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Biomedical subjects

Fay B Horak

Publications and source records attributed to Fay B Horak.

29 records · Page 2Linked to original sources
PubMed2004

Effects of cadence on the acquisition and expression of podokinetic after-rotation.

Podokinetic after-rotation (PKAR) occurs as blindfolded subjects inadvertently rotate when asked to step in-place following stepping in-place on a rotating surface. We examined the effects of using different cadences on PKAR to test the following hypothesis: the position signal indicating the amount of rotation between the trunk and the feet during each stance period of treadmill stimulation is used to determine the amount of rotation between the trunk and the feet that is expressed during each stance period of PKAR. Based on this hypothesis, we predicted that use of different cadences during treadmill stimulation would alter PKAR velocity because use of different cadences alters stance duration, thus changing the amount of limb rotation under the trunk during each stance phase of stimulation. We also predicted that use of different cadences during PKAR would alter PKAR velocity because the more steps that are taken in a given time the higher the velocity of PKAR given that the same rotation between trunk and feet occurs during each stance period. Use of different cadences during treadmill stimulation did not alter PKAR velocity, suggesting that PKAR velocity is not determined based upon a position signal regarding the relative rotation between the trunk and feet during stimulation. Use of different cadences during PKAR resulted in lower and higher velocities, respectively, than using a medium cadence. Based on these results, we now hypothesize that the PK system likely uses a velocity or acceleration signal present during stimulation to recalibrate the amount of relative rotation between the trunk and limbs that is expressed with each step during PKAR.

Adult↗
PubMed2004

Comparison between subthalamic nucleus and globus pallidus internus stimulation for postural performance in Parkinson's disease.

Nine subjects with Parkinson's disease, five subjects with electrodes implanted in the subthalamic nucleus (STN) and four with electrodes in the globus pallidus internus (GPi), were compared with a population of ten age-matched control subjects. The measures studied include a set of summary statistic scores, two stochastic parameters, the distribution of the center of pressure (CoP) displacement angles under each foot, and the distribution of bilateral patterns of CoP displacement angles. A Bayes classifier was designed to monitor the trend of postural performance in patients, with different treatments. Results suggested that the selected measures were sensitive to Parkinsonian postural sway abnormalities and highlight differences in response to treatments. Deep brain stimulation restored a more normal postural sway and levodopa increased sway abnormalities. Furthermore, the selected measures appear to detect different responses to levodopa between the STN and GPi groups: the negative side effects of levodopa on posture were less severe for STN than for GPi patients, perhaps due to the decreased need for levodopa intake in STN subjects. The measures proposed in this preliminary study may be useful adjuncts to evaluate balance and postural control strategies in patients with Parkinson's disease and may allow the comparison of DBS electrode sites, on stance posture.

Electric Stimulation Therapy↗
PubMed2003

Postural feedback responses scale with biomechanical constraints in human standing.

We tested whether human postural responses can be described in terms of feedback control gains, and whether these gains are scaled by the central nervous system to accommodate biomechanical constraints. A feedback control model can describe postural responses for a wide range of perturbations, but biomechanical constraints-such as on the torque that can be exerted on the ground-make a single set of feedback gains inappropriate for all perturbations. To observe how postural responses change with perturbation magnitude, we applied fast, backward perturbations of magnitudes 3-15 cm to 13 healthy young volunteers (4 men, 9 women, aged 20-32 years). We used a 3-segment, sagittal-plane biomechanical model and a linear state feedback controller to reproduce the observed postural responses. Optimization was used to identify the best-fit feedback control gains for each trial. Results showed that trajectories of joint angles and joint torques were scaled with perturbation magnitude. This scaling occurred gradually, rather than abruptly changing at magnitudes where biomechanical constraints became active. Feedback gains were found to fit reasonably well with data ( R(2)=0.92) and to be multivariate and heterogenic in character, meaning that the torque produced at any joint is generally a function of motions not only at the same joint, but other joints as well. Hip gains increased and ankle gains decreased nearly linearly with perturbation magnitude, in accordance with biomechanical limitations on ground reaction torque. These results indicate that postural adjustments can be described as a single feedback control scheme, with scalable heterogenic gains that are adjusted according to biomechanical constraints.

