Figure 1a makes explicit a human-analogue as executive system, a library of motor scores, cortical and spinal keyboards on which to play out a retrieved score, and various possibilities for the physiological units by which the score might be realized. With minor adjustments e. Four perspectives on motor control. See text for details. Figure 1a is adapted with permission from Turvey, M. The Bernstein perspective, I: The problems of degrees of freedom and context-conditioned variability. Kelso Ed. American Psychologist , 45 , — Figure 1b presents a more contemporary image—a coupling of Turing computation with Newtonian mechanics see Loeb, ; Turvey, Within the marionette image, the hand is the brain, the control platform is the spine, the strings are the muscles, and the marionette is the skeleton.
Explicit involvement of Turing computation decreases top-down; explicit involvement of Newtonian mechanics decreases bottom-up. In this image the embodiment of the nervous system is made explicit neural-body dynamics , as are the embeddings of nervous system neural-environment dynamics and body body-environment dynamics in the task environment. The rightward panel of Figure 1d is closely cognate with Figure 1c. Then, in comparison to the leftward panel of Figure 1d which shows movements without environmental referents , the rightward panel which shows the event of changing a tire expresses two principles.
Figure 1d brings to the forefront the ecological hypothesis that behavior is intrinsically functional rather than intrinsically mechanical and only extrinsically or secondarily functional Reed, It highlights that actions are specific to function not to mechanism see Section 2. Movements and postures are controlled and coordinated to realize functionally specific acts based on the perception of affordances that is, possibilities for action, Gibson, ; Reed, ; Turvey, In Figure 1a we begin with relatively concrete identifications of what is controlled and where control originates in terms of anatomical parts.
Below, key issues in the theory of motor control are identified by way of posing, in question form, assumptions implicit in the perspective characterized in Figure 1a or its most immediate successor, the perspective characterized in Figure 1b. The issues are divided into two sets roughly according to the schema of Figure 1a.
The first set covers the level of the executive and cortical keyboard and the second set covers the levels below. Figure 1a represents a long-standing convention to think of movement control as essentially a neural matter. Control is primarily—perhaps solely—an enterprise of the central nervous system, with different aspects divided among different cortical, subcortical and spinal subsystems.
A student of movement would be strongly inclined to this belief after reading a standard neuroscience text e. Figure 1b suggests, albeit modestly, why control cannot be understood in a strict neural sense.
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The body must make an integral contribution. As Raibert and Hodgins , p. We believe that the mechanical system has a mind of its own, governed by the physical structure and the laws of physics. Figures 1c and 1d go further in the not-strictly-neural direction. They highlight that comprehension of control and the development of a thoroughgoing theory requires inclusion of the body and the environment that embed the neural processes. In the image of Figure 1c , control resides within the triad of couplings: between nervous system and body, body and environment, and nervous system and environment.
As a consequence, one cannot assign credit for adaptive behavior to any one piece of this coupled system. Rather, they must contribute to the totality of extant forces just those muscular forces that will bend the character of an event in the right direction. For the theorist, Figure 1a identifies issues of representation , selection , and translation.
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To elaborate, it identifies issues of a defining the representational form of motor scores or programs for movement i. Responses to issues a — c have been shaped traditionally by the presumptions that the primary motor cortex contains a topographic map of the body with each point specifying muscular tension either for a single muscle or for a small collection of muscles e.
Accordingly it could be assumed that the patterning of activity in the primary motor cortex—the tune played on the cortical keyboard—is, for all intents and purposes, the planned movement. In the foregoing context of ideas, the plan and program selection constitute planning and the translation of selected programs into the cortical keyboard pattern constitutes execution. Consider an arm movement parallel to the sagittal plane. Planning could be in respect to the trajectory of the hand the end point , or the angular motions of the shoulder, elbow and wrist, or the tensions in the muscles of the shoulder, elbow and wrist.
