Modeling the Neurological Control of Human Movements

Although computer models have been extensively used in recent years to understand the way physical systems operate and interact, the enormous power of mathematical modeling and computer simulations has been difficult to implement for the benefit of neuroscientists studying the human motor control system. Nevertheless, homeomorphic models are now being used to explain and predict the neural and biomechanical aspects of different human movements. This paper argues for the importance of regarding model simulations as a supplementary approach to traditional methods of experimental investigation by drawing examples from both the experimental and the modeling literature. The discussion focuses on studies of the triphasic control signal for fast, goal-directed movements and on aspects of sampled data control for slow, tracking movements. The aim of this viewpoint article is to promote a more widespread use of modeling and simulation in the field of motor control.     

Perceptual control models of pursuit manual tracking demonstrate individual specificity and parameter consistency

Attention, Perception, & Psychophysics - Computational models that simulate individuals’ movements in pursuit-tracking tasks have been used to elucidate mechanisms of human motor control....     

Movement variability near goal equivalent manifolds: Fluctuations,control, and model-based analysis

Fluctuations in the repeated performance of human movements have been the subject of intense scrutiny because they are generally believed to contain important information about the function and health of the neuromotor system. A variety of approaches has been brought to bear to study these fluctuations. However it is frequently difficult to understand how to synthesize different perspectives to give a coherent picture. Here, we describe a conceptual framework for the experimental study of motor variability that helps to unify geometrical methods, which focus on the role of motor redundancy, with dynamical methods that characterize the error-correcting processes regulating the performance of skilled tasks. We describe how goal functions, which mathematically specify the task strategy being employed, together with ideas from the control of redundant systems, allow one to formulate simple, experimentally testable dynamical models of inter-trial fluctuations. After reviewing the basic theory, we present a list of five general hypotheses on the structure of fluctuations that can be expected in repeated trials of goal-directed tasks. We review recent experimental applications of this general approach, and show how it can be used to precisely characterize the error-correcting control used by human subjects.     

Discrete and cyclical movements: unified dynamics or separate control?

In the literature on motor control, three theoretical perspectives on the relation between discrete and cyclical movements may be discerned: (a) cyclical movements are concatenated discrete movements; (b) discrete movements are a limiting case of cyclical movements, and (c) discrete and cyclical movements are motor primitives that may be combined but are irreducible to each other. To examine the tenability of these perspectives, 16 participants performed cyclical and discrete (flexion and extension) reaching movements of various amplitudes to differently sized targets. The kinematic properties of the recorded movements were analyzed and compared in detail. The cyclical, ongoing movements differed markedly from the discrete movements as well as from the first and last half-cycles of a bout of cyclical movements, especially in terms of their symmetry ratio. These effects were largely independent of amplitude, target size and movement direction (flexion-extension). The results obtained ruled out perspective (a) and, in principle, left open perspectives (b) and (c). However, the observed kinematic features were not readily accounted for by the specific dynamical models that have been proposed under perspectives (b) and (c). Future modeling attempts should explicate the dynamics of initiation and abortion of both discrete and cyclical movements.     

Electromyographic evidence of neurological controller signals with viscous load

Ensemble averaging after pre-editing of surface EMG potentials has enabled construction of underlying controller signals from stereotyped head movements. Carefully controlled, intended time-optimal movements by trained, actively participating human subjects have been found to yield repeatable, multi-pulse controller signals. Also, adaptive changes in these horizontal head movements in response to added viscous loads showed further causal relationships between movement dynamics and EMG signals from left and right splenius muscles. These experimental results are a base from which modeling studies can be performed to explicate the neurological control strategies used in the performance of this class of movements.     

The Use of a Nonlinear Muscle Model in Explaining the Relationship Between Duration, Amplitude, and Peak Velocity of Human Rapid Movements

Rapid human movements exhibit a quasilinear relationship between their amplitude and maximum velocity and a log-like relationship between their amplitude and duration. The authors demonstrate that those well-observed relations can be obtained with a simple nonlinear muscle model and a pulse-step control scheme. That result encourages the use of nonlinear musculoskeletal models with simple control schemes for modeling human ballistic movements.     

Accurate reaching after active but not passive movements of the hand: evidence for forward modeling

Converging behavioral findings support recent models of motor control suggesting that estimates of the future positions of a limb as well as the expected sensory consequences of a planned movement may be derived, in part, from efference copies of motor commands. These estimates are referred to as forward models. However, relatively little behavioral evidence has been obtained for proposed forward models that provide on-line estimates of current position. We report data from a patient (JD) who reached accurately to visualized targets with and without vision of her hand despite substantial proprioceptive loss. Additionally, we administered a double-start reaching test to examine the possibility that efference copy information could be used to estimate current limb position. JD reached accurately, without vision, to a final target after actively reaching to a landmark, but exhibited severely impaired reaching after passive movements to the landmark. This finding suggests that forward modeling of efference copy signals may provide relatively accurate estimates of current limb position for the purpose of motor planning. The possibility that such estimates may also contribute to the awareness of body position and to self-recognition is discussed.     

