Compatibility of postural behavior induced by two aspects of visual feedback: time delay and scale display.

Fourteen healthy adults were tested to assess the potential influence on stance maintenance of two parts of the visual feedback technique (display scale and time delay). The task consisted in their keeping a spot on the screen representing their center of pressure, CoP (i.e. successive points of application of the ground reaction forces detected by the force platform on which they were standing) to a minimum size. The analysis focused on elementary motions computed from the complex CoP trajectories, that is the horizontal motion of the center of gravity (CoG(h)) and the difference between the CoP and the vertical projection of the center of gravity (CoP-CoG(v)). The former is recognized as the main variable in postural control, and several interesting features can be extracted from the latter. The results indicate that setting a delay and increasing the display scale induce substantial reductions in CoP-CoG(v) and CoG(h) displacements, respectively. Interestingly, when the two effects are combined, these single effects cohabit quite happily. Fractional Brownian motion modeling of these trajectories revealed clearly that, in each case, these effects originate principally from poor or improved control, respectively. This feature confirms that these elementary motions are involved differently in the postural system and that study of the complex CoP might not be of great interest. By generating opposing but complementary trends, the visual feedback technique should thus be perceived as a promising tool for inducing particular postural behavior in healthy and disabled individuals.