A plan expressed as an end-point trajectory places the most computational demands upon execution and the least computational demands on planning. To be implemented, a desired trajectory has to be mapped to joint motions, which in turn have to be mapped to joint torques. The implementation entails ill-defined processes of inverse kinematics and inverse kinetics addressed, theoretically, through conceptions from control theory and biomechanics Hollerbach, a , b ; Todorov, In contrast, the conceptions forming the theoretical framework for planning are closer to those of the information processing approach to cognitive psychology and the tradition of logicism see Kirsch, in artificial intelligence.
The planning-execution conceptual divide is difficult to maintain in the perspective of Figures 1c and 1d. The singular dynamical, self-organizing language required to capture the time-evolution of neural, body, and environmental states incorporates preparing and doing Beer, A ball is depicted traveling a parabolic path from right to left with its position P shown at equal temporal intervals. Vertical lines, starting on the line to P1 and ending on the line to P3, identify image planes.
Arrows indicate where the line to P2 intersects the image planes with a dashed segment above and a solid segment below each arrow. The rightward array of points expresses the successive ball locations viewed from A, B, and C. In summary, the ball will arrive at the eye if its optical image the image on the plane has zero acceleration. From Michaels, C.
Catching fly balls. Davids, G. Savelsbergh, S. Bennett, and J. London: Routledge. Reprinted with permission from Taylor and Francis Publishers. In the figure, the catcher starts at S. From McBeath, M. How baseball outfielders determine where to run to catch fly balls. Science , , — Reprinted with permission from AAAS. Historically, the lexicon of motor control theorists includes the word executive or synonyms thereof.
In Figure 1a the executive is plainly portrayed and in Figure 1b it is implicit in the hand on the control platform. A distinct executive function often seems inevitable and a need to give it concrete form has led to suggestions that it is housed in prefrontal cortex. The pertinent characteristic of an executive system is intelligence—the intelligence needed to make the right kinds of inferences and decisions, those that produce adaptive behavior.
The conception of a very intelligent executive intervening frequently has been a common although, perhaps, implicit feature of accounts of motor control fashioned in the frameworks of Figures 1a and 1b. The loans are taken to ensure the requisite competence of the inference engine s and, thereby, the means of accounting for the adaptability of movement, but it is not readily apparent as to how the loans will be repaid.
To repay such loans in full requires another kind of theory, one that explains the knowledge-like capability in a non-epistemic fashion. The devolution of executive responsibilities and, perforce, a consequent reduction in executive knowledge, has been of some concern to movement theorists e. Fundamentally, the concern is reducing executive degrees of freedom. In broader perspective, the concern is developing a theory of a minimally intelligent executive intervening minimally Kugler et al.
The desideratum is an understanding of movement control grounded, counter intuitively, in a theory of executive ignorance. In Figure 1 the challenge for the executive in framing its commands is the complicated nature of the state spaces of the to-be-commanded subsystems and the mappings among them Greene, The state spaces may not be known explicitly. And even if they were known explicitly, it is questionable whether the executive would find such knowledge useful.
The subsystem dynamics vary from moment to moment and from subsystem to subsystem. Prime examples are the family of all tunings and the family of all transition functions with individual family members depicted in Figure 2. In principle, an executive could always activate standard members of the two families, with independent processes selecting those members of the families those variants of the equivalent classes most suited to the prevailing contexts.
In this scheme, the executive would bring about felicitous outcomes in ignorance of the details the tunings and transition functions responsible for them. The family of tunings of a synergy left and the family of transitions between synergies right can be characterized, for simplicity, by means of control phase-space diagrams. The x s are the phase variables, for example, position, velocity, and the C s are the control parameters.
For a given synergy m the values assumable by its control parameter for the attainment of a specific function, invariant over circumstances, form an equivalence class. In the left panel, the change of C m from j to k changes the space-time behavior but preserves the function. In the right panel, to achieve the desired behavior, two or more synergies must be serially coordinated. At issue is the type and timing of the transition. Here, the transition functions ensuring the circumstance-invariant requisite coordination of synergies assume values that form an equivalence class.
Internal models in various guises are hypothesized as the bases for controlling movement in each of its many aspects e. They are proposed as the means for anticipating the upcoming consequences of movement forward models and for prescribing the dynamics inverse models. These models are analytical. They are typically expressed in the perspective of Figure 1b. In that perspective they require, at a minimum , explicit and accurate knowledge of the Newtonian equations governing skeletal motion, the involved quantities e.