Servo Hypotheses for the Biological Control of Movement

An analysis is made of equilibrium-point models for motor control, describing these models in the context of servo control mechanisms. We considered issues of speed and stiffness scaling that are incompatible with current formulations of the equilibrium-point models. A modification of the equilibrium-point models is proposed in which the central nervous system controls velocity as well as positions during the course of fast 1imb movements. Numerical simulations are presented that verify that such a servo control mechanism could successfully produce fast limb movements, as observed in human subjects     

Cortical cell assemblies: a possible mechanism for motor programs

The concept of a motor program has been used to interpret a diverse range of empirical findings related to preparation and initiation of voluntary movement. In the absence of an underlying mechanism, its exploratory power has been limited to that of an analogy with running a stored computer program. We argue that the theory of cortical cell assemblies suggests a possible neural mechanism for motor programming. According to this view, a motor program may be conceptualized as a cell assembly, which is stored in the form of strengthened synaptic connections between cortical pyramidal neurons. These connections determine which combinations of corticospinal neurons are activated when the cell assembly is ignited. The dynamics of cell assembly ignition are considered in relation to the problem of serial order. These considerations lead to a plausible neural mechanism for the programming of movements and movement sequences that is compatible with the effects of precue information and sequence length on reaction times. Anatomical and physiological guidelines for future quantitative models of cortical cell assemblies are suggested. By taking into account the parallel re-entrant loops between the cerebral cortex and basal ganglia, the theory of cortical cell assemblies suggests a mechanism for motor plans that involve longer sequences. The suggested model is compared with other existing neural network models for motor programming.     

Cues and control strategies in visually guided tracking

Detailed quantitative models are required to investigate the neurological basis of motor behavior. Previous studies of visually guided manual tracking have either identified a variety of control signals (cues) for planning tracking movements or analyzed how a single cue is used (i.e., one-tracking strategy). A systematic, quantitative analysis of the effects and interactions of cues in terms of human manual-tracking performance is presented here together with measurements of concomitant eye movements. These measurements help define the routes by which information reaches the CNS, and the analysis elucidates how the control signals are processed and combined. The results quantify not only the large improvement in performance observed when the target waveform being tracked is predictable but also the extent to which this improvement depends on the availability of current information about target movements and positional error. Target information is shown to provide short-term prediction independent of the error signals used in on-line negative feedback control.     

Cortical Cell Assemblies: A Possible Mechanism for Motor Programs

The concept of a motor program has been used to interpret a diverse range of empirical findings related to preparation and initiation of voluntary movement. In the absence of an underlying mechanism, its explanatory power has been limited to that of an analogy with running a stored computer program. We argue that the theory of cortical cell assemblies suggests a possible neural mechanism for motor programming. According to this view, a motor program may be conceptualized as a cell assembly, which is stored in the form of strengthened synaptic connections between cortical pyramidal neurons. These connections determine which combinations of corticospinal neurons are activated when the cell assembly is ignited. The dynamics of cell assembly ignition are considered in relation to the problem of serial order. These considerations lead to a plausible neural mechanism for the programming of movements and movement sequences that is compatible with the effects of precue information and sequence length on reaction times. Anatomical and physiological guidelines for future quantitative models of cortical cell assemblies are suggested. By taking into account the parallel, re-entrant loops between the cerebral cortex and basal ganglia, the theory of cortical cell assemblies suggests a mechanism for motor plans that involve longer sequences. The suggested model is compared with other existing neural network models for motor programming.     

The neuronal basis of bimanual coordination: recent neurophysiological evidence and functional models

Recent physiological studies of the neuronal processes underlying bimanual movements provide new tests for earlier functional models of bimanual coordination. The recently acquired data address three conceptual areas: the generalized motor program (GMP), intermanual crosstalk and dynamic systems models. To varying degrees, each of these concepts has aspects that can be reconciled with experimental evidence. The idea of a GMP is supported by the demonstration of abstract neuronal motor codes, e.g. bimanual-specific activity in motor cortex. The crosstalk model is consistent with the facts that hand-specific coding also exists and that interactions occur between the motor commands for each arm. Uncrossed efferent projections may underlie crosstalk on an executional level. Dynamic interhemispheric interactions through the corpus callosum may provide a high-level link at the parametric programming level, allowing flexible coupling and de-coupling. Flexible neuronal interactions could also underlie adaptive large-scale systems dynamics that can be formalized within the dynamic systems theory approach.

The correspondence of identified neuronal processes with functions of abstract models encourages the development of realistic computational models that can predict bimanual behavior on the basis of neuronal activity.     

