Balance is the maintenance of upright posture despite perturbations from the external environment. Three systems in the body act in concert to maintain stable orientation and the sensation of being well balanced. These three systems are the visual system, the vestibular (inner ear) system, and the proprioceptive (sensory nerves) system.
These are listed in order of importance for the situation presently under consideration. These systems all provide sensory information to the brain. The visual system provides an image of the environment surrounding the person. The inner ear senses linear (back and forth, up and down) motion as well as circular motions.
The sensory nerves sense all of the postural muscle tension, joint position, and pressure on various areas of the bottom of the foot. All of these systems are interconnected and integrate with the higher centers of the central nervous system subserving functions of cognition and conscious activities as well as the subconscious areas of the brain.
All of these systems have different frequency ranges over which they operate most efficiently. In the case of the inner ear it responds best to relatively low frequency movements. The visual system works best at higher frequencies.
These systems act in concert to tell the brain, and thus the individual, about balance and orientation in space. This information includes the person's position relative to upright and whether or not movement is taking place. Generally the visual system exerts the strongest influence on the overall balance system followed by inner ear and finally the sensory nerves. If there is no visual input then the other systems predominate.
The interaction of the balance system with the environment is not perfect and can easily be fooled. A good example of such a situation where perception is not reality is found in full visual field video presentations. This effect is achieved in a theater with a very large floor to ceiling and side to side screen.
The image is projected on to the screen and accounts for almost all of the visual field of the viewer. This type of situation can create an illusory sensory situation where the person believes they are, as an example, in the helicopter and actually in flight. If you watch a crowd at one of these films many will be seen to be swaying back and forth as the on screen image changes. The motions of the crowd will be those you would expect if the viewer was actually riding with the camera. This effect is subconscious but with effort can be reduced to some extent.
Gregory A. Ator, M.D., FACS
Neurotology, Otology, and Skull Base Surgery
Disorders of Hearing, Balance, and the Facial Nerve
