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 ~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.