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Biomedical subjects

F Hlavacka

Publications and source records attributed to F Hlavacka.

At least 19 recordsLinked to original sources
PubMed2006

Somatosensory influence on postural response to galvanic vestibular stimulation.

We investigated how postural responses to galvanic vestibular stimulation were affected by standing on a translating support surface and by somatosensory loss due to diabetic neuropathy. We tested the hypothesis that an unstable surface and somatosensory loss can result in an increase of vestibulospinal sensitivity. Bipolar galvanic vestibular stimulation was applied to subjects who were standing on a force platform, either on a hard, stationary surface or during a backward platform translation (9 cm, 4.2 cm/s). The intensity of the galvanic stimulus was varied from 0.25 to 1 mA. The amplitude of the peak body CoP displacement in response to the galvanic stimulus was plotted as a function of stimulus intensity for each individual. A larger increase in CoP displacement to a given increase in galvanic current was interpreted as an increase of vestibulospinal sensitivity. Subjects with somatosensory loss in the feet due to diabetes showed higher vestibulospinal sensitivity than healthy subjects when tested on a stationary support surface. Control subjects and patients with somatosensory loss standing on translating surface also showed increased galvanic response gains compared to stance on a stationary surface. The severity of the somatosensory loss in the feet correlated with the increased postural sensitivity to galvanic vestibular stimulation. These results showed that postural responses to galvanic vestibular stimulus were modified by somatosensory information from the surface. Somatosensory loss due to diabetic neuropathy and alteration of somatosensory input during stance on translating support surface resulted in increased vestibulospinal sensitivity.

Adult↗
PubMed2006

Human postural response to lower leg muscle vibration of different duration.

Body lean response to bilateral vibrations of soleus muscles were investigated in order to understand the influence of proprioceptive input from lower leg in human stance control. Proprioceptive stimulation was applied to 17 healthy subjects by two vibrators placed on the soleus muscles. Frequency and amplitude of vibration were 60 Hz and 1 mm, respectively. Vibration was applied after a 30 s of baseline. The vibration duration of 10, 20, 30 s respectively was used with following 30 s rest. Subjects stood on the force platform with eyes closed. Postural responses were characterized by center of pressure (CoP) displacements in the anterior-posterior (AP) direction. The CoP-AP shifts as well as their amplitudes and velocities were analyzed before, during and after vibration. Vibration of soleus muscles gradually increased backward body tilts. There was a clear dependence of the magnitude of final CoP shift on the duration of vibration. The amplitude and velocity of body sway increased during vibration and amplitude was significantly modulated by duration of vibration as well. Comparison of amplitude and velocity of body sway before and after vibration showed significant post-effects. Presented findings showed that somatosensory stimulation has a long-term, direction-specific influence on the control of postural orientation during stance. Further, the proprioceptive input altered by soleus muscles vibration showed significant changes in postural equilibrium during period of vibration with interesting post-effects also.

Adult↗
PubMed2004

Velocity of head movements and sensory-motor adaptation during and after short spaceflight.

To investigate to time course of sensory-motor adaptation to microgravity, we tested spatially-directed voluntary head movements before, during and after short spaceflight. We also tested the re-adaptation of postural responses to sensory stimulation after space flight. The cosmonaut performed in microgravity six cycles of voluntary head rotation in pitch, roll and yaw directions. During the first days of weightlessness the angular velocity of head movements increased. Over the next days of microgravity the velocity of head movements gradually decreased. On landing day a significant decrease of head rotation velocity was observed compared to the head movement velocity before spaceflight. Re-adaptation to Earth condition measured by body sway on soft support showed similar time course, but re-adaptation measured by postural responses to vestibular galvanic stimulation was prolonged. These results showed that the angular velocity of aimed head movements of cosmonauts is a good indicator of sensory-motor adaptation in altered gravity conditions.

Adaptation, Physiological↗
PubMed2001

Human balance control during cutaneous stimulation of the plantar soles.

Previous work on human postural control of upright stance, performed in the absence of visual and vestibular orientation cues, suggests that somatosensory cues in the feet enable subjects to maintain equilibrium during low-frequency platform tilts. Here we confirm earlier studies which indicated that stimulation of plantar cutaneous mechanoreceptors can lead to postural responses. Yet, this stimulation did not modify considerably the postural reactions of normal subjects and vestibular loss patients during platform tilts. We therefore suggest that it is necessary to differentiate between (i) cues from plantar cutaneous receptors involved in exteroceptive functions, like the evaluation of the support structure or of relative foot-to-surface motion, and (ii) cues from deep receptors which subserve proprioceptive functions like the control of center of pressure shifts within the limits of the foot support base.

