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Biomedical subjects

E Bizzi

Publications and source records attributed to E Bizzi.

At least 37 records · Page 2Linked to original sources
PubMed1995

Modular organization of motor behavior in the frog's spinal cord.

The complex issue of translating the planning of arm movements into muscle forces is discussed in relation to the recent discovery of structures in the spinal cord. These structures contain circuitry that, when activated, produce precisely balanced contractions in groups of muscles. These synergistic contractions generate forces that direct the limb toward an equilibrium point in space. Remarkably, the force outputs, produced by activating different spinal-cord structures, sum vectorially. This vectorial combination of motor outputs might be a mechanism for producing a vast repertoire of motor behaviors in a simple manner.

Animals↗
PubMed1994

Linear combinations of primitives in vertebrate motor control.

Recent investigations on the spinalized frog have provided evidence suggesting that the neural circuits in the spinal cord are organized into a number of distinct functional modules. We have investigated the rule that governs the coactivation of two such modules. To this end, we have developed an experimental paradigm that involves the simultaneous stimulation of two sites in the frog's spinal cord and the quantitative comparison of the resulting mechanical response with the summation of the responses obtained from the stimulation of each site. We found that the simultaneous stimulation of two sites leads to the vector summation of the endpoint forces generated by each site separately. This linear behavior is quite remarkable and provides strong support to the view that the central nervous system may generate a wide repertoire of motor behaviors through the vectorial superposition of a few motor primitives stored within the neural circuits in the spinal cord.

Animals↗
PubMed1993

Intermediate representations in the formation of arm trajectories.

Psychophysical evidence shows that the planning of an arm trajectory is specified by the central nervous system in extrinsic coordinates. The complex issue of translating the planning of arm movements into muscle forces is discussed in relation to the recent discovery of structures in the brainstem and in the spinal cord. These structures represent discrete maps of motor behavior. Remarkably, the force outputs, produced by activating different zones of the map, sum vectorially. This vectorial combination of motor outputs is a mechanism for producing a vast repertoire of motor behaviors in a simple fashion.

Animals↗
PubMed1993

Effects of dorsal root cut on the forces evoked by spinal microstimulation in the spinalized frog.

Spinalized frogs were microstimulated in the intermediate grey layers of the lumbar spinal cord; the forces evoked in the hindlimb were measured at several limb positions. The data were expressed as force fields. After the collection of many force fields, the dorsal roots were cut with the stimulating electrode in place, and the position-dependent stimulation-evoked forces were again measured repeatedly. We found that the position-dependent pattern of evoked forces--the force fields--did not change after the dorsal roots were cut. In other words, the postcut evoked forces pointed in the same direction as the precut evoked forces. This result was predicted and confirmed by the muscle activations (EMGs): Before and after the dorsal roots were cut, the same muscles were activated in the same proportions. In all limb positions, the rank ordering of the muscle activations remained fixed. The stimulation needed to evoke forces was increased by deafferentation, and there were subtle changes in the force magnitudes that were consistent with a linearization of the muscle stiffness by the afferents. We conclude that the microstimulation activated specific muscle synergies that resulted in limb forces pointing toward a particular posture. The patterns of evoked forces were predominantly attributable to feedforward activation of these muscle synergies.

Animals↗
PubMed1993

Postural force fields of the human arm and their role in generating multijoint movements.

