G. V. Kozhina

Postural responses to combined vestibular and hip proprioceptive stimulation in man

Vestibular–proprioceptive interaction in human postural control in the frontal plane was studied by analysing the lateral body sway evoked in a standing subject by a weak, near‐threshold galvanic vestibular stimulation combined with a balanced, bilateral vibration of the medial gluteus muscles. The intensities of the stimuli were adjusted so that none of them produced a consistent postural response when delivered alone. The pattern of the lateral body sway evoked by the combined stimulation was compared with postural responses to suprathreshold vestibular stimulation and asymmetric (unilateral) vibration of the hip abductors. During the vestibular stimulation alone the head movement started earlier and was larger than movement of the hip. During unilateral vibration the head movement was delayed with respect to the hip movement and the amplitude of head deviation was less than that of the hip. The pattern of postural response to combined vestibular stimulation and balanced vibration resembled that observed under unbalanced, unilateral vibration in terms of both the latencies and amplitudes of deviation of the body segments from their respective baseline positions. It is suggested that the asymmetric vestibular signal provided by galvanic stimulation of the labyrinth introduces a bias into the reference frame for central interpretation of proprioceptive signals so that a symmetric proprioceptive input gives rise to a lateral body sway when referenced to an asymmetric vestibular input.

Maintenance of the Upright Posture in Humans upon Manipulating the Direction and Delay of Visual Feedback

We examined the maintenance of the upright posture in subjects immersed in a 3D virtual visual environment, VVE. The latter consisted of two plans; the VVE foreground represented the image of a room window, while the background looked like an outside urban landscape. The position of the foreground (window) was linked, using the respective software, with body sways in the sagittal plane and shifted in accordance with body movements, while the background always remained immobile. By manipulating the direction and time delay of the relationship between the body sways and shifts of the VVE foreground, we tried to estimate the contribution of visual signals to postural control. For this purpose, we used the technique of frequency filtration of displacements of the center of feet pressure (CFP) and, in such a way, detected signals proportional to movements of the general center of gravity (CG) of the body (CG variable). After this, we calculated the difference between the CFP and CG shifts (CFP-CG-variable). The latter variable is known to be proportional to the horizontal acceleration; this is why it was used for estimation of the muscular torques correcting CG shifts. Analysis of changes in the above variables (CG and CFP-CG) demonstrated a clear dependence between the root mean square (RMS) of their amplitude spectra on the direction of feedback between body sways and shifts of the VVE foreground. With a synphase (SPh) pattern of the link between shifts of the body and those of the VVE foreground, the RMS values were closer to the range typical of standing with the eyes closed, while with an antiphase (APh) link, they were closer to the zone of values observed under conditions of normal vision. Introduction of time delays in feedback between sagittal body sways and shifts of the VVE foreground exerted nearly similar effects on the CG and CFP-CG variables. In both SPh and APh relations, 0.2- and 0.5-sec-long time delays resulted in increases in the RMS of the sway spectrum, while 0.8- and 1.0-sec-long delays led to RMS decreases. Thus, our findings show that the maintenance of upright stance under conditions of manipulations of the direction and time delay of visual feedback is performed, at least partly, with the use of an unstable VVE foreground as the reference. This can be considered proof of a special (frequently, dominating) role of the visual signals in postural control.

Dependence of joint stiffness on the conditions of visual control in upright undisturbed stance in humans

