived a mathematical model relating body segment angles and their angular velocities (i.e. state space) to CM angular and linear momentum. We used this model to partition the variability of joint angle and joint velocity configurations into combinations that leave CM momentum invariant and combinations that do not leave CM momentum invariant. The results revealed that subjects used a range of different state-space combinations from trial to trial that were equivalent with respect to producing a stable value of angular and linear momentum. In contrast, body segment combinations that changed the value of momentum were more restricted. Most interesting was the finding that, when standing up under more challenging support surface conditions, the range of state-space combinations used to stabilize momentum was increased. That is, variability increased most strongly for those angle and angular velocity combinations that left CM momentum invariant, with smaller increases registered for combinations that affected CM momentum.

Adult↗

PubMed2002

Differential joint coordination in the tasks of standing up and sitting down.

We studied similarities and differences in the use of goal-equivalent patterns of joint coordination to stand up and sit down from different support surfaces, performed without vision. Sagittal plane motion of major body segments was measured and joint angles for the left upper and lower extremities and the trunk were calculated. We used a modeling strategy relating motion in the redundant space of the joints to motion of individual performance variables, such as the center of mass (CM) or head, and determined how the variability of joint combinations across trials was structured; i.e. variations in joint combinations leading to a consistent value of a performance variable (goal-equivalent variability) and variations resulting in variability of the performance variable (non goal-equivalent variability). We found the variability of joint combinations to be selectively channeled into goal-equivalent directions, leading to stable horizontal motion of the CM and of the head, during both standing up and sitting down. In contrast, when evaluating the effect of joint combination variability on the control of vertical CM motion, we found differences in the variability components between standing up and sitting down. In general, more variable vertical CM motion occurred. An important finding was an enhanced use of goal-equivalent joint combinations under challenging task conditions, whether standing up or sitting down.

Adult↗

PubMed2002

Dynamic field theory of movement preparation.

A theoretical framework for understanding movement preparation is proposed. Movement parameters are represented by activation fields, distributions of activation defined over metric spaces. The fields evolve under the influence of various sources of localized input, representing information about upcoming movements. Localized patterns of activation self-stabilize through cooperative and competitive interactions within the fields. The task environment is represented by a 2nd class of fields, which preshape the movement parameter representation. The model accounts for a sizable body of empirical findings on movement initiation (continuous and graded nature of movement preparation, dependence on the metrics of the task, stimulus uncertainty effect, stimulus-response compatibility effects, Simon effect, precuing paradigm, and others) and suggests new ways of exploring the structure of motor representations.

Humans↗

PubMed2002

Motor control strategies revealed in the structure of motor variability.

We describe an uncontrolled manifold hypothesis, which suggests a particular solution for the notorious problem of motor redundancy. A body of recent experiments supports the uncontrolled manifold hypothesis and shows its ability to discover biological strategies of the coordination of apparently redundant motor systems. The hypothesis and associated computational apparatus have great potential for application in the areas of motor rehabilitation and motor skill acquisiton.

Biomechanical Phenomena↗

PubMed2002

Goal-equivalent joint coordination in pointing: affect of vision and arm dominance.

This study used the uncontrolled manifold (UCM) approach to study joint coordination underlying the control of task-related variables important for success at reaching and pointing to targets. More combinations of joint motions are available to the control system to achieve task success than are strictly necessary. How this abundance of motor solutions is managed by the nervous system and whether and how the availability of visual information affects the solution to joint coordination was investigated in this study. The variability of joint angle combinations was partitioned into 2 components with respect to control of either the hand's path or the path of the arm's center of mass (CM). The goal-equivalent variability (GEV) component represents trial-to-trial fluctuations of the joint configuration consistent with a stable value of the hand or CM path. The other component, non-goal-equivalent variability (NGEV), led to deviations away from the desired hand or CM path. We hypothesized a style of control in which the NGEV component is selectively restricted while allowing a range of goal-equivalent joint combinations to be used to achieve stability of the hand and CM paths. Twelve healthy right-handed subjects reached across their body to the center of a circular target with both the right and left arms and with their eyes open or closed on different trials. When repeating the task with the same arm under identical task conditions, subjects used a range of goal-equivalent joint configurations to control the entire trajectory of both the hand's and the arm's CM motion, as well as the terminal position of the pointer-tip. Overall joint configuration variability was consistently larger in the middle of the movement, near the time of peak velocity. The style of joint coordination was qualitatively similar regardless of the arm used to point or the visual condition. Quantitative differences in the structure of joint coordination were present for the non-dominant arm, however, when pointing in the absence of vision of the hand and target. The results of this study suggest that the nervous system uses a control strategy that provides for a range of goal-equivalent, rather than unique, joint combinations to stabilize the values of important task-related variables, while selectively restricting joint configurations that change these values. The possible advantage of this style of control is discussed. Absence of vision during reaching affected joint coordination only quantitatively and only for the less skilled left arm, suggesting that the role of visual information may be greater when organizing the motor components of this arm.

Adult↗