Multi-muscle control of head movements in monkeys: the referent configuration hypothesis

Reaction of human walking to transient block of vision: analysis in the context of indirect, referent control of motor actions

Human locomotion may result from monotonic shifts in the referent position, R, of the body in the environment. R is also the spatial threshold at which muscles can be quiescent but are activated depending on the deflection of the current body configuration Q from R. Shifts in R are presumably accomplished with the participation of proprioceptive and visual feedback and responsible for transferring stable body balance (equilibrium) from one place in the environment to another, resulting in rhythmic activity of multiple muscles by a central pattern generator (CPG). We tested predictions of this two-level control scheme. In particular, in response to a transient block of vision during locomotion, the system can temporarily slow shifts in R. As a result, the phase of rhythmical movements of all four limbs will be changed for some time, even though the rhythm and other characteristics of locomotion will be fully restored after perturbation, a phenomenon called long-lasting phase resetting. Another prediction of the control scheme is that the activity of multiple muscles of each leg can be minimized reciprocally at specific phases of the gait cycle both in the presence and absence of vision. Speed of locomotion is related to the rate of shifts in the referent body position in the environment. Results confirmed that human locomotion is likely guided by feedforward shifts in the referent body location, with subsequent changes in the activity of multiple muscles by the CPG. Neural structures responsible for shifts in the referent body configuration causing locomotion are suggested.

Central pattern generator and human locomotion in the context of referent control of motor actions

Unperturbed human locomotion presumably results from feedforward shifts in stable body equilibrium in the environment, thus avoiding falling and subsequent catching considered in alternative theories of locomotion. Such shifts are achieved by relocation of the referent body configuration at which multiple muscle recruitment begins. Rather than being directly specified by a central pattern generator, multiple muscles are activated depending on the extent to which the body is deflected from the referent, threshold body configuration, as confirmed in previous studies. Based on the referent control theory of action and perception, solutions to classical problems in motor control are offered, including the previously unresolved problem of the integration of central and reflex influences on motoneurons and the problem of how posture and movement are related. The speed of locomotion depends on the rate of shifts in the referent body configuration. The transition from walking to running results from increasing the rate of referent shifts. It is emphasised that there is a certain hierarchy between reciprocal and co-activation of agonist and antagonist muscles during locomotion and other motor actions, which is also essential for the understanding of how locomotor speed is regulated. The analysis opens a new avenue in neurophysiological approaches to human locomotion with clinical implications.

Eye and head movements and vestibulo-ocular reflex in the context of indirect, referent control of motor actions

Conventional explanations of the vestibulo-ocular reflex (VOR) and eye and head movements are revisited by considering two alternative frameworks addressing the question of how the brain controls motor actions. Traditionally, biomechanical and/or computational frameworks reflect the views of several prominent scholars of the past, including Helmholtz and von Holst, who assumed that the brain directly specifies the desired motor outcome and uses efference copy to influence perception. However, empirical studies resulting in the theory of referent control of action and perception (an extension of the equilibrium-point hypothesis) revealed that direct specification of motor outcome is inconsistent with nonlinear properties of motoneurons and with the physical principle that the brain can control motor actions only indirectly, by changing or maintaining the values of neurophysiological parameters that influence, but can remain independent of, biomechanical variables. Some parameters are used to shift the origin (referent) points of spatial frames of reference (FRs) or system of coordinates in which motor actions emerge without being predetermined. Parameters are adjusted until the emergent motor actions meet the task demands. Several physiological parameters and spatial FRs have been identified, supporting the notion of indirect, referent control of movements. Instead of integration of velocity-dependent signals, position-dimensional referent signals underlying head motion can likely be transmitted to motoneurons of extraocular muscles. This would produce compensatory eye movement preventing shifts in gaze during head rotation, even after bilateral destruction of the labyrinths. The referent control framework symbolizes a shift in the paradigm for the understanding of VOR and eye and head movement production.

