Postural sway adaptation induced by optic flow

Optic flow is crucial for the maintenance of quiet stance in the upright position. However, little is known about the correlation between stimulus type and postural response. Aim of this study was to investigate the optic flow effect on the distribution of the prevalent direction of postural sway. Stabilometric data were recorded in 24 right-handed young volunteers who stood with each foot on a Kirstel force platform, facing a wide screen. Optic flow stimuli were full screen expansion and contraction, while random dot motion was used as control. For each stimulus we computed the maximum variance of sway direction (the prevalent oscillation direction) and the mean vector. The vector of the prevalent direction of postural sway was 127.7° in expansion, 223.7° in contraction and 211.4° for random stimulus. All stimuli showed a uniform distribution at the Rayleigh test, but we did not find a correlation between the stimuli, meaning that each stimulus evokes a proper postural sway. Contrary to males, females showed a prevalent forward sway during contraction. A possible explanation for this might be that a different muscular activation could cause a different posture during the stimulus presentation. It is possible that the activation of either plantaflexors or plantaextensors during the trial caused a continuous oscillation in anteroposterior direction. That would require the generation of a stronger vertical force to keep postural stability and to avoid backward fall.