Spinal circuits can accommodate interaction torques during multijoint limb movements

The dynamic interaction of limb segments during movements that involve multiple joints creates torques in one joint due to motion about another. Evidence shows that such interaction torques are taken into account during the planning or control of movement in humans. Two alternative hypotheses could explain the compensation of these dynamic torques. One involves the use of internal models to centrally compute predicted interaction torques and their explicit compensation through anticipatory adjustment of descending motor commands. The alternative, based on the equilibrium-point hypothesis, claims that descending signals can be simple and related to the desired movement kinematics only, while spinal feedback mechanisms are responsible for the appropriate creation and coordination of dynamic muscle forces. Partial supporting evidence exists in each case. However, until now no model has explicitly shown, in the case of the second hypothesis, whether peripheral feedback is really sufficient on its own for coordinating the motion of several joints while at the same time accommodating intersegmental interaction torques. Here we propose a minimal computational model to examine this question. Using a biomechanics simulation of a two-joint arm controlled by spinal neural circuitry, we show for the first time that it is indeed possible for the neuromusculoskeletal system to transform simple descending control signals into muscle activation patterns that accommodate interaction forces depending on their direction and magnitude. This is achieved without the aid of any central predictive signal. Even though the model makes various simplifications and abstractions compared to the complexities involved in the control of human arm movements, the finding lends plausibility to the hypothesis that some multijoint movements can in principle be controlled even in the absence of internal models of intersegmental dynamics or learned compensatory motor signals.This work is funded by the project "eSMCs: Extending Sensorimotor Contingencies to Cognition," FP7-ICT-2009-6 no: 270212

Inter-Joint Coordination Deficits Revealed in the Decomposition of Endpoint Jerk During Goal-Directed Arm Movement After Stroke

It is well documented that neurological deficits after stroke can disrupt motor control processes that affect the smoothness of reaching movements. The smoothness of hand trajectories during multi-joint reaching depends on shoulder and elbow joint angular velocities and their successive derivatives as well as on the instantaneous arm configuration and its rate of change. Right-handed survivors of unilateral hemiparetic stroke and neurologically-intact control participants held the handle of a two-joint robot and made horizontal planar reaching movements. We decomposed endpoint jerk into components related to shoulder and elbow joint angular velocity, acceleration, and jerk. We observed an abnormal decomposition pattern in the most severely impaired stroke survivors consistent with deficits of inter-joint coordination. We then used numerical simulations of reaching movements to test whether the specific pattern of inter-joint coordination deficits observed experimentally could be explained by either a general increase in motor noise related to weakness or by an impaired ability to compensate for multi-joint interaction torque. Simulation results suggest that observed deficits in movement smoothness after stroke more likely reflect an impaired ability to compensate for multi-joint interaction torques rather than the mere presence of elevated motor noise

The Dominant Role of the Hip in Multijoint Reflex Responses in Human Spinal Cord Injury

Following a spinal cord injury (SCI), people often experience exaggerated reflexes, such that mild provocations can cause prolonged and uncontrolled muscle activity throughout the entire leg. These reflexes can be problematic and are known to interfere with functional tasks, such as transferring to and from a wheelchair, and they may interfere with locomotor function by prolonging muscle activity and/or inappropriately activating muscles during attempts to walk. While these multijoint reflexes have been shown to originate from several afferent cues, hip afferent input is a particularly potent sensory signal that readily triggers multijoint reflexes. The overall objective of this dissertation was to understand the role of hip sensory cues and the potential mechanisms associated with multijoint reflex behavior in human SCI. To evaluate this, a custom -built robot was used to impose movement of the legs about the hip joint. Joint torque and muscle activity were used as quantitative measures of reflex activity in SCI subjects. The findings from this suggest that the mutability of reflexes triggered by hip-mediated sensory signals is reduced. Voluntary effort and stretch-sensitive sensory feedback impart weak signals that do not significantly alter multijoint reflex patterns. Additionally, reflex behaviors presented with a distinct temporal response that has been associated with the disregulation of voltage-dependent depolarizing persistent inward currents (PICs). These results further elucidate the underlying mechanisms associated with hyperexcitable multijoint reflexes to help guide rehabilitation techniques for controlling unwanted muscle activity and for increasing functional gains in human SCI