Adult↗
PubMed2001

Somatosensory loss increases vestibulospinal sensitivity.

To determine whether subjects with somatosensory loss show a compensatory increase in sensitivity to vestibular stimulation, we compared the amplitude of postural lean in response to four different intensities of bipolar galvanic stimulation in subjects with diabetic peripheral neuropathy (PNP) and age-matched control subjects. To determine whether healthy and neuropathic subjects show similar increases in sensitivity to galvanic vestibular stimulation when standing on unstable surfaces, both groups were exposed to galvanic stimulation while standing on a compliant foam surface. In these experiments, a 3-s pulse of galvanic current was administered to subjects standing with eyes closed and their heads turned toward one shoulder (anodal current on the forward mastoid). Anterior body tilt, as measured by center of foot pressure (CoP), increased proportionately with increasing galvanic vestibular stimulation intensity for all subjects. Subjects with peripheral neuropathy showed larger forward CoP displacement in response to galvanic stimulation than control subjects. The largest differences between neuropathy and control subjects were at the highest galvanic intensities, indicating an increased sensitivity to vestibular stimulation. Neuropathy subjects showed a larger increase in sensitivity to vestibular stimulation when standing on compliant foam than control subjects. The effect of galvanic stimulation was larger on the movement of the trunk segment in space than on the body's center of mass (CoM) angle, suggesting that the vestibular system acts to control trunk orientation rather than to control whole body posture. This study provides evidence for an increase in the sensitivity of the postural control system to vestibular stimulation when somatosensory information from the surface is disrupted either by peripheral neuropathy or by standing on an unstable surface. Simulations from a simple model of postural orientation incorporating feedback from the vestibular and somatosensory systems suggest that the increase in body lean in response to galvanic current in subjects with neuropathy could be reproduced only if central vestibular gain was increased when peripheral somatosensory gain was decreased. The larger effects of galvanic vestibular stimulation on the trunk than on the body's CoM suggest that the vestibular system may act to control postural orientation via control of the trunk in space.

Adaptation, Physiological↗
PubMed2001

Human postural responses to different frequency vibrations of lower leg muscles.

We analyzed human postural responses to muscle vibration applied at four different frequencies to lower leg muscles, the lateral gastrocnemius (GA) or tibialis anterior (TA) muscles. The muscle vibrations induced changes in postural orientation characterized by the center of pressure (CoP) on the force platform surface on which the subjects were standing. Unilateral vibratory stimulation of TA induced body leaning forward and in the direction of the stimulated leg. Unilateral vibration of GA muscles induced body tilting backwards and in the opposite direction of the stimulated leg. The time course of postural responses was similar and started within 1 s after the onset of vibration by a gradual body tilt. When a new slope of the body position was reached, oscillations of body alignment occurred. When the vibrations were discontinued, this was followed by rapid recovery of the initial body position. The relationship between the magnitude of the postural response and frequency of vibration differed between TA and GA. While the magnitude of postural responses to TA vibration increased approximately linearly in the 60-100 Hz range of vibration frequency, the magnitude of response to GA vibration increased linearly only at lower frequencies of 40-60 Hz. The direction of body tilt induced by muscle vibration did not depend on the vibration frequency.

Adult↗
PubMed2000

Vestibular, visual, and somatosensory contributions to human control of upright stance.

We investigated the changes of human posture control of upright stance which occur when vestibular cues (VEST) are absent and visual and somatosensory orientation cues (VIS, SOM) are removed. Postural responses to sinusoidal tilts of a motion platform in the sagittal plane (+/-2 degrees, f=0.05, 0.1, 0.2 and 0.4 Hz) were studied in normal subjects (Ns) and patients with bilateral vestibular loss (Ps). We found that absence of VEST (Ps, visual reference) and removal of VIS (Ns, no visual reference) had little effect on stabilization of upright body posture in space. In the absence of both VEST and VIS (Ps, no visual reference) somatosensory graviception still provided some information on body orientation in space at 0.05 and 0.1 Hz. However, at the higher frequencies Ps qualitatively changed their behavior; they then tended to actively align their bodies with respect to the motion platform. The findings confirm predictions of a novel postural control model.

Cues↗
PubMed2000

Visual contributions to human self-motion perception during horizontal body rotation.

