A theory on the control of arbitrary movements

A theory dealing with the control of human, arbitrary movements is proposed. A schema is set up to suggest how the relevant information flows and what kind of operations affect it. A number of successive steps are distinguished in the production of a movement. It is assumed that the intended movement is carried out in the imagination, and that this imaginary movement is composed of a spatial trajectory and an intensity course, which are considered to be independent features of the intended movement. The spatial trajectory will be encoded in a special coding, which is related to the lengths of the muscles that effect the movement. From this special coding of the intended movement static and dynamic control signals can be derived. Because afferent and efferent signals are encoded in the same way in this schema, the evaluation and correction of the performed movement is quite simple. The higher levels in the control schema may function in an abstract way, i.e. the signals at these levels are barely concerned with details of the peripheral motor system. This abstract functioning of the higher levels is based on the numerous feedback mechanisms involved at all levels of control and in the peripheral motor system. Nevertheless, it is possible to incorporate specific peripheral properties in the generation of the control signals. The assumptions in this theory will be discussed and aspects of the proposed control schema will be compared with general control principles.     

A framework for considering the role of afference and efference in the control and perception of ocular position

It has been well established that extra-retinal information is used in the perception of visual direction and distance. Furthermore, a number of studies have established that both efference copy and afferent discharge contribute to the extra-retinal signal. Despite this, no model currently exists to explain how the signals which arise through oculomotor control contribute to perception. This paper attempts to provide such a framework. The first part of the paper outlines the framework [the cyclopean equilibrium point (EP) model] and considers the binoculus or cyclopean eye from the perspective of a current account of motor control (the EP hypothesis). An existing model is used to describe how the nervous system could utilise available efference copy and afferent extra retinal signals when determining the direction and distance of cyclopean fixation. Although the cyclopean EP model is speculative, it allows for a parsimonious framework when considering the oculomotor contribution to perception. The model has the additional advantage of being consistent with current theories regarding the control and perception of limb movement. The second part of the paper shows that the model is biologically plausible, demonstrates the use of the proposed model in describing the central control of eye movements with regard to non-conjugate peripheral adaptation and reconciles seemingly disparate empirical findings.     

The thalamus in the motor system.

The ventrolateral (VL) and anterior (VA) are the main thalamic relay for cerebellar and pallidal efferents going to the motor cortex. Four aspects of the function of these nuclei are briefly considered. (1) It is well known that these thalamic structures are not a simple relay on the way to the motor cortex, but that they have a gating function for the cerebellar afferents. The gating mechanism is active during slow-wave sleep, with deafferentation and with the use of various anesthetics. Possibly, it might play a role in the central organization of movement. (2) The organization at the unitary level of the projections between VL and motor cortex is examined and their role in the command of motor synergies through the motor cortex is strongly suggested. (3). It appears that unitary activity of VL neurons is not only related to movement but also to postural changes associated with movement. (4) The sensory input to VL nucleus is briefly analyzed. The inefficacy of exteroceptive stimulation in awake animals, in contrast with the effect of the same stimulation in anesthetized preparations, is discussed.     

The discontinuous nature of motor execution II. Merging discrete and rhythmic movements in a single-joint system -- the phase entrainment effect

