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Biomedical subjects

Robert J Peterka

Publications and source records attributed to Robert J Peterka.

12 recordsLinked to original sources
PubMed2006

Postural after-effects of stepping on an inclined surface.

In previous studies, blindfolded, healthy subjects exhibited an after-effect of leaning while standing on a horizontal surface after a period of standing on an inclined surface. We investigated whether this kinesthetic after-effect would transfer from one task to another by asking blindfolded subjects to stand on a horizontal surface after stepping-in-place on an incline. Results showed that all subjects demonstrated a forward trunk leaning after-effect lasting from half a minute to over 6 min after stepping on a 10 degrees -toes-up incline for 2.5 min. For 5/7 subjects, the amplitude of the leaning after-effect was very similar following stepping or standing on the inclined surface. The similarity of the post-incline lean between the standing and stepping conditions suggests a common underlying mechanism for the after-effect following standing and walking on a gradient and suggests that prolonged maintenance of a constant ankle or leg posture is not a prerequisite condition for the after-effect. The transfer of a postural effect built-up during a locomotor task to a postural after-effect during a standing task is consistent with a central adaptive mechanism that adjusts the surface-referenced set point for whole body postural orientation for both gait and posture.

Adaptation, Physiological↗
PubMed2006

Adaptation of postural orientation to changes in surface inclination.

We previously showed that standing on an inclined surface resulted in an after-effect of leaning in many healthy, blindfolded subjects when they returned to standing on a horizontal surface (Kluzik et al. in Exp Brain Res 162:474-489, 2005). The direction of leaning depended on the direction of prior surface inclination, always in a direction that preserved the relative alignment between the body and the support surface. For example, subjects leaned forward after they stood on a toes-up-inclined surface. In the present study, we investigated how the amplitude of surface inclination affected postural muscle activity, joint position, body segment orientation, and body center of mass (CoM) and foot center of pressure (CoP) locations before, during, and after subjects stood on an inclined surface. We asked whether the mechanism that underlies the lean after-effect involves regulation of local postural variables, such as the position of the ankle joint or the level of muscle activity, or whether instead, the mechanism involves regulation of global, whole-body postural variables that can only be determined by multisensory processing, such as orientation of the trunk or the body's CoM. In one experiment, we found that varying the amplitude of a toes-up surface inclination between 2.5 degrees and 10 degrees had a systematic, linear, effect on the post-incline orientation of the trunk and head, but did not systematically affect the post-incline orientation of the legs, position of the ankle joint, the level of EMG activity, or the location of the CoP. In a second experiment, we found that preventing the legs from leaning in the post-incline period did not abolish leaning of the upper body. These findings suggest that (1) the body-to-support-surface relationship is an important reference for the CNS internal representation of postural orientation which is subject to adaptive modification and (2) the adaptive mechanism underlying the post-incline after-effect of leaning acts at the level of global, whole-body postural variables.

Adaptation, Physiological↗
PubMed2006

Stimulus-dependent changes in the vestibular contribution to human postural control.

Humans maintain stable stance in a wide variety of environments. This robust behavior is thought to involve sensory reweighting whereby the nervous system adjusts the relative contribution of sensory sources used to control stance depending on environmental conditions. Based on prior experimental and modeling results, we developed a specific quantitative representation of a sensory reweighting hypothesis that predicts that a given reduction in the contribution from one sensory system will be accompanied by a corresponding increase in the contribution from different sensory systems. The goal of this study was to test this sensory-reweighting hypothesis using measures that quantitatively assess the relative contributions of the proprioceptive and graviceptive (vestibular) systems to postural control during eyes-closed stance in different test conditions. Medial/lateral body sway was evoked by side-to-side rotation of the support surface (SS) while simultaneously delivering a pulsed galvanic vestibular stimulus (GVS) through electrodes behind the ears. A model-based interpretation of sway evoked by SS rotations provided estimates of the proprioceptive weighting factor, Wp, and showed that Wp declined with increasing SS amplitude. If the sensory-reweighting hypothesis is true, then the decline in Wp should be accompanied by a corresponding increase in Wp, the graviceptive weighting factor, and responses to the GVS should increase in proportion to the value of Wp derived from responses to SS rotations. Results were consistent with the predictions of the proposed sensory-reweighting hypothesis. GVS-evoked sway increased with increasing SS amplitude, and Wp measures derived from responses to GVS and from responses to SS rotations were highly correlated.

Adult↗
PubMed2006

Determining the effectiveness of a vibrotactile balance prosthesis.