New control strategies for neuroprosthetic systems

The availability of techniques to artificially excite paralyzed muscles opens enormous potential for restoring both upper and lower extremity movements with\ud neuroprostheses. Neuroprostheses must stimulate muscle, and control and regulate the artificial movements produced. Control methods to accomplish these tasks include feedforward (open-loop), feedback, and adaptive control. Feedforward control requires a great deal of information about the biomechanical behavior of the limb. For the upper extremity, an artificial motor program was developed to provide such movement program input to a neuroprosthesis. In lower extremity control, one group achieved their best results by attempting to meet naturally perceived gait objectives rather than to follow an exact joint angle trajectory. Adaptive feedforward control, as implemented in the cycleto-cycle controller, gave good compensation for the gradual decrease in performance observed with open-loop control. A neural network controller was able to control its system to customize stimulation parameters in order to generate a desired output trajectory in a given individual and to maintain tracking performance in the presence of muscle fatigue. The authors believe that practical FNS control systems must\ud exhibit many of these features of neurophysiological systems

Neuromechanical Tuning for Arm Motor Control

Movement is a fundamental behavior that allows us to interact with the external world. Its importance to human health is most evident when it becomes impaired due to disease or injury. Physical and occupational rehabilitation remains the most common treatment for these types of disorders. Although therapeutic interventions may improve motor function, residual deficits are common for many pathologies, such as stroke. The development of novel therapeutics is dependent upon a better understanding of the underlying mechanisms that govern movement. Movement of the human body adheres to the principles of classic Newtonian mechanics. However, due to the inherent complexity of the body and the highly variable repertoire of environmental contexts in which it operates, the musculoskeletal system presents a challenging control problem and the onus is on the central nervous system to reliably solve this problem. The neural motor system is comprised of numerous efferent and afferent pathways with a hierarchical organization which create a complex arrangement of feedforward and feedback circuits. However, the strategy that the neural motor system employs to reliably control these complex mechanics is still unknown. This dissertation will investigate the neural control of mechanics employing a “bottom-up” approach. It is organized into three research chapters with an additional introductory chapter and a chapter addressing final conclusions. Chapter 1 provides a brief description of the anatomical and physiological principles of the human motor system and the challenges and strategies that may be employed to control it. Chapter 2 describes a computational study where we developed a musculoskeletal model of the upper limb to investigate the complex mechanical interactions due to muscle geometry. Muscle lengths and moment arms contribute to force and torque generation, but the inherent redundancy of these actuators create a high-dimensional control problem. By characterizing these relationships, we found mechanical coupling of muscle lengths which the nervous system could exploit. Chapter 3 describes a study of muscle spindle contribution to muscle coactivation using a computational model of primary afferent activity. We investigated whether these afferents could contribute to motoneuron recruitment during voluntary reaching tasks in humans and found that afferent activity was orthogonal to that of muscle activity. Chapter 4 describes a study of the role of the descending corticospinal tract in the compensation of limb dynamics during arm reaching movements. We found evidence that corticospinal excitability is modulated in proportion to muscle activity and that the coefficients of proportionality vary in the course of these movements. Finally, further questions and future directions for this work are discussed in the Chapter 5

Inertial Load Compensation by a Model Spinal Circuit During Single Joint Movement

Office of Naval Research (N00014-92-J-1309); CONACYT (Mexico) (63462

How Spinal Neural Networks Reduce Discrepancies between Motor Intention and Motor Realization

This paper attempts a rational, step-by-step reconstruction of many aspects of the mammalian neural circuitry known to be involved in the spinal cord's regulation of opposing muscles acting on skeletal segments. Mathematical analyses and local circuit simulations based on neural membrane equations are used to clarify the behavioral function of five fundamental cell types, their complex connectivities, and their physiological actions. These cell types are: α-MNs, γ-MNs, IaINs, IbINs, and Renshaw cells. It is shown that many of the complexities of spinal circuitry are necessary to ensure near invariant realization of motor intentions when descending signals of two basic types independently vary over large ranges of magnitude and rate of change. Because these two types of signal afford independent control, or Factorization, of muscle LEngth and muscle TEnsion, our construction was named the FLETE model (Bullock and Grossberg, 1988b, 1989). The present paper significantly extends the range of experimental data encompassed by this evolving model.National Science Foundation (IRI-87-16960, IRI-90-24877); Instituto Tecnológico y de Estudios Superiores de Monterre

