Determinants of the Perceived Vertical and Horizontal

The fact that the seen world appears upright is so obvious that it may seem axiomatic. Nevertheless this verticalhorizontal stability of phenomenal things is as much a psychological problem as their coloration, size, or distance. Stratton once asked the question, "why do things look right side up instead of inverted?" But a different and more fundamental question is, "why do things look right side up in the first place—instead of, for instance, tilted?" Some attention has been paid to the related observation that the objective visual field remains phenomenally upright when the head is tilted (23, 30). But this phenomenon presupposes a more fundamental one, the fact that the objective visual field is phenomerilally upright when the head is not tilted. Koffka has recently described phenomenal space not only as a 'ground' for the perception of objects but also as a 'framework' with reference to which they are oriented and localized (23). Space, as experienced, has the quality of 'updownness' and 'right-leftness'; vertical and horizontal reference-axes are an implicit characteristic of all seeing. Or, in other words, we seem to possess a visual 'sense' of the vertical and horizontal. In support of this assertion, innumerable experiments have proved that we can imagine these axes in a perfectly homogeneous circular visual field, and can set an adjustable line in this field to the vertical or horizontal with great precision. This perceptual ability coexists with the ability to maintain and sense deviations from an erect posture, and also to feel the normality or tilt of the terrain and other