Indirect, referent control of motor actions underlies directional tuning of neurons

Many neurons of the primary motor cortex (M1) are maximally sensitive to "preferred" hand movement directions and generate progressively less activity with movements away from these directions. M1 activity also correlates with other biomechanical variables. These findings are predominantly interpreted in a framework in which the brain preprograms and directly specifies the desired motor outcome. This approach is inconsistent with the empirically derived equilibrium-point hypothesis, in which the brain can control motor actions only indirectly, by changing neurophysiological parameters that may influence, but remain independent of, biomechanical variables. The controversy is resolved on the basis of experimental findings and theoretical analysis of how sensory and central influences are integrated in the presence of the fundamental nonlinearity of neurons: electrical thresholds. In the presence of sensory inputs, electrical thresholds are converted into spatial thresholds that predetermine the position of the body segments at which muscles begin to be activated. Such thresholds may be considered as referent points of respective spatial frames of reference (FRs) in which neurons, including motoneurons, are centrally predetermined to work. By shifting the referent points of respective FRs, the brain elicits intentional actions. Pure involuntary reactions to perturbations are accomplished in motionless FRs. Neurons are primarily sensitive to shifts in referent directions, i.e., shifts in spatial FRs, whereas emergent neural activity may or may not correlate with different biomechanical variables depending on the motor task and external conditions. Indirect, referent control of posture and movement symbolizes a departure from conventional views based on direct preprogramming of the motor outcome.

Vestibular and corticospinal control of human body orientation in the gravitational field

Body orientation with respect to the direction of gravity changes when we lean forward from upright standing. We tested the hypothesis that during upright standing, the nervous system specifies the referent body orientation that defines spatial thresholds for activation of multiple muscles across the body. To intentionally lean the body forward, the system is postulated to transfer balance and stability to the leaned position by monotonically tilting the referent orientation, thus increasing the activation thresholds of ankle extensors and decreasing their activity. Consequently, the unbalanced gravitational torque would start to lean the body forward. With restretching, ankle extensors would be reactivated and generate increasing electromyographic (EMG) activity until the enhanced gravitational torque would be balanced at a new posture. As predicted, vestibular influences on motoneurons of ankle extensors evaluated by galvanic vestibular stimulation were smaller in the leaned compared with the upright position, despite higher tonic EMG activity. Defacilitation of vestibular influences was also observed during forward leaning when the EMG levels in the upright and leaned position were equalized by compensating the gravitational torque with a load. The vestibular system is involved in the active control of body orientation without directly specifying the motor outcome. Corticospinal influences originating from the primary motor cortex evaluated by transcranial magnetic stimulation remained similar at the two body postures. Thus, in contrast to the vestibular system, the corticospinal system maintains a similar descending facilitation of motoneurons of leg muscles at different body orientations. The study advances the understanding of how body orientation is controlled. NEW & NOTEWORTHY The brain changes the referent body orientation with respect to gravity to lean the body forward. Physiologically, this is achieved by shifts in spatial thresholds for activation of ankle muscles, which involves the vestibular system. Results advance the understanding of how the brain controls body orientation in the gravitational field. The study also extends previous evidence of empirical control of motor function, i.e., without the reliance on model-based computations and direct specification of motor outcome.

Referent control and motor equivalence of reaching from standing

Motor actions may result from central changes in the referent body configuration, defined as the body posture at which muscles begin to be activated or deactivated. The actual body configuration deviates from the referent configuration, particularly because of body inertia and environmental forces. Within these constraints, the system tends to minimize the difference between these configurations. For pointing movement, this strategy can be expressed as the tendency to minimize the difference between the referent trajectory (R_T) and actual trajectory (Q_T) of the effector (hand). This process may underlie motor equivalent behavior that maintains the pointing trajectory regardless of the number of body segments involved. We tested the hypothesis that the minimization process is used to produce pointing in standing subjects. With eyes closed, 10 subjects reached from a standing position to a remembered target located beyond arm length. In randomly chosen trials, hip flexion was unexpectedly prevented, forcing subjects to take a step during pointing to prevent falling. The task was repeated when subjects were instructed to intentionally take a step during pointing. In most cases, reaching accuracy and trajectory curvature were preserved due to adaptive condition-specific changes in interjoint coordination. Results suggest that referent control and the minimization process associated with it may underlie motor equivalence in pointing.

NEW & NOTEWORTHY

Motor actions may result from minimization of the deflection of the actual body configuration from the centrally specified referent body configuration, in the limits of neuromuscular and environmental constraints. The minimization process may maintain reaching trajectory and accuracy regardless of the number of body segments involved (motor equivalence), as confirmed in this study of reaching from standing in young healthy individuals. Results suggest that the referent control process may underlie motor equivalence in reaching.