We present a quantitative method for characterizing the effectiveness of a balance prosthesis based on tactile vibrators. The balance prosthesis used an array of 12 tactile vibrators (tactors) placed on the anterior and posterior surfaces of the torso to provide body orientation feedback related to the angular position and velocity of anterior-posterior body sway. Body sway was evoked in subjects with normal sensory function and in vestibular loss subjects by rotating the support surface upon which a test subject stood with eyes closed. Tests were performed both with (tactor trials) and without (control trials) the prosthesis activated. Several amplitudes of support surface stimulation were presented with each stimulus following a pseudorandom motion profile. For each stimulus amplitude, a transfer function analysis characterized the amplitude (gain) and timing (phase) of body sway evoked by the support surface stimulus over a frequency range of 0.017 to 2.2 Hz. A comparison of transfer function results from the control trials of normal subjects with results from tactor trials of vestibular loss subjects provided a quantitative measure of the effectiveness of the balance prosthesis in substituting for missing vestibular information. Although this method was illustrated using a specific balance prosthesis, the method is general and could be applied to balance prostheses that utilize other technology.

Adult↗
PubMed2005

Differences in preferred reference frames for postural orientation shown by after-effects of stance on an inclined surface.

This study reports a postural after-effect of leaning that follows a period of stance on an inclined surface with eyes closed. This leaning after-effect maintained the body-to-surface relationship as if subjects still stood on the incline. We examined the incidence and robustness of the leaning after-effect in 51 healthy subjects. The location of the center of pressure (CoP) under the feet and the alignment of the trunk and legs were measured before, during and after blindfolded subjects stood on a 5 degrees toes-up inclined surface for 2.5 min. When the surface was inclined, all subjects stood with their trunk and legs aligned near to gravity-vertical, similar to the alignment adopted in the pre-incline period. When the surface returned to horizontal in the post-incline period, there was a continuum of postural alignment strategies across subjects. At one extreme, subjects leaned forward, with an average trunk lean near 5 degrees . The leaned posture decayed exponentially toward baseline postural alignment across a period of up to 5 min. At the other extreme, subjects did not lean in the post-incline period, but instead, stayed aligned near upright with respect to gravity. Subjects were highly consistent in their post-incline postural behaviors upon repeated testing over days to months and across different directions of surface inclination. Our results suggest that individuals have well-established, preferred, sensory strategies for controlling postural orientation when vision is not available. Subjects who leaned in the post-incline period appear to depend more on the geometry of the support surface as a reference frame and to rely more on proprioceptive information to extract kinematic relationships, whereas subjects who did not lean appear to depend more on gravity as a reference frame and to rely more on sensory information related to forces and load.

Adaptation, Physiological↗
PubMed2005

Pulse-step-sine rotation test for the identification of abnormal vestibular function.

This study illustrates the use of a novel "pulse-step-sine" (PSS) rotational stimulus to identify abnormal function of the horizontal semicircular canals in human subjects with unilateral and bilateral vestibular deficits. The cyclic PSS stimulus includes a "bias component" and a "probe component". The bias component, consisting of a short duration pulse of acceleration followed by an acceleration step, is designed to produce a large change in canal afferent discharge rates that silences the neural activity in one canal during the step portion of the PSS stimulus. The pulse and step components are then repeated with opposite sign to silence afferent activity in the opposite canal. The probe component, consisting of a approximately 1 Hz sine wave superimposed on the step portions of the stimulus, is designed to test the ability of canal afferents in the excited canal to encode the probe stimulus. Various response measures are developed that characterize the horizontal slow phase eye movements evoked by the PSS stimulus. Results show that these measures can distinguish subjects with normal vestibular function from those with unilateral and bilateral vestibular loss, can identify the side-of-lesion in subjects with well compensated unilateral vestibular loss, and can possibly identify the side-of-greater-loss in subjects with asymmetric bilateral loss.

Adult↗
PubMed2004

A new interpretation of spontaneous sway measures based on a simple model of human postural control.

This study shows that center-of-pressure (COP) traces that closely resemble physiologically measured COP functions can be produced by an appropriate selection of model parameters in a simple feedback model of the human postural control system. Variations in the values of stiffness, damping, time delay, and noise level determine the values of 15 sway measures commonly used to characterize spontaneous sway. Results from model simulations indicate that there is a high degree of correlation among these sway measures, and the measures cluster into three different groups. Only two principal components accounted for about 92% of the variation among the different sway measures analyzed. This model can be used to formulate hypotheses regarding the cause of postural control deficits reported in the literature. This is accomplished using a multidimensional optimization procedure to estimate model parameters from a diverse set of spontaneous sway measures. These model parameters describe physiologically meaningful features of the postural control system as opposed to conventional sway measures that provide only a parametric description of sway. To show the application of this method, we applied it to published data of spontaneous sway from elderly subjects and contrasted it to the data of young healthy subjects. We found that modest increases in stiffness and damping and a fairly large increase in noise level with aging could account for the variety of sway measures reported in the literature for elderly subjects.

Age Factors↗
PubMed2004

Origin of orientation-dependent asymmetries in vestibulo-ocular reflexes evoked by caloric stimulation.