Analytical models of control entail substantial intelligence-loans of the kind identified in Section 2. Additionally, in adopting them one has to presume that the versatility of the movement system and its modes of control are addressable by known variables and governing equations. The presumption might hold for some highly restrained movements but its generality can be questioned. The leading inspiration for analytical models is robotics in which the operational components, the actuators, do not have, as biological actuators do, a nested structure consisting of multiple redundant components and multiple neural, metabolic, and mechanical processes at indefinitely many length and time scales.
For an analytic model of a biological movement system there is not a single inverse transformation but many. Explicit specification of joint torques would have to be succeeded by explicit solutions to the inverse problems of specifying individual muscles, motor units, EMG signals, motoneurons, synaptic potentials, and so on. The single inverse transformation from joint kinematics to joint kinetics in the robotics case would be, at best, a first step in the biological case. Within the context of Figures 1c and 1d , perhaps more so than in the other two contexts, there is pressure for comprehending a form of control that does not require explicit knowledge.
An accurate analytical model of the interactive dynamics of nervous system, body, and environment, with their nonlinearities, nonstationarities, and nonobvious variables, is impractical, if not impossible. The requisite form of control, it would seem, has to be model-independent non-analytical, implicit. Some rough intuitions about the implementation of model-independent control might be gleaned from efforts to bring about real time experimental control of dynamical e. But in the general case of assembling control on the fly, there is no time for a learning phase, no time for pre-control analysis.
Dynamics must be determined in real time. The latter notions present an interesting option for the elegant theory of executive ignorance. A valuable theoretical exercise is addressing the question: To what extent can the idea of such variables, assembled on the spot, specific to the task, generalize to the problems of motor control? Model-based prediction via internal simulations has been referred to as weak anticipation Dubois, It is contrasted with strong anticipation in which the anticipation is a property of the system itself rather than of a symbol-manipulating model of the system.
Examples of efforts to identify strong anticipation are to be found in perception-action research. Proposed solutions to the problem of catching fly balls the outfielder problem are good instances. Figure 3 presents the two most prominent proposals. For both proposals, getting to the right place at the right time is not solved by prediction. The catcher so behaves as to nullify this optical quantity. Describing the prior two examples as instances of strong anticipation is to mislabel them, however, given that there is no explicit anticipation or prediction.
A similar lack of explicit anticipation or prediction seems to characterize the task of balancing a stick at the fingertip. The question posed raises issues continuous with those encountered in Section 2. The executive system of Figure 1a has at its disposal a library of recipes for movement.
Given an intention, preassembled movements in the form of a program of instructions can be retrieved, singly or in combination, for implementation by the musculoskeletal system. In the context of Figure 1a and the traditional division of motor cortex into pre- and primary motor areas Fulton, the posed question often translates into whether movements as such are coded in the primary area.
First, the textbook somatotopy Gleitman, ; Kandell et al. Second, every standard kinetic and kinematic descriptor of a moving limb examined so far seems to have a correlate in cortical neuronal activity Scott, No single type of control information is preeminent. Correlates of spatial goals, end effector motion, joint motion, emg activity and spindle activity are all present, bringing into question the idea often advanced in the perspective of Figure 1b of a logically ordered time-evolution of sensorimotor transformations from, say, spatial goals to emg activity.
Third, putative hierarchical orderings of the many motor areas identified to date are questionable on grounds that most, if not all, of the areas project to the spinal cord in an intricate, intermingled fashion.
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On elaboration, one can suppose that control is very much in the spirit of Figure 1 : there is a stored set of postures from which any situation-specific limb posture, and the details of its realization, can be generated by a process of linear combination e. Examples of preassembled transformations of arm posture. From Graziano, M. The cortical control of movement revisited.