Cross-validated models of the relationships between neck muscle electromyography and three-dimensional head kinematics during gaze behavior

The object of this study was to model the relationship between neck electromyography (EMG) and three-dimensional (3-D) head kinematics during gaze behavior. In two monkeys, we recorded 3-D gaze, head orientation, and bilateral EMG activity in the sternocleidomastoid, splenius capitis, complexus, biventer cervicis, rectus capitis posterior major, and occipital capitis inferior muscles. Head-unrestrained animals fixated and made gaze saccades between targets within a 60° × 60° grid. We performed a stepwise regression in which polynomial model terms were retained/rejected based on their tendency to increase/decrease a cross-validation-based measure of model generalizability. This revealed several results that could not have been predicted from knowledge of musculoskeletal anatomy. During head holding, EMG activity in most muscles was related to horizontal head orientation, whereas fewer muscles correlated to vertical head orientation and none to small random variations in head torsion. A fourth-order polynomial model, with horizontal head orientation as the only independent variable, generalized nearly as well as higher order models. For head movements, we added time-varying linear and nonlinear perturbations in velocity and acceleration to the previously derived static (head holding) models. The static models still explained most of the EMG variance, but the additional motion terms, which included horizontal, vertical, and torsional contributions, significantly improved the results. Several coordinate systems were used for both static and dynamic analyses, with Fick coordinates showing a marginal (nonsignificant) advantage. Thus, during gaze fixations, recruitment within the neck muscles from which we recorded contributed primarily to position-dependent horizontal orientation terms in our data set, with more complex multidimensional contributions emerging during the head movements that accompany gaze shifts. These are crucial components of the late neuromuscular transformations in a complete model of 3-D head-neck system and should help constrain the study of premotor signals for head control during gaze behaviors.

The equilibrium-point hypothesis--past, present and future

This chapter is a brief account of fundamentals of the equilibrium-point hypothesis or more adequately called the threshold control theory (TCT). It also compares the TCT with other approaches to motor control. The basic notions of the TCT are reviewed with a major focus on solutions to the problems of multi-muscle and multi-degrees of freedom redundancy. The TCT incorporates cognitive aspects by explaining how neurons recognize that internal (neural) and external (environmental) events match each other. These aspects as well as how motor learning occurs are subjects of further development of the TCT hypothesis.

Neck muscle synergies during stimulation and inactivation of the interstitial nucleus of Cajal (INC)

The interstitial nucleus of Cajal (INC) is thought to control torsional and vertical head posture. Unilateral microstimulation of the INC evokes torsional head rotation to positions that are maintained until stimulation offset. Unilateral INC inactivation evokes head position-holding deficits with the head tilted in the opposite direction. However, the underlying muscle synergies for these opposite behavioral effects are unknown. Here, we examined neck muscle activity in head-unrestrained monkeys before and during stimulation (50 muA, 200 ms, 300 Hz) and inactivation (injection of 0.3 mul of 0.05% muscimol) of the same INC sites. Three-dimensional eye and head movements were recorded simultaneously with electromyographic (EMG) activity in six bilateral neck muscles: sternocleidomastoid (SCM), splenius capitis (SP), rectus capitis posterior major (RCPmaj.), occipital capitis inferior (OCI), complexus (COM), and biventer cervicis (BC). INC stimulation evoked a phasic, short-latency ( approximately 5-10 ms) facilitation and later ( approximately 100-200 ms) a more tonic facilitation in the activity of ipsi-SCM, ipsi-SP, ipsi-COM, ipsi-BC, contra-RCPmaj., and contra-OCI. Unilateral INC inactivation led to an increase in the activity of contra-SCM, ipsi-SP, ipsi-RCPmaj., and ipsi-OCI and a decrease in the activity of contra-RCPmaj. and contra-OCI. Thus the influence of INC stimulation and inactivation were opposite on some muscles (i.e., contra-OCI and contra-RCPmaj.), but the comparative influences on other neck muscles were more variable. These results show that the relationship between the neck muscle responses during INC stimulation and inactivation is much more complex than the relationship between the overt behaviors.

