Development of a Mechanistic Model of Multisensory Integration and Human Postural Control

Imagine situations in which you cannot see very well, such as when you enter a darkened room or get up from bed at night to get a drink of water. In these situations, one often seeks out nearby chairs or tables with the fingertips to guide us through the room. However, we have found that lightly touching surfaces offers more than just guidance, it serves to steady upright balance as well.

Peripheral and Central Postural Disorders

Impaired balance control may arise from neurological problems such as loss of inner ear (vestibular) function. Deficits in one sensory system can also affect how intact sensory systems contribute to postural control. For example, patients without vestibular function commonly relate no difficulty walking on hard support surfaces, but report discomfort walking on “spongy” surfaces (e.g., grass) that disrupt the processing of sensory information at the feet. The daily life of such individuals is severely hampered by such balance problems.

Multisensory Integration and Human Postural Control

Standing upright on two legs is a complicated control problem for the nervous system. Humans never stand perfectly still and small muscular corrections are constantly made to keep the head and trunk in an upright posture. But sending the proper muscular commands is only half the problem. Proper corrections are possible only if you have a good estimate of body position. To get this estimate, you need information from the visual (eyes), vestibular (inner ear) and somatosensory (muscular and skin receptors) systems. We know a lot about how each of these sensory systems contribute to postural control, but very little is known about how sensory information from different senses is fused to form a estimate of body position.

Genetic Basis for Vestibular Dysfunction

n collaboration with Dr. Steve Roth, we studied individuals who have lost vestibular function due to gentamicin therapy to determine whether there is an underlying genetic basis for such loss. The loss of balance function can have severe implications for general physical function and independence, and is a common consequence of aging. The loss of balance function occurs in a significant fraction of individuals (6-16%) treated with the antibiotic gentamicin (GM), and we hypothesize that genetic susceptibility is important. To study the genetic basis of drug susceptibility, we are studying individuals with antibiotic-induced balance dysfunction. This study will provide an opportunity for studying the influence of several genes on balance function. In addition to providing an immediate and significant clinical impact for patients being considered for antibiotic therapy, the present work will provide important information about the genetic aspects of balance function in general.

The Interaction of Sensory Information, Posture, and Locomotion

The evolutionary development of bipedal stance, which freed the hands from locomotion, is considered the fundamental distinction between humans and our closest relatives. Accompanying that development is the problem of stabilizing the multilinked body over a narrow base of support. Engineered devices, such as cars and robots, typically solve the stability problem by having a wide base of support and/or concentrating the bulk of its weight lower down. However, the human body has evolved with more than just upright stability as a constraint, with most of its mass concentrated higher up in the trunk, making it inherently unstable and prone to falls. The human body has evolved a sophisticated control system to achieve upright bipedal locomotion. The characteristics of this control system are virtually unknown, with most research focusing on biomechanical and energetic aspects of human gait. We are investigating how multisensory information Is used to stabilize the gait cycle by stimulating walking subjects with multiple forms of sensory information, including visual, vestibular and proprioceptive.

Multisensory Balance Training

Falls are known to be a major source of injury and accidental death in the elderly. Of the population over 65 years of age, one-third to one-half experience falls annually; of these, half do so repeatedly. Falls are the leading cause of injury in older adults and the primary cause of accidental death in those over age 85. Five percent of falls lead to a fracture, with hip fractures being the most common (greater than 200,000 annually). Further, imbalance in older adults is strongly associated with functional decline and frailty. Certain activities of daily living can no longer be performed, or are avoided due to a fear of falling. Unstable elderly persons become increasingly sedentary, homebound and isolated. Falls and instability contribute to 40% of nursing home admissions.

Decomposing the Control Loop for Upright Balance

Human upright bipedal stance is deceivingly complex. What appears to be a relatively simple behavior in healthy individuals is the function of a complex adaptive feedback control system that remains poorly understood. The complexity becomes more apparent with the onset of balance problems due to neurological injury or disease, not only in our poor understanding of the mechanisms through which such injury/disease affects balance control, but in our ability to treat it effectively. It is crucial to decompose the balance control loop in order to trace deficits to a specific component/process.

The Influence of Light Touch Contact on Human Postural Control

Imagine situations in which you cannot see very well, such as when you enter a darkened room or get up from bed at night to get a drink of water. In these situations, one often seeks out nearby chairs or tables with the fingertips to guide us through the room. However, we have found that lightly touching surfaces offers more than just guidance, it serves to steady upright balance as well.