[Spectral Analysis of Sway of Human Body When Standing on Firm and Compliant Surfaces under Different Visual Conditions.]

The influence of different visual conditions on maintenance of vertical posture when standing on a firm and a compliant surface were compared. Motionless visual environment (MVE), closed eyes (CE) and virtual visual environment (VVE) were used as visual conditions. Displayed VVE included foreground and background. The foreground represented a room window with the walls adjoining to it, and the background - an aqueduct with the adjacent terrain. Destabilization of the VVE was produced by assignmentinphase or anti-phase relationship between the position of the foreground of visual scene and the body sway. We estimated changes in the amplitude spectra of two elementary variables calculated from trajectories of displacement of the center of pressure (CoP) in the anteroposterior and lateral directions, namely, trajectories of the center of gravity (CG variable) and differences between trajectories of CoP and CG (CoP-CG variable). The trajectory of CG was considered as a controlled variable, and a difference between CoP and CG trajectories - as the variable connected with acceleration of body and related to changes of resultant stiffness in ankle joints. In the condition of inphaserelation RMS of spectra of the CG variable were about the same, as when standing with CE, and RMS of spectra of the CoP-CG variable were significantly less, than at CE. When standing on acompliantsurface, body sway was increased significantly both in anteroposterior and lateral directions under all visual conditions. Moreover, during standing with CE RMS ofspectra of both variables were increased significantly more, than during standing with in phaserelation between the foreground of visual scene and the body sway. Furthermore, when standing on a compliant support RMS of spectra of CG variable, calculated from the body sway in the lateral direction, under the condition of antiphase relation were significantly greater than under the condition of inphaserelation, while RMS of spectra of CoP-CG variable had similar values underboth conditions. The results of the analysis of body sway showed that under somevisual conditions the transition from standing on a firm support to the standing on acompliant support have not causeda propor- tional change of amplitude characteristics of CG and CoP-CG variables. It is supposed that the revealed disproportion of changes of these two variables is probably connected with a contribution to posturalcontrol of one additional factor: the passive elastic component of muscular-articular stiffness generated by fascial-tendon tissue.

[The Influence of Light Tactile Contact on the Maintenance of Vertical Posture under the Conditions of Destabilization of the Visual Environment].

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

[Errors in pointing under various body orientation relative to the gravity vector]. / Oshibki tselevykh dvizhenii ruki pri raznoi orientatsii tela otnositel'no gravitatsionnoi vertikali.

Errors in pointing to remembered target locations were measured in healthy subjects in upright, supine and prone positions, i.e. in three positions differing in body and arm movement orientation relative to the gravity vector. The X-error was unaffected by the body position, whereas the Y-error was dependent on subject's orientation relative to the gravity vector. In two lying positions, the subjects pointed below and in the vertical posture above the remembered target locations. On the contrary, the variable error was differentially affected by orientation of the movement trajectory relative to the gravity vector. In the supine position, the variable error was larger and in the prone position, smaller than in the upright posture. The gravity bias in pointing errors suggests that the motor program generated in the lying position is not modified to adapt to altered orientation relative to the gravity vector.

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.

Effect of body orientation with respect to gravity on directional accuracy of human pointing movements.

Errors in pointing to remembered target locations were measured in normal subjects in the upright posture and in a supine or prone lying position, three conditions that differed in the orientation of the moving arm with respect to gravity. Vertical errors, or, more strictly, errors along the body axis, were significantly larger in both supine and prone lying positions as compared with the vertical posture. Subjects' orientation with respect to gravity produced a consistent error pattern, i.e. subjects pointed below the remembered target locations in the vertical posture and above them under the two lying conditions, in the body-related co-ordinates. On the contrary, variable error (the confidence ellipse) was differentially affected by the orientation of the movement trajectory with respect to gravity. In the supine body position the variable error was larger, and in the prone position it was smaller than in the upright posture. The finding of gravity-related bias in pointing errors suggests that the motor programme generated in the lying position is not modified to adapt to an altered orientation with respect to gravity.

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.

[Visual illusions during the electrical stimulation of the labyrinth]. / Zritel'nye illiuzii pri élektricheskoi stimuliatsii labirinta.