Adult↗
PubMed2003

Postural muscle responses to multidirectional translations in patients with Parkinson's disease.

The postural adaptation impairments of patients with Parkinson's disease (PD) suggest that the basal ganglia may be important for quickly modifying muscle activation patterns when the direction of perturbation or stance conditions suddenly change. It is unknown whether their particular instability to backward postural perturbations is due to specific abnormalities of parkinsonian postural muscle synergies in that direction and not present in other directions. In the present study, we test this hypothesis by comparing the patterns of leg and trunk muscle activation in 13 subjects with PD and 13 control subjects in response to eight randomly presented directions of horizontal surface translations while standing with either narrow or wide stance. The direction of maximum activation for each muscle was similar for PD and control subjects, suggesting that the basal ganglia is not critical for programming externally triggered postural synergies. However, antagonist muscle activation was earlier and larger in PD than in control subjects, resulting in coactivation. PD subjects also did not increase the magnitude of muscle activation as much as did control subjects when changing from wide to narrow stance. These results are consistent with the hypothesis that PD results in an inability to shape the pattern and magnitude of postural muscle responses for changes in perturbation direction and in stance position.

Adaptation, Physiological↗
PubMed2003

Voluntary control of postural equilibrium patterns.

The ability to voluntarily transit from one whole-body movement to another is based on the multisensory integration of visual, vestibular, and somatosensory information. The role of functional sensory ranges and mechanical constraints on the ability to voluntarily transit between whole-body movements was studied by requiring subjects to switch from a head-fixed-to-surface to head-fixed-in-space postural pattern (and vice versa). The head-fixed-to-surface pattern required an erect stance characterized by an in-phase relationship between center of pressure (CoP) and platform motion. The head-fixed-in-space pattern required subjects to fix trunk-head position in-space while producing an anti-phase relationship between CoP and platform motion. The voluntary transition was performed with and without vision while standing on a surface oscillating in the anterior-posterior (A/P) direction. The support surface oscillated at five frequencies (0.2-1Hz) with amplitude fixed at 15cm. The voluntary transition was initiated with an auditory cue. The appropriate CoP-platform phase relationship for the two postural patterns was produced for all frequencies with and without vision. Upper-trunk kinematics revealed that subjects often failed to produce the head-fixed-to-surface pattern for frequencies >/=0.6Hz, while producing the head-fixed-in-space pattern at all frequencies with vision. Without vision, neither pattern was produced consistently based on upper-trunk kinematics. These findings demonstrate separate control processes for upper- and lower-body motion and that functional sensory ranges and mechanical constraints can facilitate or inhibit voluntary production of whole-body movements based on these control processes. The results are discussed in reference to neurological substrates that may be involved in the planning and execution of motor set-switching. The experimental protocol we employ may also have application as a diagnostic tool for the evaluation of postural deficits.

Adaptation, Physiological↗
PubMed2003

Head and body center of gravity control strategies: adaptations following vestibular rehabilitation.

OBJECTIVE: We present for the first time evidence that vestibulopathy impairs coordination of the head with the body center of gravity (CG) during free speed gait over ground. Vestibulopathic individuals demonstrate uncoordinated movement and gait due, at least in part, to impaired head stability and visual fixation. Vestibular rehabilitation increases speed and stability during gait and stair climbing, although the underlying mechanisms are poorly understood. MATERIAL AND METHODS: To determine whether these locomotor improvements are due to reorganized coordination of the head with whole body CG, three-dimensional kinematics were obtained from 10 vestibulopathic individuals before and after vestibular rehabilitation and from 10 matched healthy control subjects during unconstrained, paced and in-place gait. Head control patterns were characterized using both qualitative pattern analysis and quantification of coherence between head and body CG displacements. RESULTS: Patterns of head-CG coordination differ between normal and vestibulopathic individuals in all three directions of head rotation--pitch, roll and yaw--before rehabilitation. Following vestibular rehabilitation, subjects with vestibulopathy demonstrate more normal patterns in pitch and improvements toward normal in roll and yaw. CONCLUSION: These data strongly suggest that compensatory mechanisms, obtained during vestibular rehabilitation, mediate head-CG coordination.

