Influence of visuoproprioceptive mismatch on postural adjustments

Abstract

The influence of visual input upon body stabilization was studied with subjects standing on a treadmill while a visual flow pattern was displayed. Both the treadmill and image were moved sinusoidally - backward/forward — at a frequency of 0.25 Hz. The effect of the presentation of the visual pattern was studied upon leg muscle electromyographic (EMG) activity and corresponding biomechanical signals for various phase shifts (45–360 degrees) between movements of the legs and the visual flow pattern. Around the posterior turning point of sinusoidal treadmill movement, i.e. at times when there is an anticipatory forward positioning of the body, a modulation of the tibialis anterior EMG was observed. The onset, duration, and amplitude of the latter were dependent upon the phase shift between the movements of the legs and the visual pattern. Maximum responses were recorded at phase shifts of 90270 degrees and minimum responses at phase shifts of 180360 degrees. Therefore a coincidence of fast velocities of the visual flow pattern with a phase of maximal body acceleration evoked the strongest tibialis anterior EMG responses. At times around the anterior turning point a modulation only of EMG amplitude occurred in the gastrocnemius muscle. This activity is believed to be linked to the preceding tibialis anterior EMG resulting in ‘resetting’ of the neutral body position. It is suggested that the tibialis anterior is more sensitive than the gastrocnemius muscle to a visual stimulus. The activity of the latter muscle with its antigravity function is more subject to proprioceptive input. Little adaptational changes in EMG pattern occurred between the second and fifth sway cycles after presentation of the optical flow pattern. It is assumed that task-specific factors act against early adaptation.