 Initiation of rapid discrete flexion movements is significantly altered when a secondary rhythmic movement is performed simultaneously with the same limb; the onset of a stimulus-evoked discrete movement tends to occur time-locked to the oscillation: i.e., the rhythmic movement entrains the discrete response. This nonlinear interaction may reflect a specific principle of coordination of motor tasks which are simultaneously executed with the same effector. This part II of a tripartite research report on such single-muscle multiple-task coordination investigates the contribution of the dynamic properties of the muscle and its reflex circuitry to phase entrainment. Assuming a simple threshold-linear relationship between the control signals generated by the central nervous system and the observable kinematic and electromyographic signals, a secondary rhythmic movement will cause an additional phase-dependent delay between the central “go” command and the first observable change in actual kinematics of the compound movement. Several indicators for such threshold-linear interaction are derived and tested on real data obtained in psychophysical experiments. Four healthy subjects performed rapid lateral abductions of the index finger in response to a visual “go” signal. During a portion of the experiments, subjects produced additional low-amplitude oscillatory movements before stimulus presentation with either the same finger (one-handed task), or with the index finger of the other hand (two-handed task). Results showed phase entrainment and modulation of reaction times when the cyclic and the discrete movements were simultaneously executed by the same finger. But there was no entrainment in the bimanual execution of the tasks. The model was capable of reproducing the observed effects. It is concluded that coordination of voluntary movements which are concurrently performed by the same effector involves specific discontinuous operations, which represents an essential part of the mechanism of motor coordination. Phase entrainment reflects this characteristic discontinuous behavior of the lower stages of motor execution and does not necessarily require nonlinear interaction of motor commands at higher levels of motor processing. Received: 5 September 2001 / Accepted in revised form: 19 December 2001     

The functions of the proprioceptors of the eye muscles

This article sets out to present a fairly comprehensive review of our knowledge about the functions of the receptors that have been found in the extraocular muscles--the six muscles that move each eye of vertebrates in its orbit--of all the animals in which they have been sought, including Man. Since their discovery at the beginning of the 20th century these receptors have, at various times, been credited with important roles in the control of eye movement and the construction of extrapersonal space and have also been denied any function whatsoever. Experiments intended to study the actions of eye muscle receptors and, even more so, opinions (and indeed polemic) derived from these observations have been influenced by the changing fashions and beliefs about the more general question of how limb position and movement is detected by the brain and which signals contribute to those aspects of this that are perceived (kinaesthesis). But the conclusions drawn from studies on the eye have also influenced beliefs about the mechanisms of kinaesthesis and, arguably, this influence has been even larger than that in the converse direction. Experimental evidence accumulated over rather more than a century is set out and discussed. It supports the view that, at the beginning of the 21st century, there are excellent grounds for believing that the receptors in the extraocular muscles are indeed proprioceptors, that is to say that the signals that they send into the brain are used to provide information about the position and movement of the eye in the orbit. It seems that this information is important in the control of eye movements of at least some types, and in the determination by the brain of the direction of gaze and the relationship of the organism to its environment. In addition, signals from these receptors in the eye muscles are seen to be necessary for the development of normal mechanisms of visual analysis in the mammalian visual cortex and for both the development and maintenance of normal visuomotor behaviour. Man is among those vertebrates to whose brains eye muscle proprioceptive signals provide information apparently used in normal sensorimotor functions; these include various aspects of perception, and of the control of eye movement. It is possible that abnormalities of the eye muscle proprioceptors and their signals may play a part in the genesis of some types of human squint (strabismus); conversely studies of patients with squint in the course of their surgical or pharmacological treatment have yielded much interesting evidence about the central actions of the proprioceptive signals from the extraocular muscles. The results of experiments on the eye have played a large part in the historical controversy, now in at least its third century, about the origin of signals that inform the brain about movement of parts of the body. Some of these results, and more of the interpretations of them, now need to be critically re-examined. The re-examination in the light of recent experiments that is presented here does not support many of the conclusions confidently drawn in the past and leads to both new insights and fresh questions about the roles of information from motor signals flowing out of the brain and that from signals from the peripheral receptors flowing into it. There remain many lacunae in our knowledge and filling some of these will, it is contended, be essential to advance our understanding further. It is argued that such understanding of eye muscle proprioception is a necessary part of the understanding of the physiology and pathophysiology of eye movement control and that it is also essential to an account of how organisms, including Man, build and maintain knowledge of their relationship to the external visual world. The eye would seem to provide a uniquely favourable system in which to study the way in which information derived within the brain about motor actions may interact with signals flowing in from peripheral receptors. The review is constructed in relatively independent sections that deal with particular topics. It ends with a fairly brief piece in which the author sets out some personal views about what has been achieved recently and what most immediately needs to be done. It also suggests some lines of study that appear to the author to be important for the future.     