Adaptation, Physiological↗
PubMed2002

Does the cerebellum play a role in podokinetic adaptation?

After sustained stepping in-place on a rotating disc, healthy subjects will inadvertently turn in circles when asked to step in-place on a stationary surface with eyes closed. We asked whether the cerebellum is important for this adaptive phenomenon, called podokinetic after-rotation (PKAR). Subjects with cerebellar degeneration and age-matched control subjects performed 15 min of stepping in-place with eyes open on a rotating disc, then 30 min of attempting to step in-place with eyes closed on a stationary surface. Rotational velocity of PKAR was measured during this 30-min period. All control subjects demonstrated PKAR; average initial rotational velocity for control subjects was 16.4+/-3.5 degrees /s. Five of the eight cerebellar subjects demonstrated impaired PK adaptation, defined as PKAR with an initial velocity more than two standard deviations below the control mean initial velocity. Average initial rotational velocity for cerebellar subjects was 7.8+/-0.2 degrees /s. Impaired PK adaptation was not associated with impaired time constants of decay and was not correlated with variability of PKAR velocity. Our results suggest that the cerebellum is important for regulation of the amplitude of PK adaptation and that reduced PKAR amplitude is not likely the result of dyscoordination or variability of movement in the subjects tested.

Adaptation, Psychological↗
PubMed2002

Vestibular stimulation affects medium latency postural muscle responses.

This study explores whether galvanic vestibular stimulation can alter automatic postural muscle responses triggered 100 ms after surface translations. Our previous study concluded that a step of bipolar, galvanic vestibular stimulation delivered 500 ms prior to a platform translation tilted the internal representation of vertical because subjects' final center of foot pressure and center of mass equilibrium position shifted toward the anode and this tilt was larger than the sum of effects for platform translations and galvanic alone. In the current study, we show that 0.2-0.4 mA of galvanic vestibular stimulation produced significant changes in the tilt of the trunk in space that was realized by changes in the magnitude of the medium latency postural muscle responses at the ankle. The galvanic-induced changes in latency and magnitude of the first 50 ms of gastrocnemius muscle burst in response to the backward surface translation were consistent with changes in background muscle tone induced by the direct vestibulospinal effects of the galvanic current. However, the galvanic-induced changes in the second 50 ms of the gastrocnemius response were in the opposite direction and consistent with the forward- or backward-tilted, final postural equilibrium goal. Kinematic analysis showed that galvanic-induced tilt was first initiated in the trunk and that the shank and thigh segment angles were not altered by galvanic stimulation prior to platform translation such that changes in gastrocnemius and soleus postural responses to translations were not due to an effect of galvanic stimulation on initial ankle angle. More proximal muscles and antagonist muscles involved in the postural response were not altered by the galvanic stimulation. These results suggest that galvanic vestibulospinal stimulation can alter medium latency, automatic postural responses in prime movers by changing the postural equilibrium goal.

Adult↗
PubMed2002

Vestibulospinal control of posture.

To better understand the role of the vestibular system in postural coordination, we compared the ability of subjects with complete, bilateral loss of vestibular function and age-matched control subjects to maintain equilibrium and postural orientation during sinusoidal displacements of the support surface at a variety of frequencies. We also examined the ability of visual or somatosensory-light touch information to substitute for missing vestibular information in dynamic postural coordination. The results suggest that vestibular information is used as a gravitational reference frame to prevent slow drift of the trunk in space during complex postural tasks. Furthermore, visual information or somatosensory information from light touch of a finger on a stable reference can significantly substitute for loss of vestibular function.

Gravitation↗
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↗