Perception and tonus

Abstract.— A review is given of the achievements of more than one hundred years of research concerning the influence of muscular tonus on the perceptual phenomena related to human spatial orientation. The tonal conditions of both the extrinsic eye muscles and the complex system of skeletal muscles have been shown to have decisive influence on "egocentric localization". The factual median is perceived as being displaced in the direction of the side of the body with greater tonus. This is the case whether the tonic asymmetry is experimentally induced or natural. Corresponding to this fact, a spot of light in the dark situated at the factual eye level will be perceived as being situated above the experienced eye level. The influence on egocentric localization exercised by prolonged fixation of greater or smaller parts of the muscular system (especially of the eye muscles) is pointed out. This "fixation effect" seems to be in accordance with conceiving the postural system of the body by analogy with a servo-mechanical system, the feed-back part of which is restrained.