The discontinuous nature of motor execution

Human movement control requires adequate coordination of different movements, which is particularly important when different motor tasks are simultaneously executed by the same effector(s) (e.g. a muscle or a joint). The process of movement execution involves a series of highly nonlinear elements; for instance, a motor unit of a muscle produces force only in the direction of muscle shortening, thus representing a threshold operator that transforms the bipolar (i.e. excitatory or inhibitory) information at its spinal input into a purely unipolar signal (i.e. muscle force). This tripartite research report addresses the contribution of the nonlinearity of neuromuscular elements to the coordination of different motor tasks simultaneously executed by the same limb. In this first part of the series, a new hypothesis for such a single-muscle multiple-task coordination is presented which suggests an essentially threshold-linear coordination mechanism. Control signals generated by the central nervous system for each individual movement independently and feedback information from peripheral receptors are linearly superimposed. This compound control/feedback signal is processed by a nonlinear limiter element reflecting the discontinuous properties of the muscle and its reflex circuitry. It is shown that threshold-linear interaction of descending commands and afferent feedback information can lead to complex interdependent patterns of compound motor action. This includes the possibility of gating (i.e. the ability of one movement pattern to constrain or even impede the execution of another pattern) and of delayed response initiation when simultaneously performing more than one voluntary motor task. A theoretical analysis of the threshold-linear coordination mechanism and an extensive experimental validation of the model is provided in part II and part III of the report. Received: 6 October 1998 / Accepted in revised form: 2 June 1999     

Possible contributions of CPG activity to the control of rhythmic human arm movement

There is extensive modulation of cutaneous and H-reflexes during rhythmic leg movement in humans. Mechanisms controlling reflex modulation (e.g., phase- and task-dependent modulation, and reflex reversal) during leg movements have been ascribed to the activity of spinal central pattern generating (CPG) networks and peripheral feedback. Our working hypothesis has been that neural mechanisms (i.e., CPGs) controlling rhythmic movement are conserved between the human lumbar and cervical spinal cord. Thus reflex modulation during rhythmic arm movement should be similar to that for rhythmic leg movement. This hypothesis has been tested by studying the regulation of reflexes in arm muscles during rhythmic arm cycling and treadmill walking. This paper reviews recent studies that have revealed that reflexes in arm muscles show modulation within the movement cycle (e.g., phase-dependency and reflex reversal) and between static and rhythmic motor tasks (e.g., task-dependency). It is concluded that reflexes are modulated similarly during rhythmic movement of the upper and lower limbs, suggesting similar motor control mechanisms. One notable exception to this pattern is a failure of contralateral arm movement to modulate reflex amplitude, which contrasts directly with observations from the leg. Overall, the data support the hypothesis that CPG activity contributes to the neural control of rhythmic arm movement.     

内源性大麻素系统对皮层下运动中枢的调控作用

以往研究已证明,内源性大麻素系统广泛存在于中枢和外周神经组织中,并作为逆向信号分子在突触信号传递中发挥重要调节作用。本文就内源性大麻素系统对皮层下运动中枢的调控作用及相关机制进行综述,以期系统地论述皮层下运动中枢在躯体运动、动作选择和运动技能学习等高级神经活动过程中的突触和神经环路机制,并为相关疾病的治疗和靶向药物开发提供理论依据。     

Biomechatronics--assisting the impaired motor system

Biomechatronics concerns the interdisciplinary field of interaction with the human neuromuscular-skeletal system with the objective to assist impaired human motor control. In this field technology is developed that integrates neuroscience, robotics, interface and sensor technology, dynamic systems and control theory. The primary issue in this field concerns the concepts of assisting impaired human motor function. The secondary, derived, issue concerns possible methods of interfacing with the human body at all hierarchical levels of the human motor system. The application of motor assist systems may serve several goals: it can take over part of the affected motor control, enable the physiological motor system to perform the desired function or aid in training the impaired physiological system. The progress in these issues are reviewed and their potential implications for assistance of the impaired human motor system are discussed.     

Sensory-motor coordination during grasping and manipulative actions.