It is still an enigma how human subjects combine visual and vestibular inputs for their self-motion perception. Visual cues have the benefit of high spatial resolution but entail the danger of self motion illusions. We performed psychophysical experiments (verbal estimates as well as pointer indications of perceived self-motion in space) in normal subjects (Ns) and patients with loss of vestibular function (Ps). Subjects were presented with horizontal sinusoidal rotations of an optokinetic pattern (OKP) alone (visual stimulus; 0.025-3.2 Hz; displacement amplitude, 8 degrees) or in combinations with rotations of a Bárány chair (vestibular stimulus; 0.025-0.4 Hz; +/- 8 degrees). We found that specific instructions to the subjects created different perceptual states in which their self-motion perception essentially reflected three processing steps during pure visual stimulation: i) When Ns were primed by a procedure based on induced motion and then they estimated perceived self-rotation upon pure optokinetic stimulation (circular vection, CV), the CV has a gain close to unity up to frequencies of almost 0.8 Hz, followed by a sharp decrease at higher frequencies (i.e., characteristics resembling those of the optokinetic reflex, OKR, and of smooth pursuit, SP). ii) When Ns were instructed to "stare through" the optokinetic pattern, CV was absent at high frequency, but increasingly developed as frequency was decreased below 0.1 Hz. iii) When Ns "looked at" the optokinetic pattern (accurately tracked it with their eyes) CV was usually absent, even at low frequency. CV in Ps showed similar dynamics as in Ns in condition i), independently of the instruction. During vestibular stimulation, self-motion perception in Ns fell from a maximum at 0.4 Hz to zero at 0.025 Hz. When vestibular stimulation was combined with visual stimulation while Ns "stared through" OKP, perception at low frequencies became modulated in magnitude. When Ns "looked" at OKP, this modulation was reduced, apart from the synergistic stimulus combination (OKP stationary) where magnitude was similar as during "staring". The obtained gain and phase curves of the perception were incompatible with linear systems prediction. We therefore describe the present findings by a non-linear dynamic model in which the visual input is processed in three steps: i) It shows dynamics similar to those of OKR and SP; ii) it is shaped to complement the vestibular dynamics and is fused with a vestibular signal by linear summation; and iii) it can be suppressed by a visual-vestibular conflict mechanism when the visual scene is moving in space. Finally, an important element of the model is a velocity threshold of about 1.2 degrees/s which is instrumental in maintaining perceptual stability and in explaining the observed dynamics of perception. We conclude from the experimental and theoretical evidence that self-motion perception normally is related to the visual scene as a reference, while the vestibular input is used to check the kinematic state of the scene; if the scene appears to move, the visual signal becomes suppressed and perception is based on the vestibular cue.

Adult↗
PubMed1999

The timing of galvanic vestibular stimulation affects responses to platform translation.

We compared the effects of galvanic vestibular stimulation applied at 0, 0.5, 1.5 and 2.5 s prior to a backward platform translation on postural responses. The effect of the galvanic stimulation was largest on the final equilibrium position of the center of pressure (CoP). The largest effects occurred for the 0.5 and 0-s pre-period, when the dynamic CoP pressure changes in response to both the galvanic stimulus and the platform translation coincided. The shift in the final equilibrium position was also larger than the sum of the shifts for the galvanic stimulus and the platform translation alone for the 0.5 and 0-s pre-periods. The initial rate of change of the CoP response to the platform translation was not significantly affected in any condition. Changes in the peak CoP position could be accounted for by local interaction of CoP velocity changes induced by the galvanic and translation responses alone, but the changes in final equilibrium position could only be accounted for by a change in global body orientation. These findings suggest that the contribution of vestibulospinal information is greatest during the dynamic phase of the postural response, and that the vestibular system contributes most to the later components of the postural response, particularly to the final equilibrium position. These findings suggest that a nonlinear interaction between the vestibular signal induced by the galvanic current and the sensory stimuli produced by the platform translation occurs when the two stimuli are presented within 1 s, during the dynamic phase of the postural response to the galvanic stimulus. When presented at greater separations in time, the stimuli appear to be treated as independent events, such that no interaction occurs.

Adult↗
PubMed1997

[Asymmetrical body tilt induced by vibration of the Achilles tendon in patients with unilateral vestibular hypofunction].

