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Biomedical subjects

E A Keshner

Publications and source records attributed to E A Keshner.

At least 19 recordsLinked to original sources
PubMed2006

Influences of the perception of self-motion on postural parameters.

We examined how spatial and temporal characteristics of the perception of self-motion, generated by constant velocity visual motion, was reflected in orientation of the head and whole body of young adults standing in a CAVE, a virtual environment that presents wide field of view stereo images with context and texture. Center of pressure responses from a force plate and perception of self-motion through orientation of a hand-held wand were recorded. The influence of the perception of self-motion on postural kinematics differed depending upon the plane and complexity of visual motion. Postural behaviors generated through the perception of self-motion appeared to contain a confluence of the cortically integrated visual and vestibular signals and of other somatosensory inputs. This would suggest that spatial representation during motion in the environment is modified by both ascending and descending controls. We infer from these data that motion of the visual surround can be used as a therapeutic tool to influence posture and spatial orientation, particularly in more visually sensitive individuals following central nervous system (CNS) impairment.

Adult↗
PubMed2005

Perturbation parameters associated with nonlinear responses of the head at small amplitudes.

The head-neck system has multiple degrees of freedom in both its control and response characteristics, but is often modeled as a single joint mechanical system. In this study, we have attempted to quantify the perturbation parameters that would elicit nonlinear responses in a single degree-of-freedom neuromechanical system at small amplitudes and velocities of perturbation. Twelve healthy young adults seated on a linear sled randomly received anterior-posterior sinusoidal translations with +/-15 mm and +/-25 mm peak displacements at 0.81, 1.76, and 2.25 Hz. Head angular velocity and angular position data were examined using a nonlinear phase-plane analysis. Poincare sections of the phase plane were computed and Lyapunov exponents calculated to measure divergence (chaotic behavior) or convergence (stable behavior) of system dynamics. Variability of head angular position and velocity across the entire phase plot was compared to that of the Poincare sections to quantify spatial-temporal irregularity. Multiple equilibrium points and positive Lyapunov exponents revealed chaotic behavior at 0.81 Hz at both amplitudes whereas responses at 1.76 and 2.25 Hz exhibited periodic oscillations, clustered phase points, and negative Lyapunov exponents. However, intersubject variability increased at the lowest frequency and a few subjects presented chaotic behavior at all frequencies. An inverted pendulum with position and velocity threshold nonlinearity was adopted as a simplistic model of the head and neck. Simulations with the model resulted in features similar to those observed in the experimental data. Our principal finding was that increasing the perturbation amplitude had a stabilizing effect on the behavior across frequencies. Nonlinear behaviors observed at the lowest stimulus frequency might be attributed to fluctuations in control between the multiple sensory inputs. Although this study has not conclusively pointed toward any single mechanism as responsible for the responses observed, it has revealed clear directions for further investigation. To examine if changing the sensory modalities would elicit a significant change in the nonlinear behaviors observed here, further experiments that target a patient population with some sort of sensory deficit are warranted.

Acceleration↗
PubMed2002

Effects of inertial load and cervical-spine orientation on a head-tracking task in the alert cat.

Simultaneous video-fluoroscopic and neck muscle EMG data were recorded from one cat performing +/-15 degrees sinusoidal (0.25 Hz) head-tracking movements in the sagittal plane in a standing body posture with two initial neck orientations and four inertial loads. Radio-opaque markers were inserted into the anterior/posterior and lateral aspects of the occipital ridge and C(1)-C(7) to measure vertebral displacement. Kinematic data were analyzed, and a computer model was applied to the data to characterize the limits of movement in the cervical spine and to estimate the moment arms of the neck muscles at different orientations of head-neck movement. For each initial neck orientation, the cat utilized a distinct set of vertebral alignments, relative joint movements, and muscle-activation patterns to achieve the same movement outcome. As inertial load increased, vertebral alignments and relative joint movements were constant with a vertically oriented neck but differed when the neck was more horizontally oriented. Different muscle-activation patterns were used to maintain the same kinematic pattern with increased inertial loads. Some muscle EMG response gains (rectus capitis major and splenius capitis) increased with increasing mass, while others (biventer cervicis and occipitoscapularis) demonstrated an initial increase and then a plateau. EMG phases were not affected by changing the mass of the system but were affected by changing neck orientation. The model predicted that muscle moment arms would vary little for the different vertebral alignments, suggesting a robust biomechanical system minimally compensates for small changes in task geometry.

