Effect of sway-referenced visual and somatosensory inputs on human head movement and postural patterns during stance.

The purposes of this study were to (1) evaluate the effects of sway-referenced visual and ankle somatosensory inputs on head movement and body sway during stance and (2) determine how sway-referenced sensory inputs might influence the selection of postural patterns. Seventeen healthy adult subjects were evaluated while attempting to maintain standing balance. Ankle somatosensory inputs were altered during stance by creating support surface rotations which were continuously adjusted ("referenced") to the center of force. In selected trials, ankle somatosensory or visual inputs were distorted further by increasing the platform/body sway gain or by increasing the sway-referenced gain of a rotating visual enclosure. The results showed that spontaneous body sway in a given direction was generally associated with head displacement in the opposite direction. The slope of the sway vs. sagittal head position linear regression line was statistically greater at a sway-referenced gain of 1.5 compared to a gain of 1.0. This finding was observed specifically when mechanical compliance of the support surface was increased and occurred whether the eyes were open or closed. Horizontal shear force was generated in the same direction as head displacement and the linear regression slope was greatest with eyes open and at a sway gain of 1.5. These findings indicate that (1) destabilization of the ankles altered displacement of the head independently from visual input, and (2) visual information was necessary to produce specific postural patterns utilizing horizontal shear forces to maintain stance on a mechanically compliant surface. The results provide evidence for a neural mechanism linking ankle somatosensory inputs and head control.