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.