A caloric stimulus evokes primarily a horizontal vestibulo-ocular reflex (VOR) when subjects are in a supine or prone orientation with the horizontal semicircular canal plane oriented vertically. In both monkeys and humans, the magnitude of VOR eye movements is greater in the supine than in the prone orientation, indicating that some factor or factors, other than the conventionally accepted convective stimulation of the horizontal canals, contributes to the generation of the VOR. We used long-duration caloric irrigations and mathematical models of canal-otolith interactions to investigate factors contributing to the prone/supine asymmetry. Binaural caloric irrigations were applied for 7.5 or 9.5 min with subjects in a null orientation with horizontal canals in the earth-horizontal plane (control trial), or with the subject's pitch orientation periodically changing between null, supine, and prone positions with each orientation held for 30 s (caloric step trial). The control trial responses identified a small response attributable to a direct thermal effect on vestibular afferent activity that accounted for only 15% of the observed prone/supine asymmetry. We show that the gravito-inertial force resolution hypothesis for sensory integration of canal and otolith information predicts that the central processing of canal and otolith information produces an internal estimate of motion that includes both a rotational motion component and a linear acceleration component. These components evoke a horizontal angular VOR and linear VOR, which combine additively in the supine orientation, but subtract in the prone orientation, thus accounting for the majority of the observed prone/supine asymmetry.

Adult↗
PubMed2003

Dynamic regulation of sensorimotor integration in human postural control.

Upright stance in humans is inherently unstable, requiring corrective action based on spatial-orientation information from sensory systems. One might logically predict that environments providing access to accurate orientation information from multiple sensory systems would facilitate postural stability. However, we show that, after a period in which access to accurate sensory information was reduced, the restoration of accurate information disrupted postural stability. In eyes-closed trials, proprioceptive information was altered by rotating the support surface in proportion to body sway (support surface "sway-referencing"). When the support surface returned to a level orientation, most subjects developed a transient 1-Hz body sway oscillation that differed significantly from the low-amplitude body sway typically observed during quiet stance. Additional experiments showed further enhancement of the 1-Hz oscillation when the surface transitioned from a sway-referenced to a reverse sway-referenced motion. Oscillatory behavior declined with repetition of trials, suggesting a learning effect. A simple negative feedback-control model of the postural control system predicted the occurrence of this 1-Hz oscillation in conditions where too much corrective torque is generated in proportion to body sway. Model simulations were used to distinguish between two alternative explanations for the excessive corrective torque generation. Simulation results favor an explanation based on the dynamic reweighting of sensory contributions to postural control rather than a load-compensation mechanism that scales torque in proportion to a fixed combination of sensory-orientation information.

Adaptation, Physiological↗
PubMed2002

Sensory contributions to the control of stance: a posture control model.

We present the outline of a dual kinetic-kinematic postural control model. It is based on concepts of inter-sensory interaction (sensor fusion) which we consider instrumental for sensorimotor integration. Separation into kinetic and kinematic control signals begins at the level of the sensors (e.g., vestibular system--otoliths: force field meters, canals: head angular speedometers). Sensor fusion mechanisms are used to yield separate internal representations for foot support kinematics, force fields such as gravity, and contact forces such as pull or push having impact on the body. These representations are fed as global set point signals into local proprioceptive control loops of the joints. Fed into an ankle joint proprioceptive loop for body-on-support stabilization, they yield compensation of support tilt, gravity and contact forces, even when these stimuli are combined and, furthermore, voluntary lean is superimposed. Model simulations parallel our experimental findings so far obtained.

Animals↗
PubMed2002

Diabetic neuropathy and surface sway-referencing disrupt somatosensory information for postural stability in stance.

In order to determine the type of somatosensory information for postural control that is most affected by neuropathy, we compared the relative effects of three methods of sway-referencing the surface in a group of subjects with profound loss of somatosensory function associated with sensory polyneuropathy from diabetes with age-matched control subjects. Sway-referencing disrupted somatosensory feedback for postural control by servo-controlling the dorsi- and plantar-flexion rotation of the support surface in proportion to anterior-posterior excursion of (1) ankle angle, (2) center of body mass (CoM) angle or (3) filtered center of pressure (CoP). Postural sway in subjects with somatosensory loss was significantly larger than normal on a firm surface but not on the sway-referenced surfaces, suggesting that sway-referencing disrupts somatosensory information for postural control already disrupted by neuropathy. Control subjects standing on any sway-referenced surface swayed significantly more than neuropathy subjects who stood on a firm surface, suggesting that sway-referencing disrupts more somatosensory information than disrupted by severe neuropathy. CoP sway-referencing was less sensitive than ankle or CoM sway-referencing for distinguishing postural sway in subjects with somatosensory loss from age-matched control subjects. Given that filtered CoP sway-referencing disrupts the ability to utilize somatosensory information related to surface reactive force to a greater extent than the other two methods of sway-referencing, then these results support the hypothesis that subjects with diabetic peripheral neuropathy have lost more CoP information, than ankle or CoM angle information, for controlling postural sway in stance.

Adult↗