Influence of enhanced visual feedback on postural control and spinal reflex modulation during stance.

The present study assessed the influence of visual feedback on stance stability and soleus H-reflex excitability. The centre of pressure (COP) displacement was measured in upright stance on a rigid surface (stable surface) and on a spinning top (unstable surface) while subjects either received "normal" visual feedback (without laser pointer = WLP) or pointed with a laser pointer on a target on the wall (LP). In order to verify that laser pointing influenced visual feedback, two additional experiments were conducted: (1) Subjects performed a finger reaction task which was thought to increase attention and cognitive demands without alteration of the visual feedback. (2) The effect of laser pointing on the wall was compared with pointing at a board, which was attached to the subjects themselves. In this case, the laser point could not serve as a reference for sway because the board moved in synchrony with the body. On stable and unstable surface, COP displacement was reduced in the LP compared to the WLP task (-17 cm +/- 6, P < 0.05; -14 cm +/- 6, P < 0.05). Conversely, H-reflexes were greater in the LP condition (stable: +20 microV +/- 30, not significant; unstable +115 microV +/- 40, P < 0.05). Stance stability and H-reflex modulation were negatively correlated (R(2) = -0.5; P < 0.001). The finger reaction task did neither influence COP displacement nor H-reflexes. Pointing at the body-fixed target did not alter COP displacement. These findings suggest that postural sway can be reduced by a handheld laser pointer targeting on an external reference point. It is argued that altered visual input was responsible for modulating the H-reflex.