Biomedical subjects

E Bizzi

Publications and source records attributed to E Bizzi.

At least 19 recordsLinked to original sources

PubMed2002

Modular organization of spinal motor systems.

The vertebrate nervous system produces a wide range of movement flexibly and efficiently, even though the simplest of these movements is potentially highly complex. The strategies by which the nervous system overcomes these complexities have therefore been of interest to motor physiologists for decades. In this review, the authors present a number of recent experiments that propose one strategy by which the nervous system might simplify the production of movement. These experiments suggest that spinal motor systems are organized in terms of a small number of distinct motor responses, or "modules." These distinct modules can be combined together simply to produce a wide range of different movements. Such a modular organization of spinal motor systems can potentially allow the nervous system to produce a wide range of natural behaviors in a simple and flexible manner.

Animals↗

PubMed2001

Neuronal correlates of motor performance and motor learning in the primary motor cortex of monkeys adapting to an external force field.

The primary motor cortex (M1) is known to control motor performance. Recent findings have also implicated M1 in motor learning, as neurons in this area show learning-related plasticity. In the present study, we analyzed the neuronal activity recorded in M1 in a force field adaptation task. Our goal was to investigate the neuronal reorganization across behavioral epochs (before, during, and after adaptation). Here we report two main findings. First, memory cells were present in two classes. With respect to the changes of preferred direction (Pd), these two classes complemented each other after readaptation. Second, for the entire neuronal population, the shift of Pd matched the shift observed for muscles. These results provide a framework whereby the activity of distinct neuronal subpopulations combines to subserve both functions of motor performance and motor learning.

Animals↗

PubMed2001

Modulation and vectorial summation of the spinalized frog's hindlimb end-point force produced by intraspinal electrical stimulation of the cord.

The ability to produce various force patterns at the ankle by microstimulation of the gray matter of the spinal cord was investigated in spinalized frogs. We evaluated the recruitment properties of individual spinal sites and found that forces increase linearly with activation level in the low-force range studied, while the structure of the force pattern remains invariant. We also measured the responses produced by coactivation of two spinal sites activated at two pairs of stimulation levels. Responses were measured at the mechanical level by recording forces at the ankle; and, at the muscular level by recording the electromyographic (EMG) activity of 11 hindlimb muscles. We found that for both pairs of activation, the forces under coactivation were the scaled vectorial summation of the individual responses. At the muscular level, rectified and integrated EMGs also summated during coactivation. Numerous force patterns could, thus, be created by the activation of a few individual sites. These results suggest that microstimulation of the circuitry of the spinal cord (higher order neurons than the motoneurons) holds promise as a new functional neuromuscular stimulation (FNS) technique for the restoration of multi-joint movements.

Animals↗

PubMed2001

Muscle synergies encoded within the spinal cord: evidence from focal intraspinal NMDA iontophoresis in the frog.

This paper relates to the problem of the existence of muscle synergies, that is whether the CNS command to muscles is simplified by controlling their activity in subgroups or synergies, rather than individually. We approach this problem with two methods that have been recently introduced: intraspinal N-methyl-D-aspartate (NMDA) microstimulation and a synergy-extracting algorithm. To search for a common set of synergies encoded for by the spinal cord whose combinations would account for a large range of electromyographic (EMG) patterns, we chose, rather than examining a large range of natural behaviors, to chemically microstimulate a large number of spinal cord interneuronal sites in different frogs. A possible advantage of this complementary method is that it is task-independent. Visual inspection suggested that the NMDA-elicited EMG patterns recorded from 12 leg muscles might indeed be constructed from smaller subgroups of muscles whose activity co-varied, suggestive of synergies. We used a modification of our extracting computational algorithm whereby a nonnegative least-squares method was employed to iteratively update both the synergies and their coefficients of activation in reconstructing the EMG patterns. Using this algorithm, a limited set of seven synergies was found whose linear combinations accounted for more than 91% of the variance in the pooled EMG data from 10 frogs, and more than 96% in individual frogs. The extracted synergies were similar among frogs. Further, preferred combinations of these synergies were clearly identified. This was found by extracting smaller sets of four, five, or six synergies and fitting each synergy from those sets as a combination from the set of seven synergies extracted from the same frog; the synergy combinations from each frog were then pooled together to examine their frequency of occurrence. Concordance with this method of identifying synergy combinations was found by examining how the synergies from the set of seven correlated pair-wise as they reconstructed the EMG data. These results support the existence of muscle synergies encoded within the spinal cord, which through preferred combinations, account for a large repertoire of spinal cord motor output. These findings are contrasted with previous approaches to the problem of synergies.