Animals↗
PubMed2002

Modeling head tracking of visual targets.

Control of the head involves somatosensory, vestibular, and visual feedback. The dynamics of these three feedback systems must be identified in order to gain a greater understanding of the head control system. We have completed one step in the development of a head control model by identifying the dynamics of the visual feedback system. A mathematical model of human head tracking of visual targets in the horizontal plane was fit to experimental data from seven subjects performing a visual head tracking task. The model incorporates components based on the underlying physiology of the head control system. Using optimization methods, we were able to identify neural processing delay, visual control gain, and neck viscosity parameters in each experimental subject.

Algorithms↗
PubMed2000

Modulating active stiffness affects head stabilizing strategies in young and elderly adults during trunk rotations in the vertical plane.

Healthy young and elderly adults were asked to actively modulate neck muscle stiffness during random rotations of the trunk in the vertical plane. Angular velocity of head with respect to trunk and myoelectric activity of semispinalis capitis and sternocleidomastoid muscles were recorded. A MANOVA was performed on group, condition, and frequency variables. A gain and phase drop at 2.15 Hz in young adults indicated neural (i.e. reflex) damping of system mechanics. In the elderly, a steady rise in gain and drop in phase (P<0.0002) was indicative of a second order underdamped system. Even when instructed to not intervene elderly subjects exhibited cocontraction. Ineffective reflex mechanisms may underlie the emergence of this strategy.

Adult↗
PubMed2000

The influence of an immersive virtual environment on the segmental organization of postural stabilizing responses.

We examined the effect of a 3-dimensional stereoscopic scene on segmental stabilization. Eight subjects participated in static sway and locomotion experiments with a visual scene that moved sinusoidally or at constant velocity about the pitch or roll axes. Segmental displacements, Fast Fourier Transforms, and Root Mean Square values were calculated. In both pitch and roll, subjects exhibited greater magnitudes of motion in head and trunk than ankle. Smaller amplitudes and frequent phase reversals suggested control of the ankle by segmental proprioceptive inputs and ground reaction forces rather than by the visual-vestibular signals. Postural controllers may set limits of motion at each body segment rather than be governed solely by a perception of the visual vertical. Two locomotor strategies were also exhibited, implying that some subjects could override the effect of the roll axis optic flow field. Our results demonstrate task dependent differences that argue against using static postural responses to moving visual fields when assessing more dynamic tasks.

Adult↗
PubMed1999

Predicting control mechanisms for human head stabilization by altering the passive mechanics.

The purpose of this study was to clarify the mechanisms controlling head and neck stabilization in the horizontal (yaw) and vertical (pitch) planes by changing the passive mechanics of the head-neck motor system. Angular velocities of the head and trunk in space were recorded in seated subjects during external perturbations of the trunk with pseudorandom sum-of-sines (SSN) stimuli. Four subjects in yaw and nine subjects in pitch actively stabilized their heads in the dark, and performed a mental distraction task in the dark both with and without a weight atop the head. In yaw, the behavior of the head was found to change relatively little with added inertia. As adding inertia to a passive mechanical system should cause substantial changes in dynamics, we inferred that neural mechanisms were invoked to maintain the constant response dynamics. A mathematical model of head-neck control was applied to predict the relative influence of the vestibulocollic and cervicocollic reflexes, and of inertia, stiffness, and viscosity. Using optimization methods to fit the model to experimental data, we identified stiffness and vestibulocollic reflex gain as the primary contributors to the control of head stabilization in space. In pitch, increasing inertia accentuated phase shifts at higher frequencies. Because our pitch model was insufficiently constrained, we only simulated responses due to passive mechanics. Model simulation predicted unstable head motion at all test frequencies. Subjects were able to compensate for trunk motion at most frequencies, however, suggesting that neural components were modulated to exert compensatory responses both with and without additional weight.