We recorded the sagittal and frontal components of the stabilogram of healthy humans in upright undisturbed stance under five conditions of visual control: (i) open eyes (OE); (ii) closed eyes (CE); (iii) visual inversion (VI); (iv) central vision (CV), and (v) diffused light (DL). Through a low-pass filter of trajectories of the center of pressure of feet (CPF), the vertical projection of the center of gravity (CG) and, consequently, the difference CPF-CG were estimated. The former represents the controlled variable, while the latter is proportional to the horizontal acceleration and assumed to express the resultant joint stiffness (mostly in the ankle joints). The stiffness was characterized through a method based on spectral analysis of the CPF-CG variable and subsequent calculations of the median frequency (MF) and the root mean square (RMS) of the spectra. The median frequencies of the spectra of the CPF-CG variable changed slightly under various visual conditions. At standing on a rigid support, they varied from 0.97 to 0.99 Hz and from 0.93 to 0.97 Hz for the CPF-CG, calculated from the sagittal and frontal components of the stabilogram, respectively. Under conditions of a pliable support, the corresponding frequencies varied within the limits of 0.79–0.83 Hz and 0.74–0.78 Hz. In contrast to the median frequencies, the RMSs demonstrated greater variability depending on different visual conditions. At standing on a rigid support, paired comparisons showed significant differences between the RMSs of the spectra of the CPF-CG variable of the sagittal direction under CE and OE conditions (0.14 ± 0.030 and 0.09 ± 0.020 mm, respectively) and under DL and OE conditions (0.130 ± ± 0.025 and 0.090 ± 0.020 mm, respectively). The RMS of the CPF-CG variable calculated for the frontal stabilogram differed significantly from each other for the VI and OE conditions (0.115 ± 0.020 and 0.075 ± ± 0.015 mm, respectively). In case of standing on a pliable support, a greater variability of visual influences on the CPF-CG variable was found. The RMS for its sagittal motion was the greatest under CE conditions (0.19 ± 0.03 mm); it was significantly greater than the respective values under OE, CV, and DL conditions (0.097 ± ± 0.020, 0.110 ± 0.020, and 0.140 ± 0.030 mm, respectively). The means of RMSs of the spectra of the frontal CPF-CG was also the greatest under CE conditions (0.20 ± 0.03 mm) and the smallest under OE conditions (0.095 ± 0.020 mm). In addition, the value of the RMS fluctuations under CE conditions (0.150 ± 0.025 mm) differed significantly from the respective values under OE conditions (0.095 ± 0.020 mm) and CV conditions (0.110 ± 0.020 mm). Thus, our findings support the statement that the influence of visual conditions on the maintenance of vertical stance is mediated (at least partially) by the mechanisms controlling the ankle joint stiffness. This regulation is mostly manifested in changes of a single parameter, the amplitude of fluctuations of the CPF-CG variable. We also found that the joint stiffness can be modulated by both nonspecific visual influences (which, in particular, reflect the perception of illumination) and specific visual influences, related to information on the position of the body and on its movements with respect to external objects.

Specific and Nonspecific Visual Influences on the Stability of the Vertical Posture in Humans

We studied physiological mechanisms of vision-related stabilization of the vertical posture in humans using a stabilographic technique; spontaneous deviations of the projection of the center of gravity during quiet stance and magnitudes of the postural response to vibratory stimulation of proprioceptors of the lower leg muscles under varied conditions of visual control were measured. The stability of quiet stance, as estimated according to the root mean square value of the sagittal component of the stabilogram, was the best with eyes open. Vibration-induced postural responses were the smallest also under these conditions. Spontaneous postural sway and the amplitude of response to vibratory stimulation increased when only a central sector of visual field (20 ang. deg) was preserved and, especially, under conditions of closed eyes and horizontal inversion of visual perception using prismatic spectacles. Parallel changes in the quantitative stabilographic indices and amplitude of vibration-induced postural responses show that the intensity of the latter is probably determined by the background stiffness of the musculoskeletal system. We tried to estimate separately the contributions of the stiffness factor, on the one hand, and specific visual influences, on the other hand, by testing the parameters of quiet stance and postural responses under conditions of standing while lightly touching a support with the index finger. We found that the influence of the conditions of visual control on the stability of quiet stance while touching the support was eliminated. At the same time, the magnitude of postural responses to vibratory stimulation decreased but, nonetheless, changed with visual conditions in the same manner as when standing without additional support. We conclude that vision performs a dual function in the control of the vertical posture; it forms the basis for the spatial reference system and serves the source of information on the movements of ones body.

Motor unit discharge during muscular after-contraction

Discharges of single motor units (MUs) in human triceps brachii and deltoid muscle were recorded using needle electromyography during after-contraction and voluntary contraction performed either against a small elastic load or under isometry. The steady-state firing rate of the MUs was lower under after-contraction than during voluntary movement of comparable amplitude and time course (or isometric force level), whereas variability of interspike intervals was similar under the two conditions. In the tibialis anterior muscle (where after-contraction was lacking), a weak voluntary contraction preceded by sustained strong voluntary effort also showed lower firing rate of MUs as compared to similar voluntary movement performed after a rest period. We concluded that sustained contraction gave rise to peripheral potentiation of contractile properties of the muscle, irrespective of whether it was proximal or distal, whereas after-contraction was due to a central tonic drive that differed for proximal and distal muscles.

