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Biomedical subjects

L M Nashner

Publications and source records attributed to L M Nashner.

At least 37 records · Page 2Linked to original sources
PubMed1984

Vestibulo-spinal control differs in patients with reduced versus distorted vestibular function.

Abnormal vestibular function disrupts a subject's reference to gravity (earth) vertical, and prevents resolution of conflicting or inaccurate visual and somatosensory spatial references. However, errors which patients make when attempting to resolve conflicting visual and somatosensory orientation inputs during upright stance differed markedly in patients with (1) symmetric or asymmetric reduced vestibular function, (2) benign paroxysmal positional nystagmus and vertigo, and (3) a combination of distorted and reduced function. Objective characterization of spatial orientation systems and compensatory strategies under altered sensory conditions is an essential first step toward identifying optimal treatment methods for each of these three types of vestibular deficient patients.

Humans↗
PubMed1983

Stance posture control in select groups of children with cerebral palsy: deficits in sensory organization and muscular coordination.

This study has focused upon the automatic components of posture and movement in a group of ten cerebral palsy children carefully selected to represent a spectrum of abnormalities relatively pure by clinical standards and ten age-matched normals. Each subject stood unsupported upon a movable platform and within a movable visual surround and was then exposed to external perturbations or was asked to pull with one arm upon a movable handle. In comparing the performance of cerebral palsy children in each clinical category with the age-matched normals and with normal adults assessed in previous studies, the process of maintaining stance was subdivided into two component functions: substrates which determined the onset timing, direction and amplitude of postural actions from somatosensory, vestibular, and visual stimuli were termed "sensory organization", and those establishing temporal and spatial patterns of muscular contractions appropriate to produce effective movements were termed "muscle coordination". We found among seven of the ten cerebral palsy children a clear localization of dysfunction within either sensory organization or muscle coordination mechanisms. These results are providing some new insights into the organization of each of these processes as well as suggesting methods for developing a more systematic understanding of the abnormalities of movement control.

Cerebral Palsy↗
PubMed1982

Properties of postural adjustments associated with rapid arm movements.

1. We have examined rapid postural adjustments associated with a class of voluntary movements that disturb postural equilibrium. In the text that follows, these motor activities are termed associated postural adjustments and voluntary focal movements, respectively. Standing human subjects performed a variety of movement tasks on a hand-held manipulandum, resulting in disturbances to their postural equilibrium. The experimental use of movements that interact with the subject's environment in a relatively simple was permitted a more precise comparison of the postural adjustments with their associated focal movements. 2. Subjects either pulled or pushed on a stiff interface (the handle) or they responded in a predetermined way to handle perturbations. These activities were carried out with various degrees of steady-state postural stability. Prior to and during these movements, support surface and handle forces, electromyographic (EMG) signals, and body sway were monitored. 3. In addition to previously shown postural adjustments associated with reaction-time armed movements, we have demonstrated these postural activities occur in concept with segmental stretch reflexes and self-initiated (untriggered) movements. Postural adjustments were initiated shortly before all focal movements tested except the short-latency component of the biceps stretch reflex (25- to 30-ms latency). However, this reflex component was rarely elicited by handle perturbations in free-standing subjects; therefore, postural adjustments usually preceded any biceps activity under this condition. 4. By varying the degree of steady-state postural equilibrium, a reciprocal gain/threshold relationship between postural and focal components was documented, i.e., when stability was high, postural activity was reduced or absent and focal activity enhanced. Conversely, the biceps stretch reflex was difficult to elicit under any condition where the subjects was not fully supported in the direction of movement and reaction times of focal movements were prolonged. 5. Postural activities associated with focal movements were found to share a number of organizational properties with automatic postural adjustments to support surface movements. Specifically, the postural muscle synergies were equivalent in muscle composition, relative activation magnitudes, and relative temporal sequencing. Furthermore, both types of postural adjustments were highly specific in locus and magnitude to the quality of steady-state postural equilibrium (e.g., postural "set"). 6. A conceptual model is proposed that suggests one simple way in which the reciprocal influence of postural set on postural and focal movement components and their temporal sequencing might be accomplished. Furthermore, we propose in this model a common central organization of postural adjustments associated with focal movements and those elicited by support-surface movements.

Afferent Pathways↗
PubMed1982

Adaptation to altered support and visual conditions during stance: patients with vestibular deficits.

Patients whose deficits were limited to clinically well qualified vestibular disorders have been exposed to a number of altered support surface and visual environments while standing unsupported. A six-degrees-of-freedom platform employing movable support surfaces for each foot and a movable visual surround deprived patients of normal inputs derived from a fixed level support surface and from an immobile surround. Various tests employing EMG, force, and body movement recording identified quantitative changes in the patients' strategy for the relative weighting of proprioceptive, vestibular, and visual inputs. The most dramatic performance deficit of patients was their inability to suppress the influence of visual and proprioceptive inputs appropriately whenever motions of external surface disturbed the orientation information provided by these inputs. Thus, the more mildly afflicted patients experienced instability not so much because of the loss of vestibular inputs directly to posture but because of their inappropriate responses to proprioceptive inputs and vision. Discussion is centered on the role of vestibular input as an internal reference system for orientation about which adaptive changes in proprioceptive and visual inputs are made.

Humans↗
PubMed1982

Ontogenetic development of postural control in man: adaptation to altered support and visual conditions during stance.