When a perturbation displaces the human hand from equilibrium, arm muscles respond by producing restoring forces. When a set of displacements are given at various directions from the same equilibrium position, the resulting restoring forces form a "postural force field." It is not known whether these postural forces are related to those generated when a reaching movement is executed. However, if a movement is a consequence of a shift of the equilibrium position of the hand toward the target, then, from the postural force field, predictions can be made regarding the nature of the elastic forces acting on the hand during the movement. We have taken the first steps in testing this hypothesis by measuring the postural force field of a subject's arm over relatively large distances, and comparing these forces with the static forces generated at the hand while the subject attempted a reaching movement. Using a robot manipulandum, the hand was displaced at various directions from an equilibrium position. The measured restoring forces were fitted to a nonlinear model to define a postural force field for that equilibrium position. This field was used to predict elastic forces generated when the subject attempted to move the manipulandum from a point on the circumference of a circle to a target at its center--the center corresponded to the equilibrium position at which the postural field was measured. In some of the movement trials, the manipulandum was locked during approximately the first 120 msec of the program for motion and the resulting static "evoked" forces measured. We found that (1) the evoked forces did not point to the target, but were a function of the configuration of the arm and rotated with the shoulder joint, and (2) the magnitude of the evoked forces varied systematically, even though the movements were of the same magnitude. These patterns were remarkably similar to those observed in the postural forces. Our results provide experimental evidence linking maintenance of posture in a multijoint system to that of generating a movement. The evidence is consistent with the hypothesis that the CNS programs a reaching movement by shifting the equilibrium position of the hand toward the target.

Adult↗
PubMed1993

Convergent force fields organized in the frog's spinal cord.

Microstimulation of the gray matter of the frog's spinal cord was used to elicit motor responses. Force responses were recorded with the frog's ankle clamped while EMG activity was monitored. The collections of force patterns elicited at different leg configurations were summarized as force fields. These force fields showed convergence to an equilibrium point. The equilibrium paths were calculated from the force fields with the leg clamped. These paths predicted free limb motion in 75% of trials. The force fields were separated into active and prestimulation resting responses. The active force field responses had a fixed position equilibrium. These active force fields were modulated in amplitude over time, although the balance and orientations of forces in the pattern remained fixed. The active fields grouped into a few classes. These included both convergent and parallel fields. The convergent force fields (CFFS) could be observed in deafferented preparations. Motoneuron (MN) activity underlying the force fields was marked using sulforhodamine. The marked activity covered several segments. Several simulations and MN stimulations show that topography, limb geometry, and random activation could not account for the results. It is likely that propriospinal interneurons distribute the activity that underlies the responses observed here. Experiments showed that CFFs that resemble those elicited by microstimulation also underlie natural behaviors. The full variety of fields revealed by microstimulation was larger than the repertoire elicited by cutaneous stimulation. It was concluded that fixed-pattern force fields elicited in the spinal cord may be viewed as movement primitives. These force fields could form building blocks for more complex behaviors.

Animals↗
PubMed1991

Computations underlying the execution of movement: a biological perspective.

To execute voluntary movements, the central nervous system must transform the neural representation of the direction, amplitude, and velocity of the limb, represented by the activity of cortical and subcortical neurons, into signals that activate the muscles that move the limb. This task is equivalent to solving an "ill-posed" computational problem because the number of degrees of freedom of the musculoskeletal apparatus is much larger than that specified in the plan of action. Some of the mechanisms and circuitry underlying the transformation of motor plans into motor commands are described. A central feature of this transformation is a coarse map of limb postures in the premotor areas of the spinal cord. Vectorial combination of motor outputs among different areas of the spinal map may produce a large repertoire of motor behaviors.

Animals↗
PubMed1989

A neural network model for limb trajectory formation.

This paper deals with the problem of representing and generating unconstrained aiming movements of a limb by means of a neural network architecture. The network produced time trajectories of a limb from a starting posture toward targets specified by sensory stimuli. Thus the network performed a sensory-motor transformation. The experimenters trained the network using a bell-shaped velocity profile on the trajectories. This type of profile is characteristic of most movements performed by biological systems. We investigated the generalization capabilities of the network as well as its internal organization. Experiments performed during learning and on the trained network showed that: (i) the task could be learned by a three-layer sequential network; (ii) the network successfully generalized in trajectory space and adjusted the velocity profiles properly; (iii) the same task could not be learned by a linear network; (iv) after learning, the internal connections became organized into inhibitory and excitatory zones and encoded the main features of the training set; (v) the model was robust to noise on the input signals; (vi) the network exhibited attractor-dynamics properties; (vii) the network was able to solve the motor-equivalence problem. A key feature of this work is the fact that the neural network was coupled to a mechanical model of a limb in which muscles are represented as springs. With this representation the model solved the problem of motor redundancy.

Extremities↗
PubMed1989

Kinematic strategies and sensorimotor transformations in the wiping movements of frogs.