Adaptation, Physiological↗
PubMed1997

Kinematics of the freely moving head and neck in the alert cat.

In this study we examined connections between the moment-generating capacity of the neck muscles and their patterns of activation during voluntary head-tracking movements. Three cats lying prone were trained to produce sinusoidal (0.25 Hz) tracking movements of the head in the sagittal plane, and 22.5 degrees and 45 degrees away from the sagittal plane. Radio-opaque markers were placed in the cervical vertebrae, and intramuscular patch electrodes were implanted in five neck muscles, including biventer cervicis, complexus, splenius capitis, occipitoscapularis, and rectus capitis posterior major. Videofluoroscopic images of cervical vertebral motion and muscle electromyographic responses were simultaneously recorded. A three-dimensional biomechanical model was developed to estimate how muscle moment arms and force-generating capacities change during the head-tracking movement. Experimental results demonstrated that the head and vertebrae moved synchronously, but neither the muscle activation patterns nor vertebral movements were constant across trials. Analysis of the biomechanical model revealed that, in some cases, modification of muscle activation patterns was consistent with changes in muscle moment arms or force-generating potential. In other cases, however, changes in muscle activation patterns were observed without changes in muscle moment arms or force-generating potential. This suggests that the moment-generating potential of muscles is just one of the variables that influences which muscles the central nervous system will select to participate in a movement.

Animals↗
PubMed1996

Mechanisms controlling head stabilization in the elderly during random rotations in the vertical plane.

Frequency-related response characteristics of the mechanisms controlling stabilization of the head in 10 elderly subjects were compared with response characteristics in 8 young adults. Angular velocity of the head with respect to the trunk and EMG responses of 2 neck muscles were recorded in 10 seated subjects during pseudorandom rotations of the trunk in the sagittal plane at frequencies of 0.35 to 3.05 Hz. Subjects were required to actively stabilize their heads with (VS) and without (NY) visual feedback so that voluntary mechanisms and the influence of vision could be tested. Reflex mechanisms were examined when subjects were distracted by a mental calculation task during rotations in the dark (MA). Age emerged as an influential factor in the performance of head stabilization mechanisms, and decrements in performance were even more pronounced in the older as compared with younger elderly subjects. Age effects could be seen in the (a) diminished ability to voluntarily stabilize the head, particularly with the absence of vision, (b) impaired ability to stabilize the head when cognitively distracted, and (c) appearance of a resonant response of the head. Control of head stabilization shifted from reflex mechanisms to system mechanics, probably as a result of age-related changes in the integrity of the sensory systems. The elderly's system mechanics could not effectively compensate for the disturbances, however, and instability was the result.

Journal Article↗
PubMed1995

Mechanisms controlling human head stabilization. II. Head-neck characteristics during random rotations in the vertical plane.

1. In this study we have tested the hypothesis that the mechanisms controlling stabilization of the head-neck motor system can vary with both the frequency and spatial orientation of an externally applied perturbation. Angular velocity of the head with respect to the trunk (neck) and myoelectric activity of two neck muscles (semispinalis capitis and sternocleidomastoid) were recorded in eight seated subjects during pseudorandom rotations of the trunk in the vertical (pitch) plane. Subjects were externally perturbed with a random sum-of-sines stimulus at frequencies ranging from 0.35 to 3.05 Hz. Four instructional sets were presented. Voluntary mechanisms were examined by having the subjects actively stabilize the head in the presence of visual feedback as the body was rotated (VS). Visual feedback was then removed, and the subjects attempted to stabilize the head in the dark as the body was rotated (NV). Reflex mechanisms were examined when subjects performed a mental arithmetic task during body rotations in the dark (MA). Finally, subjects performed a voluntary head tracking task while the body was kept stationary (VT). 2. In VS and NV, gains and phases of head velocity indicated good compensation for the perturbation at frequencies up to 2 Hz. Between 2 and 3 Hz, gains dropped slowly and then steeply descended above 3 Hz as phases became scattered. 3. In MA, gains were lower and exhibited more scatter than in VS and NV at frequencies < 1 Hz. Phases around -180 degrees indicated that compensatory activity was occurring even with these low gains. Between 1 and 2 Hz, response gains steeply ascended, implying that reflex mechanisms were becoming the predominant mechanism for compensation in this frequency range. Above 2 Hz, gains dropped off to 0.5 and lower, but phases remained close to -180 degrees, suggesting that the reflex mechanisms were not dominant in this frequency range, but that they were still contributing toward compensation for the trunk perturbation. 4. Neck muscle electromyographic (EMG) responses were similar in VS, NV, and MA, demonstrating decreasing gains between 0.35 and 1.5 Hz, and then increasing beyond the previous high level of activation. This U-shaped response pattern implies an enhanced participation of neural mechanisms, probably of reflex origin, in the higher frequency range. 5. Patterns observed during external perturbations of the trunk were not apparent in the response dynamics of voluntary head tracking. In VT, subjects successfully tracked the stimulus only at the lowest frequencies of head movement.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1994