Threshold position control of arm movement with anticipatory increase in grip force

The grip force holding an object between fingers usually increases before or simultaneously with arm movement thus preventing the object from sliding. We experimentally analyzed and simulated this anticipatory behavior based on the following notions. (1) To move the arm to a new position, the nervous system shifts the threshold position at which arm muscles begin to be recruited. Deviated from their activation thresholds, arm muscles generate activity and forces that tend to minimize this deviation by bringing the arm to a new position. (2) To produce a grip force, with or without arm motion, the nervous system changes the threshold configuration of the hand. This process defines a threshold (referent) aperture (R(a)) of appropriate fingers. The actual aperture (Q(a)) is constrained by the size of the object held between the fingers whereas, in referent position R(a), the fingers virtually penetrate the object. Deviated by the object from their thresholds of activation, hand muscles generate activity and grip forces in proportion to the gap between the Q(a) and R(a). Thus, grip force emerges since the object prevents the fingers from reaching the referent position. (3) From previous experiences, the system knows that objects tend to slide off the fingers when arm movements are made and, to prevent sliding, it starts narrowing the referent aperture simultaneously with or somewhat before the onset of changes in the referent arm position. (4) The interaction between the fingers and the object is accomplished via the elastic pads on the tips of fingers. The pads are compressed not only due to the grip force but also due to the tangential inertial force ("load") acting from the object on the pads along the arm trajectory. Compressed by the load force, the pads move back and forth in the gap between the finger bones and object, thus inevitably changing the normal component of the grip force, in synchrony with and in proportion to the load force. Based on these notions, we simulated experimental elbow movements and grip forces when subjects rapidly changed the elbow angle while holding an object between the index finger and the thumb. It is concluded that the anticipatory increase in the grip force with or without correlation with the tangential load during arm motion can be explained in neurophysiological and biomechanical terms without relying on programming of grip force based on an internal model.

Threshold position control and the principle of minimal interaction in motor actions

The answer to the question of how the nervous system controls multiple muscles and body segments while solving the redundancy problem in choosing a unique action from the set of many possible actions is still a matter of controversy. In an attempt to clarify the answer, we review data showing that motor actions emerge from central resetting of the threshold position of appropriate body segments, i.e. the virtual position at which muscles are silent but deviations from it will elicit activity and resistive forces (threshold position control). The difference between the centrally-set threshold position and the sensory-signaled actual position is responsible for the activation of neuromuscular elements and interactions between them and the environment. These elements tend to diminish the evoked activity and interactions by minimizing the gap between the actual position and the threshold position (the principle of minimal interaction). Threshold control per se does not solve the redundancy problem: it only limits the set of possible actions. The principle of minimal interaction implies that the system relies on the natural capacity of neuromuscular elements to interact between themselves and with the environment to reduce this already restricted set to a unique action, thus solving the redundancy problem in motor control. This theoretical framework appears to be helpful in the explanation of the control and production of a variety of actions (reaching movements, specification of different hand configurations, grip force generation, and whole-body movements such as sit-to-stand or walking). Experimental tests of this theory are provided. The prediction that several types of neurons specify referent control variables for motor actions may be tested in future studies. The theory may also be advanced by applying the notion of threshold control to perception and cognition.

Muscle coordination in complex movements during Jeté in skilled ballet dancers

The principal goal of our study is to gain an insight into the coordinative structure of a complex body movement. As a first step, this paper describes the activity of multiple skeletal muscles associated with the drawing-like movements that resemble the Jeté, performed by skilled ballet dancers. The EMG activity of 18 muscles of the trunk, pelvis, and both legs was recorded when dancers standing on the left leg moved the toe of the right leg forward and backward along a straight line. A major finding is that the EMG activity of all right muscles, despite their functional and anatomical diversity, was minimised not only at the initial, vertical position but also in the reversal phase of movement when the moving leg was maximally deviated from the vertical position. In other words, the activity was minimal when torques of the weights of limb segments were minimal as well as when these torques were maximal. In contrast, in the static task when the maximally deviated leg position was maintained, there was substantial tonic activation of leg muscles, an activity that was necessary to balance these torques. The result is consistent with the hypothesis that movements of the body result from centrally induced changes in the muscle recruitment thresholds influencing the referent configuration of the body. The existence of minima in the overall EMG activity of skeletal muscles is not the only prediction of the referent configuration hypothesis. An immediate consequence of the hypothesis is that, in movements of the limb, the EMG patterns should be a direction-dependent phenomenon known as "directional tuning" of muscles. In combination with the principle of minimal interaction of neuromuscular system, the referent configuration hypothesis offers a dynamic approach to the problems of how control levels may guide multi-muscle and multi-joint systems without redundancy problems.