Animals↗

PubMed2001

Retraining movement in patients with acquired brain injury using a virtual environment.

We report preliminary results of an ongoing study in which a virtual environment (VE) system is used to facilitate motor relearning of a pouring task in patients with Acquired Brain Injury (ABI). Four subjects were evaluated pre- and post-VE training using virtual-world and real-world tests in which subjects performed a pouring motion while holding a cup. Standard clinical tests of motor and functional ability were also used. Three of four subjects demonstrated improvement in end-point trajectories (cup path) performed during the virtual and real world tests. Clinical test scores also improved. Results indicate that subjects with ABI were able to learn a movement in VE, and generalize this ability to real-world performance of similar and unrelated tasks. VE training appears to be a feasible and promising approach to the rehabilitation of subjects with ABI.

Adolescent↗

PubMed2000

Motor learning through the combination of primitives.

In this paper we discuss a new perspective on how the central nervous system (CNS) represents and solves some of the most fundamental computational problems of motor control. In particular, we consider the task of transforming a planned limb movement into an adequate set of motor commands. To carry out this task the CNS must solve a complex inverse dynamic problem. This problem involves the transformation from a desired motion to the forces that are needed to drive the limb. The inverse dynamic problem is a hard computational challenge because of the need to coordinate multiple limb segments and because of the continuous changes in the mechanical properties of the limbs and of the environment with which they come in contact. A number of studies of motor learning have provided support for the idea that the CNS creates, updates and exploits internal representations of limb dynamics in order to deal with the complexity of inverse dynamics. Here we discuss how such internal representations are likely to be built by combining the modular primitives in the spinal cord as well as other building blocks found in higher brain structures. Experimental studies on spinalized frogs and rats have led to the conclusion that the premotor circuits within the spinal cord are organized into a set of discrete modules. Each module, when activated, induces a specific force field and the simultaneous activation of multiple modules leads to the vectorial combination of the corresponding fields. We regard these force fields as computational primitives that are used by the CNS for generating a rich grammar of motor behaviours.

Animals↗

PubMed2000

Cortical correlates of learning in monkeys adapting to a new dynamical environment.

In this paper, we describe the neural changes observed in the primary motor cortex of two monkeys while they learned a new motor skill. The monkeys had to adapt their reaching movements to external forces that interfered with the execution of their arm movements. We found a sizable population of cells that changed their tuning properties during exposure to the force field. These cells took on the properties of neurons that are involved in the control of movement. Furthermore, the cells maintained the acquired activity as the monkey readapted to the no-force condition. Recent imaging studies in humans have reported the effects of motor learning in the primary motor cortex. Our results are consistent with the findings of these studies and provide evidence for single-cell plasticity in the primary motor cortex of primates.

Adaptation, Physiological↗

PubMed2000

New perspectives on spinal motor systems.

The production and control of complex motor functions are usually attributed to central brain structures such as cortex, basal ganglia and cerebellum. In traditional schemes the spinal cord is assigned a subservient function during the production of movement, playing a predominantly passive role by relaying the commands dictated to it by supraspinal systems. This review challenges this idea by presenting evidence that the spinal motor system is an active participant in several aspects of the production of movement, contributing to functions normally ascribed to 'higher' brain regions.

Adaptation, Physiological↗

PubMed2000

Output units of motor behavior: an experimental and modeling study.

Cognitive approaches to motor control typically concern sequences of discrete actions without taking into account the stunning complexity of the geometry and dynamics of the muscles. This begs the question: Does the brain convert the intricate, continuous-time dynamics of the muscles into simpler discrete units of actions, and if so, how? One way for the brain to form discrete units of behavior from muscles is through the synergistic co-activation of muscles. While this possibility has long been known, the composition of potential muscle synergies has remained elusive. In this paper, we have focused on a method that allowed us to examine and compare the limb stabilization properties of all possible muscle combinations. We found that a small set (as few as 23 out of 65,536) of all possible combinations of 16 limb muscles are robust with respect to activation noise: these muscle combinations could stabilize the limb at predictable, restricted portions of the workspace in spite of broad variations in the force output of their component muscles. The locations at which the robust synergies stabilize the limb are not uniformly distributed throughout the leg's workspace, but rather, they cluster at four workspace areas. The simulated robust synergies are similar to the actual synergies we have previously found to be generated by activation of the spinal cord. Thus, we have developed a new analytical method that enabled us to select a few muscle synergies with interesting properties out of the set of possible muscle combinations. Beyond this, the identification of robustness as a common property of the synergies in simple motor behaviors will open the way to the study of dynamic stability, which is an important and distinct property of the vertebrate motor-control system.