Goal-directed grasping and manipulation of objects are human skills that depend on automatic sensory control in which predictive feed-forward mechanisms integrate somatosensory and visual signals with sensory-motor memory systems. Memory representations of physical and task-relevant properties of the object play a pivotal role. Anticipatory strategies are crucial when purposeful actions arise from learned relationships between afferent patterns and efferent commands. The development of even elementary precision grip skills is a protracted process not concluded until early adolescence. Not surprisingly, the neural control of manual actions engages most central nervous system areas known to be involved in motor control.     

Balance of a multijoint biomechanical system in natural and artificial environments: a simulation model

The paper is devoted to a neurobionic simulation model for controlling balance in a biomechanical pendulum. The model is realized by a complex of fuzzy regulators and an artificial neural network. Fuzzy regulators are used for simulating the physiological characteristics of the motor system and the functions of the sensory systems. The second level of control is the central integrator. It is realized as an artificial neural network (ANN), which simulates a real process of analysis and synthesis of afferent signals, formation of the model of action, etc.Equilibrium control in a multijoint biomechanical object is a specific example of a self-developing multilevel system of movement control. In the course of elaboration of the model and further examination of its behavior we have received model results which revealed correspondence with the results demonstrated by real subjects in stabilographic tests performed after long-term space flights. We concluded that the model permits us to simulate the peculiarities of human movement control and can be used for creating individual plans of recovery and rehabilitation of patients after long-term motionless or learning movement control in unknown environments.     

Increased Muscle Activation Following Motor Imagery During the Rehabilitation of the Anterior Cruciate Ligament

Motor imagery (MI) is the mental representation of an action without any concomitant movement. MI has been used frequently after peripheral injuries to decrease pain and facilitate rehabilitation. However, little is known about the effects of MI on muscle activation underlying the motor recovery. This study aimed to assess the therapeutic effects of MI on the activation of lower limb muscles, as well as on the time course of functional recovery and pain after surgery of the anterior cruciate ligament (ACL). Twelve patients with a torn ACL were randomly assigned to a MI or control group, who both received a series of physiotherapy. Electromyographic activity of the quadriceps, pain, anthropometrical data, and lower limb motor ability were measured throughout a 12-session therapy. The data provided evidence that MI elicited greater muscle activation, even though imagery practice did not result in pain decrease. Muscle activation increase might originate from a redistribution of the central neuronal activity, as there was no anthropometric change in lower limb muscles after imagery practice. This study confirmed the effectiveness of integrating MI in a rehabilitation process by facilitating muscular properties recovery following motor impairment. MI may thus be considered a reliable adjunct therapy to help injured patients to recover motor functions after reconstructive surgery of ACL.     

Laterale Inhibition in der pupillomotorischen Bahn des Menschen

The human pupillomotor pathway is considered as a model for neural interaction processes between signals elicited in a paired sense organ. A previous hypothesis is subjected to more rigorous tests applying monocular and simultaneous binocular as well as subsequent double pulse stimuli to limited central and peripheral areas of the retinas. The disturbing influence of light scattered by the dioptric media was suppressed by means of stationary background illumination. The basic assumption of the hypothesis that the interaction involves lateral inhibition of the shunting type and subsequent addition of the signal fluxes, could be verified. However, details of the previous hypothesis had to be modified on the basis of the recent findings. In addition, evidence was obtained for a long-range interaction of the pupillomotor signals within one retina and for two systems contributing independently to the light reflex.     

MODEM: a multi-agent hierarchical structure to model the human motor control system