Kinematic synergy adaptation to an unstable support surface and equilibrium maintenance during forward trunk movement

The aim of this investigation was to study the adaptation to an unstable support surface of kinematic synergy responsible for equilibrium control during upper trunk movements. Eight adult subjects were asked to bend their upper trunk forward to an angle of 35 degrees and then to hold the final position for 3 s, first in a standard condition, with two feet on the ground and the second, on a rocking platform swinging in the sagittal plane. The movement characteristics (duration, amplitude, and mean angular velocity of the trunk), the time course of the antero-posterior center of mass (CM) shift during the movement, and the EMG pattern of the main muscles involved in the movement were studied under the two experimental conditions. Kinematic synergy was quantified by performing a principal component analysis on the hip, knee, and ankle angle changes occurring during the movement. The results indicate that (1) the CM shift from the very onset of the movement remains controlled during performance of the forward trunk movement when the equilibrium constraints were increased; (2) the principal component analysis of the hip, knee, and ankle angle changes occurring during the movement showed a transition from one principal component (PC(1)) in the standard condition to two components in the rocking platform condition; (3) the greatest contribution of PC(1) (weight coefficients) was located at the hip level in both the standard and rocking platform conditions, while the greatest contribution of PC(2) in the rocking platform condition was located at the ankle level; and (4) the EMG pattern underlying kinematic synergy is modified. It is concluded that a simple adaptation of kinematic synergy by changing the weight coefficients of each pair of joints participating in the movement is no longer sufficient when the equilibrium constraints increase and, rather, disturbs equilibrium. The CNS has to provide two parallel controls, one to perform the trunk movement and the other to preserve equilibrium.

Basic elements of arm postural control analyzed by unloading

To address the question of how arm posture is controlled, we analyzed shoulder-elbow unloading responses in the horizontal plane for different directions of the initial load. The initial load, produced by a double-joint manipulandum, was suddenly diminished to 1of 12 randomly presented levels (60 to -10% of the initial load; in 6 out of 12 cases the final load direction varied by +/-20 degrees ). Subjects were instructed "not to intervene" in response to unloading. Neither the unloading onset nor the final load level was predictable and we assumed that the responses to rapid unloading were involuntary. Unloading elicited a smooth hand movement characterized by a bell-shaped velocity profile. The changes in hand position, joint angles, and joint torques generally increased with greater amounts of unloading. For each direction of the initial load, tonic electromyographic activity of the shoulder and elbow muscles also changed, depending on the amount of unloading. The shoulder and elbow joint torques before and after unloading were a function of the difference between the actual configuration of the arm and its referent configuration (R) described by the angles at which each joint torque was zero. The R configuration changed depending on the direction of the initial load. Our electromyographic data imply that these changes result from a central modification of muscle activation thresholds. The nervous system may thus control the R configuration in a task-specific way by leaving it unchanged to generate involuntary responses to unloading or modifying it to accommodate a new load direction at the same initial position. It is concluded that the R configuration is a major variable in both intentional and involuntary control of posture.