Neuron , 36 , — Reprinted with permission from Elsevier Publishing. In an overlap of the perspectives conveyed by Figures 1b and 1c is found a different kind of answer to the question of whether there are preassembled movements. The focus for the present, at least is posture and locomotion and the central idea is that of a movement template : the simplest model exhibiting a given targeted behavior.
For the target behavior of hopping, the template is a spring-loaded inverted pendulum SLIP. The minimal morphology and physiology needed to embed or anchor the template is sought by asking what is essential to the implementation of the SLIP dynamics. On the flip side, the template itself is an instance of the derivation of simplicity from complexity, a condensing of degrees of freedom to derive a low dimensional form.
The issue of executive intelligence Section 2. In respect to the current topic of preassembled movements, the template-anchor approach promises a base set of templates. Ontologically, they are not of like kind with the motor scores retrieved by the executive of Figure 1a. They are abstractions of the stabilities or attractors of tasks, defined at the task level and generalized across species and, perforce, nervous systems. Relationship between template and anchor see text for details. After Full, R. Templates and anchors: Neuromechanical hypotheses of legged locomotion on land.
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The latter assessment can be taken a step further. As underscored in the introduction, the strong implication of Figure 1d is that all movements are specific to the problems of realizing goals in ever-changing animal-environment settings. The focus implicit in the tire-changing act depicted in the right panel of Figure 1d is abstract, task-specific organizations of movements, not a repertoire of preassembled movements as might be inferred from the left panel of Figure 1d.
It can be hypothesized that the many parallel and successive activities on view in the right panel of Figure 1d possess intrinsic dynamics Kelso, with attractor states modified by optic, acoustic and haptic variables via their influences on the control parameters of those dynamics. In Figure 1a , the executive produces movement by instructions sent to pieces of anatomy. This emphasis on anatomy in defining what is controlled follows from the Cartesian machine metaphor that has been the mainstay of theorizing on movement for over years.
Descartes advocated that all physical things inanimate and animate be explained in the way machines or automata are explained, through the properties of their independent parts. Subsystem functions are presumed to be context independent. Functional units are the alternative to anatomical units. They are contingent—that is, context dependent—with no existence outside the larger system. Whereas in the conception of anatomical components, C entails F , in the conception of functional units, F entails C. The latter direction of entailment is integral to the theory behind the perspective captured by Figure 1d and implicit in that portrayed by Figure 1c.
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Consider muscles, the frequently proposed prime targets of executive instructions. In vivo work-loops reveal that, depending upon the action context, muscles function as brakes, struts, tuners, meters, and springs as well as functioning as motors Dickinson et al. They also function in less obvious ways for which we do not as yet have names e. One implication is that muscles in a single anatomical group e. The implication, stated more generally, is that redundancy in a multiple muscle group may represent diversity in muscle function.
If such is the case, then the coordination that characterizes a synergy or coordinative structure see Kelso in the present volume is not so much the coordination of individuals muscles as it is the coordination of roles context-dependent functions. The most hard-worked conception of an anatomical unit is the reflex: a stereotyped, context-independent response to a specific or proper or adequate stimulus. It served to ground notions of motor control throughout the past century despite the early cautionary remarks of Sherrington The complex interactions among neural elements at multiple locations within the CNS and their ongoing changes during any functionally meaningful motor activity, makes it difficult to accept that reflexes conceptualized in strictly anatomical terms are a basic form of stimulus—response mechanism.
Uncomplicated reflexes are highly influenced by oral instructions Gurfinkel, Kots, Krinskiy et al. To be purposely redundant, instead of fixed responses, reflexes are context-dependent and probably functionally related. Gurfinkel, Kots, Krinskiy et al. On cue, participants extended a knee while surface electromyography EMG of the rectus femoris two-joint and vastus lateralis one-joint muscles were recorded.
During the interval between the oral command and the observed EMG voluntary response, taps of the patellar tendon were applied at different times and the reflex EMG activity recorded. It was observed that the size EMG amplitude of the patellar reflex was greatest at to ms after giving the command to extend the knee. When the command given to the participants was to flex the hip, the same amplification of the reflex response was observed in the bi-articular rectus femoris muscle that participates in the movement , but was absent in the vastus lateralis muscle that, as a single-joint muscle, cannot contribute to the solicited movement.