Vertebral orientations and muscle activation patterns during controlled head movements in cats.

The focus of these experiments was to determine the relationships between head movement, neck muscle activation patterns, and the positions and movements of the cervical vertebrae. One standing cat and one prone cat were trained to produce voluntary sinusoidal movements of the head in the sagittal plane. Video-opaque markers were placed on the cervical vertebrae, and intramuscular patch electrodes implanted in four muscles of the head and neck. Cinefluoroscopic images of cervical vertebral motion and electromyographic responses were simultaneously recorded. Analysis of the spinal movement revealed that the two cats used different strategies to keep their heads aligned with the tracker. In the standing cat, vertebral motion described a more circular arc, compared to a forward diagonal in the prone cat. Intervertebral motion was limited, but more acute angles appeared between the vertebrae of the prone lying than of the standing animal. Data revealed that the central nervous system could control several axes of motion to keep the cervical spine matched to the moving stimulus. Phase relations between the sinusoidal motion of the vertebral column, peak activation of the neck muscles, and that of the stimulus were examined, and several different control strategies were observed both between and within animals. The results suggest that the central nervous system engages in multiple strategies of musculo-skeletal coordination to achieve a single movement outcome.

Animals↗
PubMed1993

Predictors of less stable postural responses to support surface rotations in healthy human elderly.

Balance corrections elicited in response to a rotation of the support-surface were compared between healthy elderly and young normal subjects using surface EMG records from the soleus, tibialis anterior, and neck extensor muscles, and measurements of trunk angular acceleration and ankle torque. Three differences were observed. First, EMG response latencies were significantly longer in the elderly. Second, the normal linear correlation between stabilizing ankle muscle activity and ankle torque was disturbed. These two differences were presumably responsible for the diminished ankle torque exerted on the support surface by the elderly subjects. Third, the magnitude of neck muscle activation was increased in elderly subjects, indicating an increased compensation at the head for trunk angular acceleration. The findings suggest that there are both neural and mechanical changes that may impact on postural corrections in elderly subjects, and that more than one factor needs to be identified when predicting an individual's risk for falling.

Adolescent↗
PubMed1992

Patterns of neck muscle activation in cats during reflex and voluntary head movements.

When the head rotates, vestibulocollic reflexes counteract the rotation by causing contraction of the neck muscles that pull against the imposed motion. With voluntary head rotations, these same muscles contract and assist the movement of the head. The purpose of this study was to determine if an infinite variety of muscle activation patterns are available to generate a particular head movement, of if the CNS selects a consistent and unique muscle pattern for the same head movement whether performed in a voluntary or reflex mode. The relationship of neck muscle activity to reflex and voluntary head movements was examined by recording intramuscular EMG activity from six neck muscles in three alert cats during sinusoidal head rotations about 24 vertical and horizontal axes. The cats were trained to voluntarily follow a water spout with their heads. Vestibulocollic reflex (VCR) responses were recorded in the same cats by rotating them in an equivalent set of planes with the head stabilized to the trunk so that only the vestibular labyrinths were stimulated. Gain and phase of the EMG responses were calculated, and data analyzed to determine the directions of rotation for which specific muscles produced their greatest EMG output. Each muscle exhibited preferential activation for a unique direction of rotation, and weak responses during rotations orthogonal to that preferred direction. The direction of maximal activation could differ for reflex and voluntary responses. Also, the best excitation of the muscle was not always in the direction that would produce a maximum mechanical advantage for the muscle based on its line of pull. The results of this study suggest that a unique pattern of activity is selected for VCR and tracking responses in any one animal. Patterns for the two behaviors differ, indicating that the CNS can generate movements in the same direction using different muscle patterns.