Control of double-joint arm posture in adults with unilateral brain damage

It has been suggested that multijoint movements result from the specification of a referent configuration of the body. The activity of muscles and forces required for movements emerge depending on the difference between the actual and referent body configurations. We identified the referent arm configurations specified by the nervous system to bring the arm to the target position both in healthy individuals and in those with arm motor paresis due to stroke. From an initial position of the right arm, subjects matched a force equivalent to 30% of their maximal voluntary force in that position. The external force, produced at the handle of a double-joint manipulandum by two torque motors, pulled the hand to the left (165 degrees ) or pushed it to the right (0 degrees ). For both the initial conditions, three directions of the final force (0 degrees , +20 degrees , and -20 degrees ) with respect to the direction of the initial force were used. Subjects were instructed not to intervene when the load was unexpectedly partially or completely removed. Both groups of subjects produced similar responses to unloading of the double-joint arm system. Partial removal of the load resulted in distinct final hand positions associated with unique shoulder-elbow configurations and joint torques. The net static torque at each joint before and after unloading was represented as a function of the two joint angles describing a planar surface or invariant characteristic in 3D torque/angle coordinates. For each initial condition, the referent arm configuration was identified as the combination of elbow and shoulder angles at which the net torques at the two joints were zero. These configurations were different for different initial conditions. The identification of the referent configuration was possible for all healthy participants and for most individuals with hemiparesis suggesting that they preserved the ability to adapt their central commands-the referent arm configurations-to accommodate changes in external load conditions. Despite the preservation of the basic response patterns, individuals with stroke damage had a more restricted range of hand trajectories following unloading, an increased instability around the final endpoint position, altered patterns of elbow and shoulder muscle coactivation, and differences in the dispersion of referent configurations in elbow-shoulder joint space compared to healthy individuals. Moreover, 4 out of 12 individuals with hemiparesis were unable to specify referent configurations of the arm in a consistent way. It is suggested that problems in the specification of the referent configuration may be responsible for the inability of some individuals with stroke to produce coordinated multijoint movements. The present work adds three findings to the motor control literature concerning stroke: non-significant torque/angle relationships in some subjects, narrower range of referent arm configurations, and instability about the final position. This is the first demonstration of the feasibility of the concept of the referent configuration for the double-joint muscle-reflex system and the ability of some individuals with stroke to produce task-specific adjustments of this configuration.

Testing hypotheses and the advancement of science: recent attempts to falsify the equilibrium point hypothesis

Criticisms of the equilibrium point (EP) hypothesis have recently appeared that are based on misunderstandings of some of its central notions. Starting from such interpretations of the hypothesis, incorrect predictions are made and tested. When the incorrect predictions prove false, the hypothesis is claimed to be falsified. In particular, the hypothesis has been rejected based on the wrong assumptions that it conflicts with empirically defined joint stiffness values or that it is incompatible with violations of equifinality under certain velocity-dependent perturbations. Typically, such attempts use notions describing the control of movements of artificial systems in place of physiologically relevant ones. While appreciating constructive criticisms of the EP hypothesis, we feel that incorrect interpretations have to be clarified by reiterating what the EP hypothesis does and does not predict. We conclude that the recent claims of falsifying the EP hypothesis and the calls for its replacement by EMG-force control hypothesis are unsubstantiated. The EP hypothesis goes far beyond the EMG-force control view. In particular, the former offers a resolution for the famous posture-movement paradox while the latter fails to resolve it.

Posture-based or trajectory-based movement planning: a comparison of direct and indirect pointing movements

Various models have been proposed in the literature to explain the control of human arm movements. To make a quantitative comparison between the predictions of various models, we tested subjects for movements to targets on a vertical screen in various conditions. Subjects were asked to move directly from one target to another, or to move by a via-point, at various movement velocities and in a condition with a weight of 0.6 kg attached to the forearm. This set of experimental data was used for comparison with the predictions by various posture-based and trajectory-based models on 3-D movement planning and control. Small but significant effects of starting position and path towards the target were found on the torsion of the arm at the end of the movement. No effects of movement velocity and weight attached to the forearm were found. The experimental results differed significantly from the predictions by any of the models considered. Of the models considered, Donders' law best predicts the experimental data. Our data indicate that future tests of models for motor control (1) should compare the predictions of not just one, but several models to a data set, and (2) should include not only planar, but rather 3-D movements in such a comparison.

Governing coordination: behavioural principles and neural correlates

The coordination of movement is governed by a coalition of constraints. The expression of these constraints ranges from the concrete--the restricted range of motion offered by the mechanical configuration of our muscles and joints; to the abstract--the difficulty that we experience in combining simple movements into complex rhythms. We seek to illustrate that the various constraints on coordination are complementary and inclusive, and the means by which their expression and interaction are mediated systematically by the integrative action of the central nervous system (CNS). Beyond identifying the general principles at the behavioural level that govern the mutual interplay of constraints, we attempt to demonstrate that these principles have as their foundation specific functional properties of the cortical motor systems. We propose that regions of the brain upstream of the motor cortex may play a significant role in mediating interactions between the functional representations of muscles engaged in sensorimotor coordination tasks. We also argue that activity in these "supramotor" regions may mediate the stabilising role of augmented sensory feedback.