Animals↗

PubMed1999

Responses to spinal microstimulation in the chronically spinalized rat and their relationship to spinal systems activated by low threshold cutaneous stimulation.

We describe the responses evoked by microstimulation of interneuronal regions of the spinal cord in unanesthetized rats chronically spinalized at T10-T12. One to three weeks after spinalization, sites in the lumbar spinal cord were stimulated using trains of low current microstimulation. The isometric force produced by stimulation of a spinal site was measured at the ankle. Responses were reliably observed from stimulation of a region within the first 1250 microm from the dorsal surface of the spinal cord. These responses were clearly not due to direct motoneuronal activation and were maintained after chronic deafferentation. The force evoked by microstimulation and measured at the ankle varied smoothly across the workspace. Simultaneous stimulation of two sites in the spinal cord produced a response that was a simple linear summation of the responses evoked from each of the sites alone. Microstimulation generally produced a highly non-uniform distribution of response directions, biased toward responses which pulled the limb toward the body. Within these distributions there appeared to be two main types of responses. These different types of responses were preferentially evoked by microstimulation of different rostrocaudal regions of the spinal cord. This anatomical organization paralleled the spinal cutaneous somatotopy, as assessed by recording cutaneous receptive fields of neurons at sites to which the microstimulation was applied. This relationship was maintained after chronic deafferentation. The findings described here in the rat spinal cord in large part replicate those previously described in the frog.

Animals↗

PubMed1999

Motor systems.

Explore the source record for details and available documents.

Animals↗

PubMed1999

The construction of movement by the spinal cord.

We used a computational analysis to identify the basic elements with which the vertebrate spinal cord constructs one complex behavior. This analysis extracted a small set of muscle synergies from the range of muscle activations generated by cutaneous stimulation of the frog hindlimb. The flexible combination of these synergies was able to account for the large number of different motor patterns produced by different animals. These results therefore demonstrate one strategy used by the vertebrate nervous system to produce movement in a computationally simple manner.

Animals↗

PubMed1998

Low dimensionality of supraspinally induced force fields.

Recent experiments using electrical and N-methyl-D-aspartate microstimulation of the spinal cord gray matter and cutaneous stimulation of the hindlimb of spinalized frogs have provided evidence for a modular organization of the frog's spinal cord circuitry. A "module" is a functional unit in the spinal cord circuitry that generates a specific motor output by imposing a specific pattern of muscle activation. The output of a module can be characterized as a force field: the collection of the isometric forces generated at the ankle over different locations in the leg's workspace. Different modules can be combined independently so that their force fields linearly sum. The goal of this study was to ascertain whether the force fields generated by the activation of supraspinal structures could result from combinations of a small number of modules. We recorded a set of force fields generated by the electrical stimulation of the vestibular nerve in seven frogs, and we performed a principal component analysis to study the dimensionality of this set. We found that 94% of the total variation of the data is explained by the first five principal components, a result that indicates that the dimensionality of the set of fields evoked by vestibular stimulation is low. This result is compatible with the hypothesis that vestibular fields are generated by combinations of a small number of spinal modules.

Animals↗

PubMed1998

Spinal cord modular organization and rhythm generation: an NMDA iontophoretic study in the frog.

Previous work using electrical microstimulation has suggested the existence of modules subserving limb posture in the spinal cord. In this study, the question of modular organization was reinvestigated with the more selective method of chemical microstimulation. N-methyl--aspartate (NMDA) iontophoresis was applied to 229 sites of the lumbar spinal cord gray while monitoring the isometric force output of the ipsilateral hindlimb at the ankle. A force response was elicited from 69 sites. At 18 of these sites, tonic forces were generated and rhythmic forces at 44. In the case of tonic forces, their directions clustered along four orientations: lateral extension, rostral flexion, adduction, and caudal extension. For the entire set of forces (tonic and rhythmic), the same clusters of orientations were found with the addition of a cluster directed as a flexion toward the body. This distribution of force orientations was quite comparable to that obtained with electrical stimulation at the same sites. The map of tonic responses revealed a topographic organization; each type of force orientation was elicited from sites that grouped together in zones at distinct rostrocaudal and depth locations. In the case of rhythmic sequences of force orientations, some were distinctly more common, whereas others were rarely elicited by NMDA. Mapping of the most common rhythms showed that each was elicited from two or three regions of the cord. These regions were close in location to the tonic regions that produced those forces that represented components specific to that rhythm. There was an additional caudal region from which the different rhythms also could be elicited. Taken together, these results support the concept of a modular organization of the motor system in the frog's spinal cord and delineate the topography of these modules. They also suggest that these modules are used by the circuitry underlying rhythmic pattern generation by the spinal cord.