1. Spinal frogs are known to make coordinated and successful wiping movements to almost all places on the body and legs. Such wiping movements involve a sensorimotor transformation. Information from both the spatial locations of stimuli on the skin and the body configuration of the frog is transformed into a set of motor commands that generate body movements adequate to successfully remove the irritant. The spinal cord itself therefore has a limited capacity for sensorimotor transformations. 2. We examined the kinematics of wiping motions in both spinal and intact leopard frogs and bullfrogs. This data was used to assess the flexibility, precision, and strategy of the kinematic sensorimotor transformations used during wiping. The movements involved the use of redundant degrees of freedom in the limbs. Thus many possible movements or solutions could generate successful wiping. This redundancy allows motor-equivalent movements to be used by the frog. 3. Movements were examined in two dimensions by the use of VHS shuttered-video recording and in three dimensions with the use of a WATSMART system of infrared diodes and cameras. The kinematic analysis was applied to those motions in which the limbs did not interact with kinematic constraints, such as the surface of the substrate or body. These unconstrained motions are directly related to motor commands and thus more easily interpreted. 4. Wiping movements to the back were retained in essentially the same form in both spinal and intact frogs. In both cases wiping had four phases with a fifth occasionally present. The phases included flexion, placing, aiming, and whisking, with occasional extension and multiply repeated wipes. However, the aiming phase was often very brief or absent in this data, and flexion was sometimes omitted in multiple wipes. We found that the placing posture was adjusted in a simple way in response to variations in the location of the target stimulus. The rostrocaudal position of the foot tip was strongly and linearly related to the rostrocaudal stimulus location. 5. During the placing posture, joint angles as well as the limb tip in back wipes had linear relationships to the stimulus' rostrocaudal coordinate. The limb configuration used by the frog allowed a strategy of linear (and potentially independent) postural adjustment of joint angle to stimulus position to generate almost linear endpoint adjustments in the placing phase of wiping. This solution to the ill-posed problem of choosing a joint angle for the placing posture in back-wiping may be computationally simple.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1987

Controlling multijoint motor behavior.

Much can be learned about the central nervous system from a study of motor coordination, but its true richness and complexity become evident only in a multiarticular system. Despite the intrinsic complexity of multiarticular actions, they offer an unparalleled opportunity to learn about the central nervous system in a quantitative and experimentally testable way. For example, the observation that unconstrained, unperturbed arm movements are coordinated in terms of hand motion shows that motor control is organized in a hierarchy of increasing levels of abstraction. These arm motions are organized as though a disembodied hand could be moved in space; the details of how this is to be achieved must then be supplied by a different level in the hierarchy. The essence of human behavior is its adaptability. Just as the true complexity of coordination is evident only in multiarticular actions, the sophistication and subtlety of adaptive behavior are evident only in dynamic, interactive tasks. A study of movement alone is not sufficient to understand this behavior. The dynamic response of the limbs becomes the overriding concern and must be controlled by the central nervous system. The dynamic response of a limb is usually associated with its posture, rather than its movement, but in a functional task such as the use of a tool, the postural dynamics are an integral part of the action. This perspective on motor behavior leads to some useful insights. Coordination is not a problem for movement alone; in a multiarticular system, even posture requires coordination and control. Muscles do not merely act reciprocally to generate forces about the joints; the net mechanical impedance of the limb may be controlled by synergistic activation of all muscles, including antagonists. Controlling dynamic behavior is a far more demanding task than controlling motion. Consequently, features of the neuromusculoskeletal system that appear to be redundant or unnecessary for movement control can play a functional role in controlling dynamic behavior. Polyarticular muscles contribute to the mechanical impedance in a unique way. Skeletal redundancies have a profound influence on all aspects of dynamic behavior, including the apparent inertia of the limbs. Redundancies are commonly perceived as a complicating factor in the control of motion, a problem that must be solved by the central nervous system. Rather than presenting a problem requiring solution, they may present a solution to a problem. Posture is not merely the outcome of a motor act; it is one of the important preparatory stages in the production of motor behavior.