Pointing movements may be produced in different frames of reference depending on the task demand

Movements are likely guided by the nervous system in task-specific spatial frames of reference (FRs). We tested this hypothesis by analyzing fast arm pointing movements involving the trunk made to targets located within the reach of the arm. In the first experiment, subjects pointed to a motionless target and, in the second experiment, to a target moving synchronously with the trunk. Vision of the arm and targets was prevented before movement onset. Each experiment started after three to five training trials. In randomly selected trials of both experiments, an electromagnet device unexpectedly prevented the trunk motion. When the trunk was arrested, the hand trajectory and velocity profile remained invariant in an FR associated with the experimental room in the first or in an FR moving with the trunk in the second experiment. Substantial changes in the arm interjoint coordination in response to the trunk arrest were observed in the first but not in the second experiment. The results demonstrate the ability of the nervous system to rapidly adapt behavior at the joint level to transform motor performance from a spatial FR associated with the environment to one associated with the body. A theoretical framework is suggested in which FRs are considered as pre-existing neurophysiological structures permitting switching between different FRs and guiding multiple joints and muscles without redundancy problems.

Sequential control signals determine arm and trunk contributions to hand transport during reaching in humans

When reaching towards objects placed outside the arm workspace, the trunk assumes an active role in transport of the hand by contributing to the extent of movement while simultaneously maintaining the direction of reach. We investigated the spatial-temporal aspects of the integration of the trunk motion into reaching. Specifically, we tested the hypothesis that the efficiency ('gain') of the arm-trunk co-ordination determining the contribution of the trunk to the extent of hand movement may vary substantially with the phase of reaching. Sitting subjects made fast pointing movements towards ipsi- and a contralateral targets placed beyond the reach of the right arm so that a forward trunk motion was required to assist in transporting the hand to the target. Sight of the arm and target was blocked before the movement onset. In randomly selected trials, the trunk motion was unexpectedly prevented by an electromagnet. Subjects were instructed to make stereotypical movements whether or not the trunk was arrested. In non-perturbed trials, most subjects began to move the hand and trunk simultaneously. In trunk-blocked trials, it was impossible for the hand to cover the whole pointing distance but the hand trajectory and velocity profile initially matched those from the trials in which the trunk motion was free, approximately until the hand reached its peak velocity. The arm inter-joint co-ordination substantially changed in response to the trunk arrest at a minimal latency of 40 ms after the perturbation onset. The results suggest that when the trunk was free, the influence of the trunk motion on the hand trajectory and velocity profile was initially neutralized by appropriate changes in the arm joint angles. Only after the hand had reached its peak velocity did the trunk contribute to the extent of pointing. Previous studies suggested that the central commands underlying the transport component of arm movements are completed when the hand reaches peak velocity. These studies, together with the present finding that the trunk only begins to contribute to the hand displacement at peak hand velocity, imply that the central commands that determine the contributions of the arm and the trunk to the transport of the hand are generated sequentially, even though the arm and trunk move in parallel.

Neck muscles in the rhesus monkey. II. Electromyographic patterns of activation underlying postures and movements

Electromyographic (EMG) activity was recorded in < or = 12 neck muscles in four alert monkeys whose heads were unrestrained to describe the spatial and temporal patterns of neck muscle activation accompanying a large range of head postures and movements. Some head postures and movements were elicited by training animals to generate gaze shifts to visual targets. Other spontaneous head movements were made during orienting, tracking, feeding, expressive, and head-shaking behaviors. These latter movements exhibited a wider range of kinematic patterns. Stable postures and small head movements of only a few degrees were associated with activation of a small number of muscles in a reproducible synergy. Additional muscles were recruited for more eccentric postures and larger movements. For head movements during trained gaze shifts, movement amplitude, velocity, and acceleration were correlated linearly and agonist muscles were recruited without antagonist muscles. Complex sequences of reciprocal bursts in agonist and antagonist muscles were observed during very brisk movements. Turning movements of similar amplitudes that began from different initial head positions were associated with systematic variations in the activities of different muscles and in the relative timings of these activities. Unique recruitment synergies were observed during feeding and head-shaking behaviors. Our results emphasize that the recruitment of a given muscle was generally ordered and consistent but that strategies for coordination among various neck muscles were often complex and appeared to depend on the specifics of musculoskeletal architecture, posture, and movement kinematics that differ substantially among species.