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 one's body.     

Postural reactions of humans in upright stance: Dependence on different visual conditions and the level of stiffness in the ankle joints

We checked on the supposition that the magnitude of postural reactions to an unexpected postural disturbance in upright stance in humans can be determined to a considerable extent by the level of background stiffness in the ankle joints. For this purpose, we estimated changes in the joint stiffness under different conditions of visual control; these values were estimated within the period of background body oscillations (i.e., before the beginning of a compensatory motor reaction) and compared with those in the course of postural reactions evoked by vibrational stimulation of the ankle (shin) muscles. Experiments were carried where the subjects stood with open and closed eyes (OE and CE, respectively) and while standing wearing spectacles with frosted glass passing only diffuse light (DL). In the course of the tests, the subjects stood in the usual comfortable vertical position (hereafter, standard stance) or in the same position but with the possibility to lightly touch an immobile object by a finger (stance with additional support). Such technique was used to weaken the effects of CE and DL on background sways of the body and to lead these sways close to the level typical of OE conditions. The joint stiffness was estimated using an approach based on frequency filtration of oscillations of the center of pressure of the feet (CPF) that allowed us to select signals proportional to displacements of the total center of gravity (CG) of the body and to calculate the difference between oscillations of the CPF and CG (a CPF-CG variable). The CPF-CG variable is proportional to the horizontal acceleration of the CG and, therefore, can be used for estimation of the changes in stiffness in the ankle joints. Under conditions of standard stance, the usual conditions rather similarly influenced both variables (CG and CPF-CG) in the course of both background body oscillations and a postural response. The examined variables were the greatest under CE conditions, decreased under conditions of perception of DL, and became smallest with OE. At standing with additional support, the dependence of the examined variables on visual conditions disappeared within the period of background body oscillations (before the beginning of postural reactions). In this case, the magnitude of oscillations of the CPF-CG variable under CE and DL conditions decreased to the level observed at standing under OE conditions. The magnitude of CG displacements induced by vibrational stimulations of the muscles remained, nevertheless, clearly dependent on visual conditions (the same regularities were observed as in the case of standing with no additional support). Thus, our findings demonstrate that the correlation between the characteristics of postural reactions in the upright stance and the level of ankle joint stiffness is not single-valued. Neirofiziologiya/Neurophysiology, Vol. 39, No. 2, pp. 146–153, March–April, 2007.     

Dependence of the Magnitude of Postural Reactions in Humans on the Strength of Unilateral Galvanic Anodal and Cathodal Vestibular Stimulations

In 13 healthy volunteers, we recorded stabilographic postural reactions (side inclinations of the body) to unilateral galvanic vestibular stimulation (GVS) by rectangular current pulses (4 sec long, 2, 3, 4, 5, or 6 mA). For the cathodal GVS, the dependence of the magnitude of reaction was linear within this range. The corresponding dependence for the anodal GVS was close to linear at small currents, but the increment of the magnitude became smaller with further increase in the stimulation intensity, and a plateau was formed. A significant divergence between the two curves was observed with stimulation currents 4 mA and higher. This difference can be explained considering modern concepts on the mechanism of GVS-induced effects (an increase or a decrease in the level of tonic impulsation in fibers of the vestibular nerve under the influence of polarization). Anodal GVS continues to suppress tonic activity up to the moment where all GVS-sensitive vestibular afferents stop to generate impulses; a further increase in the intensity of hyperpolarizing current is not accompanied by a decrease in the activity in the vestibular nerve and, consequently, by an increase in the magnitude of postural reactions. The tested approach can be used for qualitative estimation of the vestibular tone in humans. Neirofiziologiya/Neurophysiology, Vol. 37, No. 2, pp. 169–176, March–April, 2005.     

Maintenance of upright stance in humans in normal and virtual visual environments

We recorded in 16 healthy subjects the sagittal and frontal components of the stabilogram when standing on a rigid motionless or movable (oscillating) platform under four conditions of visual control: (i) open eyes, OE; (ii) closed eyes, CE; (iii) central vision, CV, and (iv) virtual visual environment, VVE. Under the latter condition, subjects observed the 3D image of a room, which was generated by the computer; the image was adhered to head movements in such a manner that a peculiar connection for normal visual conditions between movements of the head and displacements of the visible visual environment was reproduced. Through a low-pass filtration of a trajectory of the center of pressure of feet (CPF), two elementary variables were received, horizontal motions of the center of gravity (CG) and the difference between the CPF and the CG (CPF-CG). Changes in these variables (CG and CPF-CG) were estimated using spectral analysis and subsequent calculation of the median frequency (MF) and root mean square value (RMS) of the spectra. The MFs of the spectra of the investigated variables were approximately identical under conditions of standing on oscillating and motionless supports and showed no clear dependence on various visual conditions. Unlike MFs, the RMSs of the spectra of body sways appeared more dependent on changes of conditions of standing and the mode of visual control (differing from each other in the higher sensitivity to modifications of conditions of standing and visual control). With standing on the motionless support, the RMSs of the spectra of both variables were the greatest under VVE and CE conditions and the smallest under OE condition. The body oscillations were considerably amplified under conditions of standing on a movable support, and a different pattern of visual influences on the RMS of both investigated variables was revealed. The RMSs had the greatest value under CE condition and were much smaller (50–40%) under other visual conditions, including the VVE condition. Therefore, our findings show that, under VVE conditions, visual sensory afferentation is ignored by cerebral structures controlling postural adjustments if standing occurs on a motionless support but is effectively used at the maintenance of upright stance on an oscillating support. Neirofiziologiya/Neurophysiology, Vol. 39, No. 6, pp. 476–485, November–December, 2007.     

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. Neirofiziologiya/Neurophysiology, Vol. 38, No. 2, pp. 157–166, March–April, 2006.     

Adaptational Modifications of the Vestibular Postural Response in Humans under Conditions of Horizontal Visual Reversal

In healthy volunteers, we recorded stabilograms and studied postural responses evoked by galvanic stimulation of the labyrinth (binaurally applied 1-mA current, 4 sec) with the subjects' eyes open and closed and under conditions of reversed visual perception. Horizontal reversal of the visual space was provided by using spectacles with the Dove's prisms. In series consisting of 10 sequential tests with eyes open, we observed a gradual drop in the response amplitude, while there were practically no changes in the maximum velocity of the displacement. Postural responses with eyes closed were considerably greater than those with eyes open, but their amplitude and velocity demonstrated no changes with sequential tests. Under conditions of reversal of the visual perception, both the amplitude and maximum velocity of the postural responses decreased with successive testing. Under the above conditions, at the beginning of a test series responses to vestibular stimulation were greater than those with eyes closed, but in repeated tests they decreased and attained the same magnitude as in the tests with eyes closed. Therefore, the effect of short-term adaptation to visual reversal on the system controlling vertical posture resulted in simple rejection of the information coming via the visual input. In another experimental mode, we studied the adaptation effects at longer (3 h long) visual reversal. Postural responses to galvanic stimulation of the labyrinth (monaurally applied, 2-mA current, 4 sec) were tested with 1-h-long intervals; tests with visual reversal and with eyes closed were made in a random order with each other. A 3-h-long interval with the prismatic spectacles on did not modify the amplitude and velocity of the vestibular postural responses when the tests were made with the eyes closed. When the tests were performed with the eyes open, but in the inverting spectacles, postural responses significantly decreased (by about 50-60%) to the 2nd and 3rd h of the experiment. Such selective suppression of the vestibular input under conditions of visual reversal can be interpreted as a result of adaptational transformation of the visual-vestibular relation directed toward minimization of the visual-vestibular conflict.