Hierarchical neural control of human postural balance and bipedal walking in sagittal plane

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2006.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 177-192).The cerebrocerebellar system has been known to be a central part in human motion control and execution. However, engineering descriptions of the system, especially in relation to lower body motion, have been very limited. This thesis proposes an integrated hierarchical neural model of sagittal planar human postural balance and biped walking to 1) investigate an explicit mechanism of the cerebrocerebellar and other related neural systems, 2) explain the principles of human postural balancing and biped walking control in terms of the central nervous systems, and 3) provide a biologically inspired framework for the design of humanoid or other biomorphic robot locomotion. The modeling was designed to confirm neurophysiological plausibility and achieve practical simplicity as well. The combination of scheduled long-loop proprioceptive and force feedback represents the cerebrocerebellar system to implement postural balance strategies despite the presence of signal transmission delays and phase lags. The model demonstrates that the postural control can be substantially linear within regions of the kinematic state-space with switching driven by sensed variables.(cont.) A improved and simplified version of the cerebrocerebellar system is combined with the spinal pattern generation to account for human nominal walking and various robustness tasks. The synergy organization of the spinal pattern generation simplifies control of joint actuation. The substantial decoupling of the various neural circuits facilitates generation of modulated behaviors. This thesis suggests that kinematic control with no explicit internal model of body dynamics may be sufficient for those lower body motion tasks and play a common role in postural balance and walking. All simulated performances are evaluated with respect to actual observations of kinematics, electromyogram, etc.by Sungho JoPh.D

Embodied Skillful Performance: Where the Action Is

When someone masters a skill, their performance looks to us like second nature: it looks as if their actions are performed smoothly without explicit, knowledge-driven, online monitoring of their performance. Contemporary computational models in motor control theory, however, are instructionist. That is, they cast skilful performance as a knowledge-driven process, one that is driven by explicit motor representations of the action to be performed skillfully, which harness instructions for performance. Optimal control theory, a popular representative of such approaches, casts skillful performance as the execution of motor commands, the deliverances of a motor control system implemented by separable forward and inverse models that work in tandem with a state estimator to control the motor plant. These models rest on the principle that motor control is realized by the concerted action of separate modular subsystems, which transform an explicit motor representation into a sequence of physical movements. This paper aims to show the limitations of such instructionist approaches to skillful performance. Specifically, we address whether the assumption of modular knowledge-driven motor control in optimal control theory (based on motor commands computed by separable state estimators, forward models, and inverse models) is warranted. The first section of this paper examines the instructionist assumption, according to which skillful performance consists in the execution of instructions invested in motor representations. The second and third sections characterize the implementation of motor representations as motor commands, with a special focus on formulations from optimal control theory. The final sections of this paper examine predictive coding and active inference – behavioral modeling frameworks that descend, but are distinct, from optimal control theory – and argue that the instructionist assumption is ill-motivated in light of new developments in motor control theory, which cast motor control and motor planning as a form of (active) inference

Embodied Skillful Performance: Where the Action Is

When someone masters a skill, their performance looks to us like second nature: it looks as if their actions are performed smoothly without explicit, knowledge-driven, online monitoring of their performance. Contemporary computational models in motor control theory, however, are instructionist. That is, they cast skilful performance as a knowledge-driven process, one that is driven by explicit motor representations of the action to be performed skillfully, which harness instructions for performance. Optimal control theory, a popular representative of such approaches, casts skillful performance as the execution of motor commands, the deliverances of a motor control system implemented by separable forward and inverse models that work in tandem with a state estimator to control the motor plant. These models rest on the principle that motor control is realized by the concerted action of separate modular subsystems, which transform an explicit motor representation into a sequence of physical movements. This paper aims to show the limitations of such instructionist approaches to skillful performance. Specifically, we address whether the assumption of modular knowledge-driven motor control in optimal control theory (based on motor commands computed by separable state estimators, forward models, and inverse models) is warranted. The first section of this paper examines the instructionist assumption, according to which skillful performance consists in the execution of instructions invested in motor representations. The second and third sections characterize the implementation of motor representations as motor commands, with a special focus on formulations from optimal control theory. The final sections of this paper examine predictive coding and active inference – behavioral modeling frameworks that descend, but are distinct, from optimal control theory – and argue that the instructionist assumption is ill-motivated in light of new developments in motor control theory, which cast motor control and motor planning as a form of (active) inference