Human standing posture is inherently unstable; even when one is standing quietly, small disturbances and corrections of posture occur continually. Control of human upright stance requires sensory input to perceive the distinction between movements of one's own body and movement of the environment. These inputs include (1) visual information from the eyes; (2) vestibular (inner ear) information about head orientation; and (3) somatosensory (touch) information about position of body segments and external surfaces, from receptors in the muscles and skin. Most postural control studies have focused on isolating a particular sensory input through experimental manipulation or pathology (e.g., patients without a functioning inner ear), to determine how an individual sensory input influences postural control. However, a single input rarely works in isolation and there is a need to understand how multiple inputs interact. The present study analyzes human postural control while simultaneously manipulating two sensory inputs, vision and touch. Previous studies have shown that vision can influence postural sway: A person standing quietly in front of a moving visual pattern sways at the frequency of the visual display. More recently, experiments in our lab have shown that the sense of touch can similarly influence sway. Postural stability was enhanced when people lightly touched a rigid surface with a single fingertip; and, if the touched surface was moving, postural sway assumed the frequency of the moving surface. The intent of the present study is to provide visual and touch information simultaneously, to determine how these two sensory inputs interact in the control of postural sway. Individuals will stand in front of a visual display consisting of a random dot pattern. At the same time they will touch a rigid metal plate. The experiments will explore how postural sway is influenced when 1) visual cues are moving and touch cues are static, and vice versa; and 2) visual and touch cues are both moving. The question is whether postural responses are dominated by vision or touch, or whether instead a more complex relationship obtains. These studies should lead to a better theoretical understanding of how the central nervous system processes complex sensory information, and eventually to better rehabilitative techniques for patients who have lost sensory function due to neurological injury (e.g., stroke).