Unilateral vestibular lesion results in postural balance deficits, which progressively vanish with time compensation. This functional recovery is caused due to the reorganization of the CNS structures and afferent inputs, mainly of the proprioceptive afferentation. Our aim was to determine the postural effect of leg proprioceptive input induced by the Achilles tendon (AT) vibration in standing patients with unilateral vestibular hypofunction. The examined patients (9 patients unilateral vestibular neuritis and 3 patients with Meniere's disease) had unilaterally decreased caloric responses. The control were 20 healthy volunteers with intact vestibular and motor functions. The postural responses evoked by AT vibration were evaluated by the symmetry of centre of pressure (COP) of the subject. The postural responses induced by the AT vibration in the healthy were bilaterally symmetrical. In the patients the body lean evoked by vibration on the side of intact vestibular apparatus was significantly decreased. The AT vibration on the lesioned side evoked practically identical response to the response of healthy subjects. In the repeated examination after 6 months the asymmetry disappeared which was in accordance with the recovered clinical state. The findings of asymmetry of postural COP displacement in patients with acute unilateral vestibular hypofunction documented transitory asymmetry of influence from leg proprioceptive inputs. The direction of decreased postural response to the proprioceptive stimuli was the same as pathological body lean of patient to the side of lesioned vestibular apparatus. This fact allows us to hypothesize that the postural responses evoked by leg proprioceptive inputs, directionally identical with the pathological body lean induced by asymmetry of vestibular afferentation are inhibited. (Fig. 3, Ref. 15.)

Achilles Tendon↗
PubMed1996

Human self-motion perception during translatory vestibular and proprioceptive stimulation.

Self-motion perception in space was studied in normal human subjects during passive vestibular stimulation (lateral translation of whole body in space), proprioceptive stimulation (of feet relative to trunk) and combinations thereof with the eyes closed. Stimulation was sinusoidal, +/- 10 cm, over a frequency range of 0.025-0.4 Hz. Vestibular self-motion perception became increasingly underestimated at low frequency, due to a rather high detection threshold. Proprioceptive stimulation at low frequency elicited a small self-motion illusion. During body translation relative to the stationary feet (vestibular-proprioceptive combination) the magnitude of perceived self-motion was constant across frequency and its threshold was low, as if determined by proprioception alone. Nevertheless, the results can be interpreted in terms of a vestibular-proprioceptive interaction, in analogy to previous findings for rotational stimuli.

Evoked Potentials, Somatosensory↗
PubMed1996

Control of the body vertical by vestibular and proprioceptive inputs.

The study examines the influence of vestibular and leg proprioceptive cues on the maintenance of the body vertical in human stance. Vestibular body orientation cues were changed by applying bipolar currents to both mastoid bones (cosine-bell wave form of 3.3 s duration, 1 mA current intensity). Proprioceptive input was modified by vibrating the tibialis anterior muscle (at f = 90 Hz, step of 5 s duration and 1 mm amplitude). Furthermore, the vestibular stimulus was paired with the muscle vibration using three different temporal relationships between the stimuli. Body lean responses were analyzed in terms of sway trajectories of the center of foot pressure on the body support surface (horizontal plane). With the anode on the right mastoid, vestibular body lean response was essentially straight towards the right side, and with the anode on left mastoid towards the left side. Vibration of right tibialis anterior muscle induced an almost straight body lean forward and to the right. Upon combined stimulation, responses with complex trajectory resulted, which depended on the stimulus interval. These responses reflected a superposition of the individual vestibular and proprioceptive effects. The results show that the body vertical is under the continuous control of leg proprioceptive and vestibular inputs, which sum linearly. We present a concept according to which these inputs are used for establishing a reference system for the control of the body vertical.

Adult↗
PubMed1995

Modification of human postural response to leg muscle vibration by electrical vestibular stimulation.

In order to understand proprioceptive and vestibular contributions to human stance posture, the effect of electrical vestibular stimulation on body lean induced by leg muscle vibration was investigated. The magnitudes and directions of postural responses were registered as changes in the center of foot pressure (COP) with a force platform. Vestibular stimulation consisted of 1 mA, binaural, bipolar galvanic current and proprioceptive input from tibialis anterior or soleus muscles was altered vibratory stimulation. The body lean induced by combined vibratory and galvanic stimulation could be largely considered as a summation of responses evoked by the galvanic and vibratory stimulation alone. The results of the present study showed that both vestibular and proprioceptive signals play important roles in the estimation of internal representation of the body vertical.

Adult↗
PubMed1995

Effect of galvanic vestibular stimulation on human postural responses during support surface translations.