Animals↗

PubMed1997

Augmented Feedback Presented in a Virtual Environment Accelerates Learning of a Difficult Motor Task.

One can use a number of techniques (e.g., from videotaping to computer enhancement of the environment) to augment the feedback that a subject usually receives during training on a motor task. Although some forms of augmented feedback have been shown to enhance performance on isolated isometric tasks during training, when the feedback has been removed subjects have sometimes not been able to perform as well in the "real-world" task as controls. Indeed, for realistic, nonisometric motor tasks, improved skill acquisition because of augmented feedback has not been demonstrated. In the present experiments, subjects (Experiment 1, N = 42; Experiment 2, N = 21) performed with a system that was designed for teaching a difficult multijoint movement in a table tennis environment. The system was a fairly realistic computer animation of the environment and included paddles for the teacher and subject, as well as a virtual ball. Each subject attempted to learn a difficult shot by matching the pattern of movements of the expert teacher. Augmented feedback focused the attention of the subject on a minimum set of movement details that were most relevant to the task; feedback was presented in a form that required the least perceptual processing. Effectiveness of training was determined by measuring their performance in the real task. Subjects who received the virtual environment training performed significantly better than subjects who received a comparable amount of real-task practice or coaching. Kinematic analysis indicated that practice with the expert's trajectory served as a basis for performance on the real-world task and that the movements executed after training were subject-specific modifications of the expert's trajectory. Practice with this trajectory alone was not sufficient for transfer to the real task, however: When a critical component of the virtual environment was removed, subjects showed no transfer to the real task.

augmented feedback↗

PubMed1996

Consolidation in human motor memory.

Learning a motor skill sets in motion neural processes that continue to evolve after practice has ended, a phenomenon known as consolidation. Here we present psychophysical evidence for this, and show that consolidation of a motor skill was disrupted when a second motor task was learned immediately after the first. There was no disruption if four hours elapsed between learning the two motor skills, with consolidation occurring gradually over this period. Previous studies in humans and other primates have found this time-dependent disruption of consolidation only in explicit memory tasks, which rely on brain structures in the medial temporal lobe. Our results indicate that motor memories, which do not depend on the medial temporal lobe, can be transformed by a similar process of consolidation. By extending the phenomenon of consolidation to motor memory, our results indicate that distinct neural systems share similar characteristics when encoding and storing new information.

Adult↗

PubMed1996

Motor learning by field approximation.

We investigated how human subjects adapt to forces perturbing the motion of their ams. We found that this kind of learning is based on the capacity of the central nervous system (CNS) to predict and therefore to cancel externally applied perturbing forces. Our experimental results indicate: (i) that the ability of the CNS to compensate for the perturbing forces is restricted to those spatial locations where the perturbations have been experienced by the moving arm. The subjects also are able to compensate for forces experienced at neighboring workspace locations. However, adaptation decays smoothly and quickly with distance from the locations where disturbances had been sensed by the moving limb. (ii) Our experiments also how that the CNS builds an internal model of the external perturbing forces in intrinsic (muscles and / or joints) coordinates.

Adult↗

PubMed1996

The control of stable postures in the multijoint arm.

The stiffness that is measured at the hand of a multijoint arm emerges from the combined effects of the elastic properties of the muscles and joints, the geometry of the linkages and muscle attachments, and the neural control circuits that act on the arm. The effective stiffness of a nonlinear linkage such as a two-joint arm depends on the force acting on the system as well as the intrinsic stiffness of the actuators. This paper presents an analysis of the factors that affect limb stiffness, including the effects of external forces. Three potential strategies for controlling the stability of the limb are proposed and demonstrated by computer simulations. The predictions from the simulations are then compared experimentally with measured stiffness values for human subjects working against an external force. These experiments were directed toward understanding what strategies are used by the CNS to control limb stiffness and stability. The experimental evidence showed that human subjects must increase the stiffness at the joints in order to maintain limb stability in the presence of applied external forces at the hand. In the process we identified a precise role for muscles which span two or more joints in the control of over all limb stiffness. A local strategy may be used to achieve limb stability, in which the muscle stiffness increases with muscle force. Multijoint muscles are shown to provide mechanical couplings which are necessary for the maintenance of stability. By utilizing these muscles, the neuro-musculo-skeletal system can control a global property of the system (stability) with a passive local strategy.

Adult↗