The relation between visual and postural determinants of the phenomenal vertical.

Fifteen years ago Koffka pointed out that perceived space could be characterized as constituting what he called a "framework" (6). Implicit in every visual perception were reference-axes of vertical and horizontal, somewhat analogous to the coordinate axes of abstract geometrical space. Objects in phenomenal space, he said, are seen to be upright or tilted or inverted only by virtue of this frame of reference. It is as if the air surrounding the objects and surfaces in our visual environment contained an invisible coordinate system with respect to which their orientation is visible. Not only the perception of the position of objects but also the perception of one's own bodily posture depended on this framework. In Koffka's theory the "ego" was a part of the phenomenal field, an entity in perceived space, and consequently the maintaining of bodily equilibrium was for him essentially a perceptual process, of which the postural reflexes were simply an expression. Koffka reached the conclusion that this phenomenal framework, this sense of the vertical which a man possesses, was determined by visual stimulation.