1. We investigated the role of the vestibular system in postural control by combining galvanic vestibular stimulation (0.2-0.5 mA) with platform translations in standing subjects. Vestibular stimulation delivered 500 ms before and continuously during the platform translation produced little change in the earliest center of pressure (COP) and center of mass (COM) movements in response to platform translations, but resulted in large changes during the execution of the postural movement and in the final equilibrium position. 2. Vestibular stimulation produced anterior or posterior shifts in the position of COP and COM, depending on the polarity of the galvanic current. These shifts were larger during platform translations than during quiet stance. The peak of these shifts in COP and COM occurred at 1.5-2.5 s after the onset of platform translation, and increased in magnitude with increasing platform velocity. The final equilibrium positions of COP and COM were also shifted, but these shifts were smaller and not dependent on platform velocity. 3. These results imply that a tonic step of galvanic current to the vestibular system can change the final equilibrium position for an automatic postural response. Furthermore, these results indicate that the vestibular system may play a larger role in interpreting sensory reafference during postural movements, and especially during fast postural movements, than in controlling quiet stance. Finally, these results indicate that the vestibular system does not play a critical role in triggering the earliest postural responses, but it may be critical in establishing an internal reference for verticality.

Adult↗
PubMed1994

Binaural monopolar galvanic vestibular stimulation reduces body sway during human stance.

The influence of monopolar binaural galvanic stimulation of the vestibular system was studied on body sway. Subjects, with eyes closed, were standing on a hard support or on foam rubber. Their body sway was registered on a force platform at intervals of 50 s. Both polarities of direct current with intensity 1 mA were used as a galvanic stimulus during the whole recording interval. Changes of body sway amplitude and velocity were analyzed in situations with and without galvanic stimulation on two different support surfaces. In stance on the hard support, the cathodal polarization of labyrinths (in most subjects) reduced body sway velocity and decreased body sway slightly in the anteroposterior direction. Anodal polarization of labyrinths during 50 s did not affect the body sway parameters. The results on the foam rubber platform exhibited a significant reduction of body sway velocity induced by both anodal and cathodal polarization of the labyrinths. The decrease of body sway in the anteroposterior direction was also observed during cathodal polarization. The stabilizing effect of vestibular binaural monopolar stimulation on the upright stance was mainly observed in the postural control situation where the leg proprioceptive input was changed (stance on soft surface) and the role of vestibular input was more important.

Adolescent↗
PubMed1993

Interaction of vestibular and proprioceptive inputs.

The study investigates the interaction of leg proprioceptive and vestibular afferents for human self-motion perception in space. Stimulation consisted of sinusoidal (0.025-0.4 Hz) and transient horizontal rotations of trunk and head in space (vestibular stimulus, VEST) and of the feet relative to the trunk (leg proprioceptive stimulus, LEG-PROP). Measures of the perception were obtained with the help of a pointing procedure. Leg proprioception. The perception of relative motion between feet and trunk during LEG-PROP was veridical across the frequencies tested and had a low detection threshold (0.2 degree/s). Perception of trunk turning in space. Trunk turning during VEST was underestimated, especially at low frequencies, and the threshold of the perception was > or = 1.0 degree/s. LEG PROP evoked an illusion of trunk turning, which reached a considerable magnitude at low frequencies. During VEST-LEG-PROP combinations, the perception varied monotonously as a function of both inputs. Reflecting the deficiencies of its constituents, it was erroneous with 1 exception. During trunk rotation about the stationary feet, the perception was approximately veridical across frequency and its threshold was down to 0.2 degree/s, suggesting that it was determined essentially by leg proprioception in this condition. These findings resemble those previously obtained for neck proprioception and, therefore, were incorporated into a conceptual model of vestibular-proprioceptive interaction in general. In this model, first an internal notion of foot in space is created by summing the following high-threshold signals: head in space (vestibular), trunk relative to head (neck proprioceptive), and foot relative to trunk (leg proprioceptive). Second, further addition of low-threshold proprioceptive signals of trunk on foot and head on trunk yields the perception of trunk in space and head in space, respectively. Not included in the model is the finding that subjects' perceptual mode may change in certain conditions. When foot excursion exceeds a certain magnitude, for instance, vestibular input alone may determine the self-motion perception.

Adult↗
PubMed1993

Visual control of human stance on a narrow and soft support surface.

The influence of additional visual feedback (VF) on stance control was studied under conditions of changed afferent information from the foot sole and ankle joint due to different support surfaces. The changes of body sway amplitudes were analyzed and their frequency spectrum was established. The effect of visual feedback on the amplitude and frequency characteristics of human stance was manifested as: a) a decrease of the mean amplitude of body sway during visual feedback, corresponding to the decrease of power spectrum density (PSD) of stabilograms in the frequency range below 0.05 Hz, b) an increase of mean velocity of body sway corresponding to the increase of PSD of stabilograms in the frequency range of 0.4-1.5 Hz. The results showed that the improvement of the upright stance by additional visual feedback is mainly mediated through activation of postural muscles at the ankle level, or ankle strategy. The stabilization effect of VF on stance control is slight or negligible if the performance part in ankle joint (narrow support) was reduced.

Adult↗