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

Abstract

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).