Effects of Manipulations with Visual Feedback on Postural Responses in Humans Maintaining an Upright Stance

We studied postural reactions evoked by vibrational stimulation of the anterior tibial and posterior neck muscles under three different conditions of visual control (in a darkened room): (i) upon standing with the eyes open, EO, with perception of a stationary 2D image of the visual environment on the screen, (ii) under conditions of perception of a 3D virtual visual environment, VVE, and (iii) upon standing with the eyes closed, EC. Vibrational stimulation of both muscle groups evoked forward inclinations of the body; average values of the latter under control conditions (EC) were close to each other. The VVE mimicking a real visual environment possessed two planes, a mobile foreground one, whose shifts were programmed in such a manner that they correlated with oscillations of the body, and a stable background one. The tested subjects were asked to use the latter as a visual reference. Under VVE conditions, the amplitude of postural reactions depended on the feedback coefficient between the body movements and shifts of the VVE foreground and the direction of this feedback (its synphase or antiphase, sph or aph, mode). Postural responses at the feedback sph direction became greater with increase in the feedback coefficient (i.e., with increases in the magnitude of shifts of the VVE foreground) and reached values typical of standing under EC conditions. In the case of the aph type of feedback, the responses changed insignificantly. If the lowest feedback coefficient, 1.0, was used, the postural responses tended to decrease, as compared with those under EO conditions. The difference between the values observed at the sph and aph types of feedback with similar coefficients was manifested more intensely in the case of stimulation of the neck muscles. This fact shows that postural reactions triggered by afferent signals from the neck muscles depend more considerably on the ongoing visual afferentation.

Postural Responses Evoked by Vibrational Stimulation of the Shin Muscles under Conditions of Virtual Visual Environment

We studied the effects of unexpected shifts of the visually perceived artificial surroundings (virtual visual environment, VVE) on postural reactions evoked by vibrational stimulation of proprioceptors of the shin muscles; tests were performed in a standing position of the subject. The VVE possessed two planes, a mobile foreground, whose displacements correlated with oscillations of the body, and a stationary background. The subjects were asked to use the latter as a reference system in corrections of the posture. The VVE parameters were controlled by a computer; shifts of the VVE foreground were combined with similar, in their duration and profile, stimulation-evoked displacements of the body. Despite the fact that the subjects had a possibility to use the stationary background as the reference system, the magnitudes of the evoked postural responses under conditions of perception of the VVE significantly exceeded the respective magnitudes upon standing with the eyes open in front of a completely stationary visual image. Postural responses progressively increased with increases in relative values of the shifts of the VVE foreground but always remained smaller than the responses under conditions of testing with the eyes closed. Augmentation of the postural responses at a synphase pattern of interrelations between the body movements and VVE shifts was more significant than at antiphase relations. Thus, shifts of the VVE foreground, on the one hand, destabilized the maintenance of the vertical posture, which resulted in intensification of the postural responses. On the other hand, such shifts allowed the subject to use them as feedback signals and to modulate the magnitude of postural responses when there was a change in the direction of interrelations between the body movements and the perceived visual image.

Spectral analysis of the human body sway during standing on firm and compliant surfaces under different visual conditions

Effects of different visual conditions on the vertical posture maintenance were compared in subjects standing on a firm or compliant surface. These visual conditions included a motionless visual environment (MVE), eyes-closed condition (EC), and a virtual visual environment (VVE). The VVE consisted of two planes: the foreground and background. The foreground displayed a room window with adjacent walls, and the background was represented by an aqueduct with the adjacent landscape. The VVE was destabilized by inducing either the cophased or the antiphased relation between the foreground of the visual scene and the body sway. We evaluated changes in the amplitude spectra of two elementary variables calculated from the trajectories of the plantar center of pressure (CoP) displacements in the anteroposterior and lateral directions, namely, the trajectories for the center of gravity projections on the support (the CG variable) and the differences between the CoP and CG trajectories (the CoP–CG variable).The CG trajectory was considered as a controlled variable, and the difference between the CoP and CG trajectories were considered as a variable related to the body acceleration and reflecting changes in the resultant stiffness in ankle joints. The rootmean-square (RMS) values for the spectra of both variables calculated from the body sway in the anteroposterior direction in standing on a firm support decreased proportionately with antiphased relation between the foreground and the body sway and increased with the cophased relation, compared with the RMS calculated for the MVE conditions. RMS for the spectra of the CG variable in the cophased relation were nearly the same, as in standing with eyes closed (EC), while the RMS for the spectra of the CoP–CG variable were significantly less than with EC. The body sway during standing on a compliant support significantly increased in both the anteroposterior and the lateral directions under all visual conditions. RMS for the spectra of both variables with EC increased considerably higher than in the cophased relation. Furthermore, the RMS for the spectra of the CG variable calculated from the body sway in the lateral direction on a compliant support was substantially higher in the antiphased relation than in the cophased relation, whereas the RMS for the spectra of the CoP–CG variable under both conditions had similar values. The analysis of body sway and the results under some visual conditions have shown that the amplitude characteristics of the CG and CoP–CG variables changed not always proportionately with the passage from standing on a firm support to a compliant support. It is suggested that the found disproportion of changes in these two variables is probably associated with the contribution of another additional factor to the process of postural control, the passive elastic component of musculo-articular stiffness generated by fascial-tendon tissues.