Similar context dependence of reflexes is observed during muscular and cutaneous afferent nerve stimulation. Stimulation of these afferents has been shown to induce complex excitatory and inhibitory effects on leg muscles that are dependent on postural orientation, the task or the phase of rhythmic movements e. The observed changes in reflex responses reflect the complex neural interactions, producing context-dependent adaptations during human movements.
Surprisingly, perhaps, contemporary inquiry into the functional nature of muscles and reflexes, the traditional staples of motor control theory, may turn motor control on its head. Muscles and reflexes are compelling examples of how context-dependence allows the very same parts the same degrees of freedom to implement different, multiple, and often higher-level functions. Both muscles and reflexes express a separation of two distinct characterizations of a functional subsystem, one descriptive, one interpretive. The subsystem can be described in terms of locally determined, intrinsic, descriptive properties and it can be described in terms of properties that are non-locally determined, relational, and interpretive—the properties it has by virtue of its role in the embedding system McClamrock, Coming to terms with the interpretive, semantic, impredicative status of muscles and reflexes is not likely within standard logical formulations.
Rampant context-dependence may require radical rethinking of foundational assumptions in motor control. If F entails C , by what principles is C assembled? Theory and research within the perspectives of Figures 1c and 1d have taken preliminary steps toward answering such a question. Common assumptions are that muscles are functionally independent of one another and functionally independent of the tissues the fascia that envelop them.
Demonstrations of different proximal and distal forces are suggestive of muscle dependence on surrounding tissues and they are suggestive of modes of force transmission additional to the myotendinous route Huijing, Collectively, these additional modes compose myofascial force transmission. Fascia in different forms connects muscle fibers to muscle fibers, muscle to muscle, and muscle to bone, to yield possibilities for intramuscular, intermuscular, and extramuscular force transmissions, respectively.
The involvement of forces manifest in muscle-fascia complexes means that neighboring muscles are more functionally bound, more unified on strictly mechanical grounds, than heretofore considered. Schematic of effects of changes in relative position of a muscle with respect to adjacent muscles and extramuscular connective tissue. Based on Huijing, B and C. Relation between mechanoreceptors and fascia in the antebrachial extensor muscles of the rat.
The gray bands in the seven muscles are the sites of the mechanoreceptors. The location within the extensor digitorum communis muscle is shown in cross-sectional view; note the proximity of the mechanoreceptors to the fascia. Figures B and C are taken from Figures 6. The afference underwriting nonvisual perception of the body and its segments is typically depicted in terms of signals transmitted over non-interacting linear pathways from mechanoreceptors to spinal neurons to brain. The signals are typically understood as referring to the states of individual muscles, tendons, and ligaments.
Feigenberg and L. For a long time it was believed that the book had been destroyed; however, this was not the case. Fortunately, the severity of the laws of the Soviet Union was softened by their notorious ineffectiveness, and one of Bernstein's students, Professor I.
Feigenberg, found the manuscript and restored the book. The book was eventually published in , 25 years after the death of its author. Very few scientific works remain interesting to the reader 50 years after they were written. This rule is particularly true for books in which the authors try to present the state of a scientific field in a popular style, understandable not only to professionals but also to people who are generally curious but lack the particular scientific background, including college and even high-school students.
Bernstein's work, however, is a rare exception to this rule, and we are sure that you will enjoy it as much as it would have been enjoyed in the s had it been published at that time. Bernstein's original book, presented in Part I, was directed at a wide audience ranging from specialists in biomechanics and motor behavior to coaches, neurologists, physical therapists, athletes, and even inquisitive college and high-. An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while. View access options below. You previously purchased this article through ReadCube.
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Learn more Check out. Abstract The ecological approach to studying perception and action is one in which researchers view the structure in ambient energy arrays surrounding a given organism as being sufficient to specify meaningful environmental properties to that organism. Citing Literature. Volume 42 , Issue 4 May Pages Related Information. Close Figure Viewer. Browse All Figures Return to Figure. Previous Figure Next Figure.
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