COUPLING OF FINGERTIP SOMATOSENSORY INFORMATION TO HEAD AND BODY SWAY

Light touch contact of a fingertip with a stationary surface can provi de orientation information that enhances control of upright stance. Sl ight changes in contact force at the fingertip provide sensory cues ab out the direction of body sway, allowing attenuation of sway. In the p resent study, we asked to which extent somatosensory cues are part of the postural control system, that is, which sensory signal supports th is coupling? We investigated postural control not only when the contac t surface was stationary, but also when it was moving rhythmically (fr om 0.1 to 0.5 Hz). In doing so, we brought somatosensory cues from the hand into conflict with other parts of the postural control system. O ur focus was the temporal relationship between body sway and the conta ct surface. Postural sway was highly coherent with contact surface mot ion. Head and body sway assumed the frequency of the moving contact su rface at all test frequencies. To account for these results, a simple model was formulated by approximating the postural control system as a second-order linear dynamical system. The influence of the touch stim ulus was captured as the difference between the velocity of the contac t surface and the velocity of body sway, multiplied by a coupling cons tant. Comparison of empirical results (relative phase, coherence, and gain) with model predictions supports the hypothesis of coupling betwe en body sway and touch cues through the velocity of the somatosensory stimulus at the fingertip. One subject, who perceived movement of the touch surface, demonstrated weaker coupling than other subjects, sugge sting that cognitive mechanisms introduce flexibility into the postura l control scheme.