Influence of a light tactile contact on vertical posture maintenance under the conditions of destabilization of visual environment

The influence of a light contact between index finger and a stationary external surface on the maintenance of upright posture in healthy subjects “immersed” in unstable virtual visual environment has been studied. Under these conditions, the subjects saw a screen with a visual scene consisting of a foreground and a background. In the foreground, there was a window of a room with the adjacent walls; in the background, there was an aqueduct with the adjacent terrain. The virtual visual environment was destabilized by setting inphase or antiphase couplings between the foreground and body oscillations. The analysis of upright posture maintenance was focused on the assessment of amplitude–frequency characteristics of two elementary variables calculated from the trajectories of the center of pressure of feet (CoP) in mediolateral and anteroposterior directions: the trajectory of vertical projection of the center of gravity (the CG variable) and the differences between the CoP and CG trajectories (the CoP-CG variable). Both in case of normal posture and the posture with a fingertip contact, the root mean square (RMS) values of the spectra of both variables were the lowest in motionless visual environment with antiphase coupling between the foreground and the body oscillations and the highest with inphase coupling and with eyes closed. In the cases with fingertip contact, the intensity of body oscillations in both directions was considerably lower; the influence of different visual conditions on RMS values of the spectra of both variables decreased. This effect was more significant for the CG variable. The frequency of body oscillations decreased as well. We observed the effect of tactile contact on the frequency of the spectra of both variables. The median frequencies of the spectra of the CoP-CG variable calculated from body oscillations in the anteroposterior and mediolateral directions increased under the conditions of tactile contact. On the contrary, the median frequencies of the spectra of the CG variable increased only for body oscillations in the mediolateral direction. Our results show that a light tactile contact (providing no mechanical support) significantly improves vertical posture maintenance, inter alia, under the conditions of destabilization of virtual visual environment. This improvement is provided by multidirectional and independent effects on the amplitude–frequency characteristics of elementary variables (CG and CoP-CG).

Human upright posture control in a virtual visual environment

The sagittal and frontal components of the stabilogram were monitored in 14 healthy subjects standing on a rigid or pliant support under three different conditions of visual control: with the eyes opened (EO), with the eyes closed (EC), or in a virtual visual environment (VVE). Under the VVE conditions, the subjects looked at a three-dimensional image of elements of a room (a 3-D artificial room) that was generated by a computer and locked to the fluctuations of the body center of gravity (CG) so that the visual connection between body sway and shifts of the visual environment typical of normal visual conditions was reproduced. Frequency filtration of the fluctuations of the foot’s center of pressure (FCP) was used to isolate the movements of the vertical projection of the CG and determine the difference between these two variables. The changes in the variables (CG and FCP-CG) were estimated using spectral analysis followed by the calculation of the root mean square (RMS) amplitudes of their spectral fluctuations. In subjects standing on a rigid support, the RMS amplitudes of the spectra of both variables were the highest under the VVE and EC conditions and the lowest under the EO conditions. In subjects standing on a pliant support, body sway was considerably enhanced, which was accompanied by a different pattern of visual influences. The RMS values were the highest under the EC conditions and were lower by a factor of 2–2.5 under the EO and VVE conditions. Thus, it has been demonstrated that the cerebral structures controlling posture ignore the afferent input from the eyes under VVE conditions, if the subject is standing on a rigid support and the CG fluctuations are relatively small; however, this afferentation is efficiently used for maintaining the posture on a pliable support, when the body sway is substantially enhanced.