Graphonomics and its contribution to the field of motor behavior: A position statement

The term graphonomics was conceived in the early 1980s; it defined a multidisciplinary emerging field focused on handwriting and drawing movements. Researchers in the field of graphonomics have made important contribution to the field of motor behavior by developing models aimed to conceptualize the production of fine motor movements using graphical tools. Although skeptics have argued that recent technological advancements would reduce the impact of graphonomic research, a shift of focus within in the field of graphonomics into fine motor tasks in general proves the resilience of the field. Moreover, it has been suggested that the use of fine motor movements due to technological advances has increased in importance in everyday life. It is concluded that the International Graphonomics Society can have a leading role in fostering collaborative multidisciplinary efforts and can help with the dissemination of findings contributing to the field of human movement sciences to a larger public.     

Testing models of object substitution with backward masking

A briefly presented visual target stimulus can be difficult to identify if shown in the context of a mask stimulus. There are several quantitative models that have been used to explain many different masking effects. Here, we look at modeling what has been called object substitution. We analyze four models and show that three of the models work with a common principle, whereas the fourth behaves quite differently. The three similar models have previously been used to explain many other aspects of visual masking, whereas the fourth model was designed exclusively to deal with object substitution masking effects. We identify an experimental test on the basis of which to choose between the models. The experimental results support the behavior of the fourth model.     

Synchronizing movements with the metronome: nonlinear error correction and unstable periodic orbits.

The control of human hand movements is investigated in a simple synchronization task. We propose and analyze a stochastic model based on nonlinear error correction; a mechanism which implies the existence of unstable periodic orbits. This prediction is tested in an experiment with human subjects. We find that our experimental data are in good agreement with numerical simulations of our theoretical model. These results suggest that feedback control of the human motor systems shows nonlinear behavior.     

The man who executed "imagined" movements: evidence for dissociable components of the body schema

We examined the nature of representations underlying motor imagery and execution in a patient (CW) with bilateral parietal lesions. When imagining hand movements, CW executed the imagined motor act but was unaware of the movements. These movements were significantly more accurate than volitional movements for the left but not right hand. CW also exhibited preserved motor imagery for the left but not right hand. Consistent with previous accounts, these findings suggest that motor imagery may normally involve the inhibition of movements. CW's unawareness of movements during motor imagery may reflect inattention or misattribution of the unexpected sensory feedback. Furthermore, in line with current models of motor control, motor imagery may depend on the integrity of a "forward model" derived from motor outflow information to generate a prediction of the consequences of a motor command. Such predictions appear to be preserved for imagery of left but not right hand movements in CW. Action may additionally depend on precise updating of effector position derived from the comparison of predicted and actual sensory information. We propose that CW's impaired volitional movements may be attributable to the degradation of such an updating mechanism.     

Response to Commentaries: Actions as Perceptual-Conceptual "Gestalts"

The author clarifies the objective and essence of the psychological, or perceptual-cognitive, approach to voluntary movement. According to that approach, voluntary movements are organized and performed as meaningful, perceptible events with bodily and environmental aspects. Body-oriented control is thus not neglected in the perceptual-cognitive approach but is actually an important issue. He further clarifies how material and psychological factors relate to each other in motor control and why they are not considered a coalition of constraints. The central importance of the sparse coding principle for the perceptual-cognitive approach is underlined. Finally, the author argues that the psychological information format is particularly suitable for motion control and stresses the power of the perceptual-cognitive approach to possibly provide a unifying framework for understanding human voluntary movements.     

A new book by N. A. Bernstein: "On dexterity and its development"

A book, "On Dexterity and Its Development," written by N. A. Bernstein in the 1940s has recently been published in Moscow. We describe briefly the amazing fate of this miraculously saved book and of its famous author. The book is unique in its reader-friendly handling of the most complicated issues of motor control, motor development, motor skill acquisition, and more general aspects of brain activity. It presents the basis for what is now called motor control and for some of the more fundamental issues of theoretical neuroscience. Bernstein described his views on multilevel control of movements, the role of sensory feedback, preprogramming, feedback and feedforward components in motor control, the role of exercise, and other subjects. Despite the fact that the book was written nearly half a century ago, its deep philosophical content remains beyond any competition.     

A New Book by N. A. Bernstein: “On Dexterity and Its Development”

A book, ‘On Dexterity and its Development,’ written by N. A. Bernstein in the 1940s has recently been published in Moscow. We describe briefly the amazing fate of this miraculously saved book and of its famous author. The book is unique in its reader-friendly handling of the most complicated issues of motor control, motor development, motor skill acquisition, and more general aspects of brain activity. It presents the basis for what is now called motor control and for some of the more fundamental issues of theoretical neuroscience. Bernstein described his views on multilevel control of movements, the role of sensory feedback, preprogramming, feedback and feedforward components in motor control, the role of exercise, and other subjects. Despite the fact that the book was written nearly half a century ago, its deep philosophical content remains beyond any competition.