Animals↗
PubMed1990

Controlling stability of a complex movement system.

Human movement systems have frequently been treated as one-dimensional, single-axis, rigid bodies in order to simplify the gathering, analysis, and interpretation of data. The problem with this approach is that the results of such assumptions often lead to conclusions about the production and control of movement that do not relate to the control demands placed on the central nervous system. In order to truly understand how the central nervous system plans and produces movements to match environmental demands, we must take into account the many variations available within the body. The purpose of this article is to examine two movement systems that have the potential to act in multiple spatial dimensions with variable muscle action patterns when performing a stabilizing task. Methods of analyzing how the systems operate under differing task constraints and results of the experiments will be presented. Hypothetical models that have been proposed to explain how complex movement systems operate will also be discussed.

Biomechanical Phenomena↗
PubMed1989

Neck muscle activation patterns in humans during isometric head stabilization.

A musculoskeletal system with more muscles than there are motions could be programmed in alternative ways to produce a single movement. In this case, the muscles would have the potential to be maximally responsive in multiple directions rather than responding preferentially in a single direction. To determine the response patterns of muscles in the head-neck motor system, the simultaneous activation of four of the 23 neck muscles acting on the head was recorded with both surface and intramuscular electrodes. Fifteen human subjects were tested during an isometric head stabilization task. When the EMG response patterns were plotted, each muscle demonstrated a preferred direction of activation. This preferred activation direction was consistent in all of the subjects for three of the muscles tested. The fourth muscle, splenius, was preferentially activated during neck flexion in half of the subjects and during neck extension in the other half. Increasing the force parameters of the task suggested a linear relationship between force and the EMG output in the preferred response directions. Responses in the nonpreferred directions were produced by a nonlinear change in EMG activation of the muscle. This finding could have implications for theories of how reciprocal activation and cocontraction patterns of response are elicited. Results from this study, that the CNS programs neck muscles to respond in specific orientations rather than generating an infinite variety of muscle patterns, are in agreement with our findings in the cat.

Adult↗
PubMed1989

Comparison of neck muscle activation patterns during head stabilization and voluntary movements.

The motor system that controls the neck musculature serves two major functions: stabilization of the head in the face of external perturbations or body movements, and generation of voluntary or orientating head movements. Typically the latter are thought to be mediated by complex pathways involving cerebral cortex and superior colliculus while stabilization is thought to be mediated by simple short-loop pathways that generate vestibulocollic and cervicocollic reflexes (VCR and CCR). Our work has been directed towards evaluating the extent to which the VCR and CCR are in fact responsible for head stabilization, and to determining how the motor patterns produced by these reflexes compare with those produced by the voluntary head movement system. To address these questions we have analysed the dynamic and spatial (kinematic) properties of the head movement system in cats and humans.

Head↗
PubMed1989

Current concepts of the vestibular system reviewed: 1. The role of the vestibulospinal system in postural control.

This paper reviews the research findings that support the presence of vestibulospinal reflexes in corrections for head and body instability. Studies of the importance of labyrinthine inputs to the central nervous system organization of eye, head, and body movements demonstrate that the vestibular nuclei are more than a simple relay station for labyrinthine activity. At all levels of the vestibular system beyond the primary vestibular afferents, parallel processing of labyrinthine signals occurs with input from other sensory systems. Thus, output of the vestibular nuclear complex (VNC) is not equivalent to the labyrinthine input. It is the VNC output that influences motor behavior. Various sensory inputs are available to the nervous system to detect and correct postural instability. Most notably, vestibular, visual, and proprioceptive signals contribute significantly to the stabilizing responses in humans. The intent of this paper is to review experimental results rather than to discuss treatment interventions. Wherever possible, conclusions are drawn as to the clinical implications of current research findings.

Ear, Inner↗