Normal young children ranging in age from 1 1/2 to 10 years were assessed in a number of experimental paradigms testing the ability to adapt quickly their strategy of control to altered support surface and visual conditions. The experimental protocols, using a movable platform and visual surround, and the analytic techniques, using EMGs and measures of reaction forces and body motions, were identical to those employed in a complementary study in this issue (Nashner, L. M., F. O. Black, and C. Wall, III (1982) J. Neurosci. 2: 536-544). The structure of automatic postural adjustments in young children was, with the exception of greater variability, similar to that of adult subjects studied previously. However, young children below the age of 7 1/2 years were unable to suppress systematically the influence of inputs derived from the support surface or from vision when these provided inappropriate orientation information due to the motion of these surfaces. The discussion emphasizes that the automatic postural adjustments and the context-dependent reweighting of support surface, vestibular, and visual inputs are organizationally separate processes and that the hierarchically lower level automatic process matures before the higher level adaptive processes.

Acclimatization↗
PubMed1981

Relation of automatic postural responses and reaction-time voluntary movements of human leg muscles.

This study contrasts the properties of compensatory postural adjustments in response to movements of the support surface with those of reaction-time voluntary movements in human subjects. Subjects stood upon a six degrees-of-freedom movable platform and performed tone and movement-triggered voluntary sways about the ankle joints both under conditions of postural stability and instability. These triggered movements could be executed as rapidly as postural adjustments to support surface perturbations (80-120 ms), but only when the former were well practiced, single-choice (direction) and were performed under conditions of postural stability. Evaluation of the properties of postural adjustments and reaction-time voluntary movements revealed a number of clear organizational differences between the two categories of movement, but most interesting was the finding that, when reaction-time movements were triggered by or at the onset of platform movement, the postural adjustments always occurred first. Only when subjects were given a tone trigger 50 ms in advance of platform movement were they able to execute the reaction-time movement first. We found that the dichotomous voluntary/reflexive classification of movements was not consistent with all of the identified properties of postural adjustments and reaction-time movements. Instead, we find a system which classifies movements by function, as either stabilizating or orientational adjustments, to be more useful. In the context of whole-body movement then, intentional focal components would be closely associated with others directed towards postural stabilization.

Electromyography↗
PubMed1979

Organization of rapid responses to postural and locomotor-like perturbations of standing man.

This study has described the organization of EMG activities among the muscles of a standing subject's legs during rapid postural adjustments (95--120 ms latencies). Adjustments were elicited by the horizontal translation of both feet (causing antero-posterior sway), by the synchronous vertical displacement of both feet (causing changes in height) and by the reciprocal vertical displacement of the feet (causing changes in height) and by the reciprocal vertical displacement of the feet (causing a locomotor-like motion of the legs and lateral sway of the body). The resulting patterns of EMG activity were highly specific for each kind of displacement, and all subjects completely reorganized the pattern of activity from one form to another within the first trials, even immediately following unexpected stimulus changes. The organization of EMG activities during reciprocal vertical displacements was qualitatively quite similar to those observed during the comparable swing and stance phases of the locomotor step cycle; flexor muscles of the ankle and knee (those being shortened by the displacement) contracted in the upwardly displaced leg while extensor muscles were active in the downwardly displaced leg. This pattern was in marked contrast to the activation of lengthening muscles during synchronous vertical and antero-posterior sway displacements. Finally, electrical cutaneous stimulation of the dorsum of one foot during reciprocal vertical displacements always enhanced the EMG activity of the agonist leg muscles, in-phase with the vertical movement.

Ankle Joint↗
PubMed1976

Adapting reflexes controlling the human posture.

Doubt about the role of stretch reflexes in movement and posture control has remained in part because the questions of reflex "usefulness" and the postural "set" have not been adequately considered in the design of experimental paradigms. The intent of this study was to discover the stabilizing role of stretch reflexes acting upon the ankle musculature while human subjects performed stance tasks requiring several different postural "sets". Task specific differences of reflex function were investigated by experiments in which the role of stretch reflexes to stabilize sway doing stance could be altered to be useful, of no use, or inappropriate. Because the system has available a number of alternate inputs to posture (e.g., vestibular and visual), stretch reflex responses were in themselves not necessary to prevent a loss of balance. Nevertheless, 5 out of 12 subjects in this study used long-latency (120 msec) stretch reflexes to help reduce postural sway. Following an unexpected change in the usefulness of stretch reflexes, the 5 subjects progressively altered reflex gain during the succeeding 3-5 trials. Adaptive changes in gain were always in the sense to reduce sway, and therefore could be attenuating or facilitating the reflex response. Comparing subjects using the reflex with those not during so, stretch reflex control resulted in less swaying when the task conditions were unchanging. However, the 5 subjects using reflex controls oftentimes swayed more during the first 3-5 trials after a change, when inappropriate responses were elicited. Four patients with clinically diagnosed cerebellar deficits were studied briefly. Among the stance tasks, their performance was similar to normal in some and significantly poorer in others. Their most significant deficit appeared to be the inability to adapt long-latency reflex gain following changes in the stance task. The study concludes with a discussion of the role of stretch reflexes within a hierarchy of controls ranging from muscle stiffness up to centrally initiated responses.

Adaptation, Physiological↗
PubMedDate not supplied

Postural disturbance in patients with benign paroxysmal positional nystagmus.

Abnormal vestibular function disrupts postural and ocular muscle control system references to gravity (earth) vertical. Vestibular disorders also prevent satisfactory resolution of normally redundant, but often conflicting, visual and somatosensory spatial references required for normal postural control during active and passive body motion. Using a moving platform and visual surrounds posturography technique to systematically interact visual, somatosensory, and vestibular inputs, it was clearly demonstrated that patients with the benign paroxysmal positional nystagmus type of distorted vestibular function employ an unstable, visually dependent postural sway distinct from the postural instability associated with unilateral or bilateral vestibular functional deficits. These findings have important clinical implications for diagnosis and management of patients with vestibular disorders.

Adult↗