Hand↗
PubMed1987

Motor control mechanisms. An overview.

Recent experiments suggest a simple relationship between posture and movement. Posture appears to result from the CNS setting the activity levels of agonist and antagonist muscles around the joint, resulting in an equilibrium force, the ratios of which are different for different positions. Control of movement is accomplished by changing the set of opposing length-tension curves. However, the experimental evidence also indicates that during movement there is active control of the trajectory in addition to control of final position. A simple strategy of choosing a new posture via the length-tension curves and letting a limb move until equilibrium is established is not the control algorithm used by the vertebrate motor system.

Animals↗
PubMed1985

Neural, mechanical, and geometric factors subserving arm posture in humans.

When the hand is displaced from an equilibrium posture by an external disturbance, a force is generated to restore the original position. We developed a new experimental method to measure and represent the field of elastic forces associated with posture of the hand in the horizontal plane. While subjects maintained a given posture, small displacements of the hand along different directions were delivered by torque motors. The hand was held in the displaced positions and, at that time, we measured the corresponding restoring forces before the onset of any voluntary reaction. The stiffness in the vicinity of the hand equilibrium position was estimated by analyzing the force and displacement vectors. We chose to represent the stiffness both numerically, as a matrix, and graphically, as an ellipse characterized by three parameters: magnitude (the area), shape (the ratio of axis) and orientation (direction of the major axis). The latter representation captures the main geometrical features of the elastic force field associated with posture. We also evaluated the conservative and nonconservative components of this elastic force field. We found that the former were much larger than the latter and concluded that the behavior of the neuromuscular system of the multiarticular arm is predominantly spring-like. Our data indicated that the shape and orientation of the stiffness were invariant over subjects and over time. We also investigated the ability of our subjects to produce voluntary and adaptive changes in the stiffness. Our findings indicated that, when a disturbance acting along a fixed and predictable direction was imposed, the magnitude of the stiffness was increased but only minor changes in shape and orientation occurred. Taken together, all of these experiments represent a step toward the understanding of the interactions between geometrical and neural factors involved in maintaining hand posture and its interactions with the environment.

Adaptation, Physiological↗
PubMed1984

Behavior of preoculomotor burst neurons during eye-head coordination.

Single-unit recordings from the pontine reticular formation in four monkeys have shown the presence of two classes of short-lead burst neurons firing during coordinated eye-head movements. The activity of one class showed a correlation with the size of saccadic movements performed during head movements; the other showed a correlation between firing pattern and the combined eye-head movement. Anatomical reconstructions of the recording sites point to an intermingling of the two cell types.

Animals↗
PubMed1984

Posture control and trajectory formation during arm movement.

One hypothesis for the generation of spatially oriented arm movements by the central nervous system is that a desired joint position is determined by the ratio of the tensions of agonist and antagonist muscles. According to this hypothesis, the transition between equilibrium states should be solely a function of the contraction time of the motor units and the mechanical properties of the arm. We tested this hypothesis in intact and deafferented rhesus monkeys by holding the forearm and measuring the accelerative transient after release of the forearm and by directly measuring the time course of the increase in torque during the movement. Both methods indicated an average time of 400 msec for attaining peak torque in a movement with a duration of 700 msec. In addition, by displacing the arm from its normal trajectory during the movement, we observed that the arm returned neither to the initial nor to the final equilibrium positions, but to points intermediate between them. We conclude that the processes underlying trajectory formation must be more complex than a simple switch between one equilibrium position and another.

Afferent Pathways↗
PubMed1982

Arm trajectory formation in monkeys.

The formation of forearm trajectories of moderate velocities (0.3-1.3 rad/s) was studied in monkeys performing a simple visuomotor task. The experiments were designed to test the hypothesis that the transition from one position to another is subserved by a rapid shift to a final equilibrium of forces in agonist and antagonist muscles. This idea is attractive because it suggests the possibility that in simple movements the trajectory is determined by the inherent inertial and viscoelastic properties of the limb and muscles around a joint. The results indicate that these moderate speed movements are controlled by a gradual, and not a step-like, shift to the final equilibrium position.

Afferent Pathways↗