THE ROLE OF HAPTIC CUES FROM ROUGH AND SLIPPERY SURFACES IN HUMAN POSTURAL CONTROL

Haptic information is critically important in complex sensory-motor ta sks such as manipulating objects. Its comparable importance in spatial orientation is only beginning to be recognized. We have shown that po stural sway in humans is significantly reduced by lightly touching a s table surface with a fingertip at contact force levels far below those physically necessary to stabilize the body. To investigate further th e functional relationship between contact forces at the hand and postu ral equilibrium, we had subjects stand in the tandem Romberg stance wh ile being allowed physically supportive (force contact) and non-physic ally supportive (touch contact) amounts of index fingertip force on su rfaces with different frictional characteristics. Mean sway amplitude (MSA) was reduced by over 50% with both touch and force contact of the fingertip, compared to standing without fingertip contact. No differe nces in MSA were observed when touching rough or slippery surfaces. Th e amplitude of EMG activity in the peroneal muscles and the timing rel ationships between fingertip forces, body sway and EMG activity sugges ted that with touch contact of the finger or with force contact on a s lippery surface, long-loop ''reflexes'' involving postural muscles wer e stabilizing sway. With force contact of the fingertip on a rough sur face, MSA reduction was achieved primarily through physical support of the body. This pattern of results indicates that light touch contact cues from the fingertip in conjunction with proprioceptive signals abo ut arm configuration are providing information about body sway that ca n be used to reduce MSA through postural muscle activation.