B. N. Smetanin

The interaction of visual and proprioceptive inputs in pointing to actual and remembered targets.

Errors in pointing to actual and remembered targets presented in three-dimensional (3D) space in a dark room were studied under various conditions of visual feedback. During their movements, subjects either had no vision of their arms or of the target, vision of the target but not of their arms, vision of a light-emitting diode (LED) on their moving index fingertip but not of the target, or vision of an LED on their moving index fingertip and of the target. Errors depended critically upon feedback condition. 3D errors were largest for movements to remembered targets without visual feedback, diminished with vision of the moving fingertip, and diminished further with vision of the target and vision of the finger and the target. Moreover, the different conditions differentially influenced the radial distance, azimuth, and elevation errors, indicating that subjects control motion along all three axes relatively independently. The pattern of errors suggest that the neural systems that mediate processing of actual versus remembered targets may have different capacities for integrating visual and proprioceptive information in order to program spatially directed arm movements.

Body Scheme in the Control of Postural Activity

The changes in the amplitude and direction of vestibulo-motor responses were studied during illusory body tilt and altered perception of the head position. The results obtained under these conditions were similar to those recorded during the real body movement and real head rotation. These results are discussed in the context of the involvement of the body scheme in the control of involuntary spatially oriented movements.

Spatial perception and vestibulomotor responses in man

A study was made on normal human subjects, using a stabilograph to investigate changes in posture produced in response to transcutaneous galvanic stimulation of the right labyrinth. Results were obtained for different head positions and under the illusion of head and trunk rotation produced by stimulating (vibrating) the gulteus maximus muscle. In the absence of illusion of movement, the direction of the vestibulomotor response was determined by the position of the head in relation to the feed: with the normal head position, the body swayed on a frontal plane, and on a sagittal plane when the heat turned through 90°. Vestibulomotor responses were sagittally oriented, as with real head turning, when illusory head and trunk turning through 90° was produced by vibration. When the illusion of head rotation (in relation to the feet) was not produced by this stimulus, the direction of the postural response was not produced by this stimulus, the direction of the postural response was determined by the real orientation of the head. It is concluded that the spatial perception system plays a major part in controlling spatially oriented vestibulomotor responses.

Effect of movement and illusion of movement on human vestibulomotor response

Lateral stabilographic response to galvanic labyrinth stimulation was investigated in healthy subjects in the standing position. Vestibulomotor response increased during forwards volitional body tilt as well as involuntary tilt occurring in response to stimulating (by vibration) the proprioceptors of the anterior tibial muscles. An illusion of the forward body tilt induced by stimulating (vibrating) the proprioceptors of the triceps surae muscles with the trunk fastened in a fixed position was accompanied by practically the same intensification of vestibulomotor response as during actual body movement. It was concluded that reinforcement of vestibulomotor response during volitional movements is brought about by the spatial perception system.

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.

Changes in the direction of vestibulomotor response in the course of adaptation to protracted static head turning in man

Vestibulomotor response during the course of adaptation to prolonged (10 min) static head turning to the furthest limit was investigated in healthy subjects standing upright with the eyes closed. The head was either actively or passively maintained in this position. The sensation of a decline in the angle of head turning was experienced during adaptation to the position by five of the 12 subjects tested. Error in appreciating this angle ranged up to 70–80°. Matching changes occurred in the direction of vestibulomotor response to electrical stimulation of the vestibular apparatus. When true and perceived head position conflict, direction of vestibulomotor response thus matches spatial perception rather than actual orientation of the head.

Azimuth errors in pointing to remembered targets under extreme head rotations

Errors in pointing to remembered target locations in 3-D space were studied when subjects were free to move their heads, and when they rotated their heads to the extreme right or left. Relative to pointing when the head was free to move, head rotations to the right shifted the final position of the responding arm to the left, whereas head rotations to the left shifted the final position of arm to the right. Horizontal rotation of the head had no systematic influence on elevation and radial distance errors. The influence of head rotations on pointing errors may be mediated by small shifts in the internal representation of external space, shifting the presentation of space in the opposite direction of the head rotation.

Postural responses to vibrostimulation of the neck muscle proprioceptors in man

Postural responses to vibrostimulation (50–100 Hz, 0.5 mm, 4–8 sec) of muscles of the back surface of the neck were studied in healthy subjects. In the sitting position, vibrostimulation evoked local displacements (backward head deflection), but global postural responses (forward inclination of the whole body) developed in the standing position. The amplitude of the evoked body inclination was dependent upon the site of the vibrostimuli application along the vertebral column. Asymmetrical application of vibrostimuli to the muscles of the right or left neck side was accompanied by development of a lateral component in the postural response. Changes in the spatial orientation of the head led to the changes in postural response direction: head turning to the right resulted in right-side body deviation during vibration, and vice versa. Illusions of head bend caused by habituation to its static turning were accompanied by precisely the same changes in the direction of body deviation. It is assumed that “neck-evoked” motor events are mediated via central mechanisms that are involved in perception of the head and body position in space.

Changes in vestibular postural response determined by information content of visual feedback

Electrical unipolar monoaural stimulation of the labyrinth led to body sway mainly on a frontal plane in normal human subjects in a standing position. Early and late stages of response with latencies of 120–200 and 200–500 msec respectively changing in size in accordance with conditions of visual control were distinguished in the stabilographic response. Maximum response was recorded when the eyes were closed. Response declined upon opening the eyes, fixing the gaze on a static target, and with visual feedback according to stabilograms. The early and late components declined by 10–20 and 50–70% respectively in all cases. Fixing the gaze, in darkness, on an illuminated light spot stationary in relation to the head had no effect on level of response. Once the expected direction of body sway had been imparted, a significant and almost identical decrease of 70–80% in both components took place with the gaze fixed, however. Early and late components of vestibulomotor response are thought to be mediated by regulatory mechasisms with differing time courses and functional connections.

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.