Time course of haptic stabilization of posture.

Contact of the index finger with a stable surface greatly attenuates postural sway in blindfolded subjects. The time course of postural stabilization was measured after subjects made finger contact with a surface. Subjects (n=12) were tested standing in a heel-to-toe stance in 25 s duration trials. The subject stood with the index finger above but not contacting a laterally placed surface, and made finger contact when cued midway through the trial. Fingertip contact forces stabilized with a time constant of less than 0.5 s and postural stabilization occurred rapidly following fingertip contact. Sway amplitude of center of pressure of the feet decreased by half with a time constant of less than 1.6 s. The stereotypical pattern of force changes at the fingertip leading correlated changes in center of pressure by approximately 300 ms was evident within the first 0.5 s of finger contact. We conclude that the fingertip can serve as a sensory-motor probe that is stabilized nearly immediately on contact with a surface and that from the moment of contact the fingertip contributes sensory signals used to control sway.