The perceived slant of visual surfaces-optical and geographical.

One of the properties of a visual surface along with hardness, distance, and color-with-illumination, is that of slant. This term must be understood to include not-slanted as well as slanted; in other words the variable consists of opposite qualities having zero slant as a norm. There is evidence that optical slant, so-called, is determined by stimulation. When vision is monocular and the head is motionless, this quality seems to depend on the gradient of the density of the "texture" of the retinal image (1). The experiment which appeared to demonstrate this psychophysical correspondence, however, is defective in that the procedure failed to isolate the quality of optical slant from a congruent quality of geographical slant which accompanied it. This failure should be amended if possible. Moreover the two kinds of slant need to be denned and their relevance to spaceperception discussed. Consider first the impression of slant embodied in the face of an object—a bounded surface, or a segment of an array of surfaces. It can be studied in the following situation. The 0 sits in an ordinary room with his gaze horizontally straight ahead and fixates the center of a surface such as a sheet of textured cardboard. This surface is then rotated by E around a horizontal axis, either forward or back1 The experimental study here reported was carried out by Janet Crum Cornsweet under the general supervision of the senior author. The experiment was planned with the assistance of Howard Gruber. The research is part of a project carried out under Contract AF 41(128)-42 between Cornell University and the USAF School of Aviation Medicine. ward. The quality of slant will increase as rotation increases, either ceilingwise or floorwise, until just after reaching the greatest possible slant the surface suddenly becomes an edge (3). Putting aside the question of change in shape, this situation provides variation in slant without variation in distance or any of the other qualities of a surface. It also shows that the quality of slant has an upper absolute threshold at the point where the surface becomes parallel to the line of sight. It should be noted, however, that in this experiment the inclination of the surface to the line of sight is so arranged as to have the same value as its inclination to the physical horizontal. Consider next the impressions of slant embodied in a continuous plane surface filling most of the visual field. Take as an example the visual experience of a man standing on a level desert plain and looking about. This example is particularly significant since it is a kind of minimum perception for any sort of spatial behavior. What he sees is a level ground extending to the horizon with himself standing on it. No impression of slant seems to be evident. But this perception of the earth is almost certainly a product of the integration of successive eye-fixations (2, ch. 8). Ordinarily the man is unaware of his saccadic eyemovements, but if he attempts to in-