Transcutaneous electrical stimulation of the labyrinth induced apparent motion of a stationary light source in darkness in normal subjects. This effect is similar to the oculogyric illusion induced by rotatory vestibular stimulation. Monaural anodal stimulation of the right labyrinth evoked apparent movement of the target to the left, whereas cathodal stimulation induced opposite illusion. The threshold current was 0.35-0.6 mA. Binaural bipolar stimuli induced illusory target motion directed to the side of the cathode, the threshold decreased 1.5-2.5 times. Binaural monopolar stimulation induced vertical apparent displacement of a target, the threshold being 1.4-3.0 mA. The amplitude and velocity of illusory target motion increased with current. The subject eyes began to move with much higher currents than those necessary for illusory sensation. It is therefore suggested that the visual illusion is related not to vestibulo-visual interaction but to vestibular effects on the spatial perception system.

[The dependence of the changes in vestibular postural reactions on the information content of visual feedback]. / Zavisimost' izmenenii vestibuliarnykh poznykh reaktsii ot informatsionnogo soderzhaniia zritel'noi obratnoi sviazi.

Electrical stimulation of the labyrinth in standing subjects induced the body sway predominantly in the frontal plane. Stabilographic response included both early (latency 120-200 ms) and late (200-500 ms) components. Their magnitudes depended on the visual control condition. Maximal responses were recorded in the eyes-closed condition. The response decreased when subjects maintained erect posture with their eyes closed, when fixing a stable visual target, and when tracking the frontal stabilogram displayed on an oscilloscope screen (visual feedback). In all the conditions the early component decreased by 10-20 percent whereas the late one decreased by 50-70 percent. Visual fixation of a small light stationary relative to the head did not influence the response. On the other hand, information on the direction of the expected body sway given in the visual fixation condition resulted in a considerable and approximately equal decrease of the two components (by 70-80 percent). It is concluded that the early and late components of the vestibulo-motor response are mediated via different mechanisms with specific temporal and functional characteristics.

[Changes in the direction of vestibulomotor responses in the process of adaptation to prolonged static head turning in man]. / Izmeneniia napravleniia vestibulomotornykh reaktsii v protsesse privykaniia k dlitel'nomu staticheskomu povorotu golovy u cheloveka.

Vestibulomotor responses were studied in normal blindfold subjects in the upright posture during adaptation to maintaining the head turned to the left up to the extreme position for 10 min. The head was maintained in that position either passively or actively. In 5 out of 12 subjects tested, adaptation to the unusual head position resulted in a gradual decrease of the appreciated angle of the head position. The error in appreciating the head position reached as much as 70-80 degrees. The direction of the vestibulomotor response was found to change in parallel with the appreciated head orientation. Thus, under mismatch between the perceived and actual head positions the direction of the vestibulomotor response corresponded to the spatial perception rather than to the actual head orientation.

[Effect of real and illusory movements on the human vestibulomotor reaction]. / Vliianie real'nykh i illiuzornykh dvizhenii na vestibulomotornuiu reaktsiiu u cheloveka.

Lateral stabilographic responses to galvanic labyrinth stimulation were studied in healthy subjects in the standing posture. The responses increased during the body tilt forward performed either voluntarily or involuntarily as a result of vibration of the tibialis anterior muscles. The illusory body tilt forward induced by the vibration of the triceps surae muscles under mechanical fixation of the trunk resulted in similar increase of the vestibulomotor response. It is concluded that the enhancement of the vestibulomotor responses during voluntary movements is mediated by the spatial perception system.

[Various characteristics of the control of cyclic movements]. / Nekotorye osobennosti upravleniia tsiklicheskimi dvizheniiami.

It was shown by studying control forces and phasic trajectories during oscillation of human forearm, locomotion and rocking of the body on the support that there was an image of accomplished movement in the central nervous system. This image seems to be realized by linear connected displacements of the muscle tension level and threshold of tonic stretch reflex. During the control process, velocity of the threshold and tension level is similarly transformed to that of the body part. The piece constant similarity coefficient is regulated centrally. The main result of such control is moving of the body along energy optimal trajectories.

[Spatial perception and vestibulomotor reactions in man]. / Prostranstvennoe vospriiatie i vestibulomotornye reaktsii u cheloveka.

Postural responses to transcutaneous galvanic stimulation of the right labyrinth were recorded by means of a stabilograph in normal human subjects in various head positions as well as under the illusion of head and trunk rotation induced by vibration of the gluteus maximum muscle. The direction of the vestibulo-motor response was determined by the position of the head: in a normal head position the body swayed in the frontal plane, whereas with the head turned 90 degrees it moved in the sagittal plane. During the illusory head and trunk 90 degrees rotation the responses were sagittally directed like those recorded in the real head-turned-sideways position. When the vibration did not induce the illusion of the head rotation with respect to the feet, the direction of the postural response was determined by the real head orientation. It is concluded that the spatial perception system is involved in the control of spatially oriented vestibulo-motor responses.