In this study, based on behavioral and neurophysiological facts, a new hierarchical multi-agent architecture is proposed to model the human motor control system. Performance of the proposed structure is investigated by simulating the control of sit to stand movement. To develop the model, concepts of mixture of experts, modular structure, and some aspects of equilibrium point hypothesis were brought together. We have called this architecture MODularized Experts Model (MODEM). Human motor system is modeled at the joint torque level and the role of the muscles has been embedded in the function of the joint compliance characteristics. The input to the motor system, i.e., the central command, is the reciprocal command. At the lower level, there are several experts to generate the central command to control the task according to the details of the movement. The number of experts depends on the task to be performed. At the higher level, a “gate selector” block selects the suitable subordinate expert considering the context of the task. Each expert consists of a main controller and a predictor as well as several auxiliary modules. The main controller of an expert learns to control the performance of a given task by generating appropriate central commands under given conditions and/or constraints. The auxiliary modules of this expert learn to scrutinize the generated central command by the main controller. Auxiliary modules increase their intervention to correct the central command if the movement error is increased due to an external disturbance. Each auxiliary module acts autonomously and can be interpreted as an agent. Each agent is responsible for one joint and, therefore, the number of the agents of each expert is equal to the number of joints. Our results indicate that this architecture is robust against external disturbances, signal-dependent noise in sensory information, and changes in the environment. We also discuss the neurophysiological and behavioral basis of the proposed model (MODEM).     

The distribution of cholinergic neurons in the human central nervous system

Choline acetyltransferase (ChAT), the enzyme responsible for the biosynthesis of acetylcholine, is presently the most specific marker for identifying cholinergic neurons in the central and peripheral nervous systems. The present article reviews immunohistochemical and in situ hybridization studies on the distribution of neurons expressing ChAT in the human central nervous system. Neurons with both immunoreactivity and in situ hybridization signals of ChAT are observed in the basal forebrain (diagonal band of Broca and nucleus basalis of Meynert), striatum (caudate nucleus, putamen and nucleus accumbens), cerebral cortex, mesopontine tegmental nuclei (pedunculopontine tegmental nucleus, laterodorsal tegmental nucleus and parabigeminal nucleus), cranial motor nuclei and spinal motor neurons. The cerebral cortex displays regional and laminal differences in the distribution of neurons with ChAT. The medial septal nucleus and medial habenular nucleus contain immunoreactive neurons for ChAT, which are devoid of ChAT mRNA signals. This is probably because there is a small number of cholinergic neurons with a low level of ChAT gene expression in these nuclei of human. Possible connections and speculated functions of these neurons are briefly summarized.     

Plasticity of central autonomic neural circuits in diabetes

Regulation of energy metabolism is controlled by the brain, in which key central neuronal circuits process a variety of information reflecting nutritional state. Special sensory and gastrointestinal afferent neural signals, along with blood-borne metabolic signals, impinge on parallel central autonomic circuits located in the brainstem and hypothalamus to signal changes in metabolic balance. Specifically, neural and humoral signals converge on the brainstem vagal system and similar signals concentrate in the hypothalamus, with significant overlap between both sensory and motor components of each system and extensive cross-talk between the systems. This ultimately results in production of coordinated regulatory autonomic and neuroendocrine cues to maintain energy homeostasis. Therapeutic metabolic adjustments can be accomplished by modulating viscerosensory input or autonomic motor output, including altering parasympathetic circuitry related to GI, pancreas, and liver regulation. These alterations can include pharmacological manipulation, but surgical modification of neural signaling should also be considered. In addition, central control of visceral function is often compromised by diabetes mellitus, indicating that circuit modification should be studied in the context of its effect on neurons in the diabetic state. Diabetes has traditionally been handled as a peripheral metabolic disease, but the central nervous system plays a crucial role in regulating glucose homeostasis. This review focuses on key autonomic brain areas associated with management of energy homeostasis and functional changes in these areas associated with the development of diabetes.     

Involving the motor system in decision making

The control of behaviour is usually understood in terms of three distinct components: sensory processing, decision making and movement control. Recently, this view has been questioned on the basis of physiological and behavioural data, blurring the distinction between these three stages. This raises the question to what extent the motor system itself can contribute to the interpretation of behavioural situations. To investigate this question we use a neural model of sensory motor integration applied to a behaving mobile robot performing a navigation task. We show that the population response of the motor system provides a substrate for the categorization of behavioural situations. This categorization allows for the assessment of the complexity of a behavioural situation and regulates whether higher-level decision making is required to resolve behavioural conflicts. Our model lends credence to an emerging reconceptualization of behavioural control where the motor system can be considered as part of a high-level perceptual system.     

