Introduction

Upright bipedal stance in humans may appear to be primarily an issue of musculoskeletal control; however, information from multiple sensory sources is crucial for normal balance function. Deficits in sensory integration processes are often suspected as an underlying source of balance disorders in individuals who have sustained brain changes due to disease, trauma, or aging. Rather than merely enhancing the resolution of a percept, sensory information from multiple modalities is essential to resolve ambiguities inherent to controlling the upright, multilinked, unstable human body. The focus of this chapter is a particularly important aspect of sensory integration for postural control: multisensory reweighting. Balance maintenance involves constant updating and prioritization of the sensory information generated by the surrounding environment (e.g., visual flow) and by self-motion (e.g., vestibular, proprioceptive). This updating and prioritizing process, which we refer to as multisensory reweighting, remains poorly understood. The first part of the discussion, then, reviews recently developed techniques that allow precise quantification of multisensory reweighting in the context of human postural control.

Postural control is achieved through a complex process involving the reception and integration of sensory inputs, motor planning, and muscular execution. To prevent balance loss, sensory systems must determine and monitor the position of the center of mass (COM) in relation to the body's base of support so that motor systems can control the COM and prevent it from exceeding its limits of stability. Correct estimation of COM position is reliant on multisensory integration, for two reasons. First, the COM is a calculated point in space that cannot be directly detected by any single sense. COM position must be derived from information gathered from three separate sensory modalities, the visual, vestibular, and somatosensory systems. Second, each of these individual sensory modalities provides information that is often ambiguous; information from other senses is required to resolve these ambiguities. Despite the critical dependence of postural control on multisensory integration for COM estimation, the mechanisms underlying multisensory integration for postural control are essentially unknown.

The three primary peripheral sensory inputs contributing to postural control are the bilateral receptors of the somatosensory, visual, and vestibular systems. Each of the three sensory systems provides both unique and redundant information for postural control. Somatosensory receptors located in the joints, ligaments, muscles, and skin provide information about muscle length, stretch, tension, and contraction; pain, temperature, and pressure; and joint position. Central (or focal) vision allows environmental orientation, contributing to the perception of verticality and object motion. Peripheral (or ambient) vision detects the motion of the self in relation to the environment, including head movements and postural sway. Central visual inputs tend to receive more conscious recognition, but both are normally used for postural control. The vestibular system provides information about the motion of the head and its position in relation to gravity and other inertial forces.

Yet ambiguities exist within each sense (intrasensory conflict). Figure 49.1 illustrates inherent ambiguities associated with sensory systems that are important for postural control. The somatosensory system alone cannot distinguish between a change in surface tilt and changes in body inclination. The visual system alone cannot necessarily discriminate motion in the environment from self-motion. The vestibular system alone cannot determine whether head motion signaled by the semicircular canals is caused by flexion at the neck or flexion at the hips, or whether head motion signaled by the otoliths is due to head tilt or to linear acceleration or deceleration.

Figure 49.1.  

Each sensory system alone provides ambiguous information about body dynamics. (A) The vestibular system provides similar information when the head is tilted and when the trunk tilts at the waist. Somatosensory information from the neck or trunk is needed to resolve the ambiguity. (B) The visual system alone cannot resolve visual field motion generated from extrinsic movements in the surround versus intrinsic self-motion. Somatosensory and vestibular information about self-movement may resolve this ambiguity. (C) A change in ankle inclination due to forward body sway cannot be distinguished from the same ankle angle produced by support surface tilt without visual flow or vestibular information.

Multisensory integration permits resolution of these ambiguities by using information received simultaneously through other senses that may or may not be consistent with the information gained from a single modality. For example, somatosensation from neck and trunk muscles can resolve the problem of whether head motion is caused by neck versus hip flexion, and vision can resolve the problem of whether head motion is caused by head tilt versus linear acceleration or deceleration. Typically, body movements in space produce consistent information from all three senses. When one leans forward, the sensations experienced include pressure on the toes and stretch of the calf muscles (somatosensation), posterior peripheral visual flow and retinal image enlargement (vision), and forward head acceleration/deceleration. These sensations are collectively perceived as a forward body lean. Thus, accurate estimates of the COM are based on information from all three sensory systems involved in postural control.

Sensory ambiguities can also arise when information between systems is not synchronous (intersensory conflict). Multisensory integration must then function to resolve the ambiguity by recognizing any discrepancies and selecting the most useful inputs on which to base motor commands. For example, a driver stopped still at a red light suddenly hits the brake when an adjacent vehicle begins to roll: movement of the other car detected by the peripheral visual system is momentarily misperceived as self-motion. In this situation, the vestibular and somatosensory systems do not detect motion, but the forward visual flow is interpreted as backward motion. Because the brain failed to suppress the (mismatched) visual inputs, the braking response was generated. To resolve conflicts between two inconsistent sensory inputs, information from a third source may be necessary.