A model of the neuro-musculo-skeletal system for anticipatory adjustment of human locomotion during obstacle avoidance

Theoretical studies on human locomotion have shown that a stable and flexible gait emerges from the dynamic interaction between the rhythmic activity of a neural system composed of a neural rhythm generator (RG) and the rhythmic movement of the musculo-skeletal system. This study further explores the mechanism of the anticipatory control of locomotion based on the emergent properties of a neural system that generates the basic pattern of gait. A model of the neuro-musculo-skeletal system to execute the task of stepping over a visible obstacle with both limbs during walking is described. The RG in the neural system was combined with a system referred to as a discrete movement generator (DM), which receives both the output of the RG and visual information regarding the obstacle and generates discrete signals for modification of the basic gait pattern. A series of computer simulations demonstrated that an obstacle placed at an arbitrary position can be cleared by sequential modifications of gait: (1) modulating the step length when approaching the obstacle and (2) modifying the trajectory of the swing limbs while stepping over it. This result suggests that anticipatory adjustments are produced not by the unidirectional flow of the information from visual signals to motor commands but by the bi-directional circulation of information between the DM and the RG. The validity of this model is discussed in relation to motor cortical activity during anticipatory modifications in cats and the ecological psychology of visuo-motor control in humans. Received: 19 September 1996 / Accepted in revised form: 21 March 1997     

Comparing information about arm movement direction in single channels of local and epicortical field potentials from monkey and human motor cortex.

Cortical field potentials have been used for decades in neurophysiological studies to probe spatio-temporal activity patterns of local populations of neurons. Recently, however, interest in these signals was spurred as they were proposed as potential control signals for neuronal motor prostheses, i.e., for devices fit to record and decode brain activity to restore motor functions in paralyzed patients. Little is known, however, about the functional significance of these cortical field potentials. Here we compared information about arm movement direction in two types of movement related cortical field potentials, obtained during a four direction center-out arm reaching paradigm: local field potentials (LFPs) recorded with intracortical micro-electrodes from monkey motor cortex, and epicortical field potentials (EFPs) recorded with macro-electrode arrays subdurally implanted on the surface of the human cerebral cortex. While monkey LFPs showed a typical sequence of positive and negative potential peaks, an initial negative peak was the most salient feature of human EFPs. Individual contralateral LFPs from the monkey motor cortex carried approximately twice as much decoded information (DI) about arm movement direction (median 0.27 bit) as did individual EFPs from the contralateral hand/arm area of primary motor cortex in humans (median 0.12 bit). This relation was similar to the relation between median peak signal-to-noise ratios for directional modulation of movement related potentials (MRPs) of both types of signals. We discuss possible reasons for the observed differences, amongst them epi- vs. intracortical recording and the different electrode dimensions used to measure EFPs and LFPs.     

On the neurodynamics of the creation of consciousness

Consciousness is expected to have a specific temporal dynamics. The COrollary Discharge of Attention Movement (CODAM) model of consciousness is deduced from an engineering approach to attention and motor attention. This model is briefly described, as is support arising from brain dynamics, especially that for the attentional blink. The understanding of known temporal dynamics in the brain associated with the emergence of consciousness is then developed from CODAM, and specifically related to the N2 ERP brain signal. How the pre-reflective self, as content-free, interacts with the content of experience is discussed in terms of the possibility that such experience arises from some proto-self generated by body signals; experiments are described which indicate that no pre-reflective self based on body signals is observable. Only a content-free pre-reflective self is consistent with this data, as CODAM suggests. How such a pre-reflective self can be further fused to give temporal continuity of a sense of self is considered in terms of various mechanisms which could be present for preserving the sense of self. The observation of the N2 signal in hippocampal encoding is proposed as providing a justification for the encoding of the N2–P3 sequence of brain signals. This would correspond to episodic encoding of the sequence of experiences of the pre-reflective self; this will thereby provide the necessary control signals in time so that ‘I’ is experienced as part of the retrieval of such memories.