March 1969
A Further Note on Occlusion
J. J. Gibson, Cornell University
The World Wide Web distribution of James Gibson’s “Purple Perils” is for scholarly use with the understanding that Gibson did not intend them for publication. References to these essays must cite them explicitly as unpublished manuscripts. Copies may be circulated if this statement is included on each copy.
I treat the perception of space as the perception of the “layout” of environmental surfaces. The layout of such surfaces is considered relative to a point of observation, and the ambient optic array at that point. This treatment escapes the fault of being concerned only with the perception of “subjective” space because it understands that a moving point of observation is normal for animals, and recognizes that the transformations of the optic array, not its form, contain the information about the objective layout of the world. Perception is only achieved over time. The constant sizes and shapes of things become clear only when the invariants under transformation emerge.
This is all very well, but we must now go on to recognize that, to perceive the objective layout of the environment, one must be aware of temporarily hidden portions of the environment. We must take account of the occlusion of surfaces as well as their distances and inclinations to the line of sight. If one is really to perceive an object, one must perceive the ground extending behind the object, as Koffka pointed out. And this is not all, for we must also perceive the back surface of the object. In the case of a window instead of an object, we have to perceive the surfaces of the world outside the window and even the back side of the wall containing the window.
In short, although we have faced up to the fact that changing slant and distance of a surface does not destroy its phenomenal rigidity, we must now face the more radical fact that changing occlusion of a surface does not affect its phenomenal existence.
There is a great deal of current confusion about occlusion, and about the philosophical implications of the perception of hidden things, and about its development in the child. It would be useful therefore to describe it in the terms of ecological optics. No description in terms of physical optics exists. We have been content to say that “light travels in straight lines” and let it go at that.
1. Occlusion occurs only to the extent that surfaces are opaque. There would be no occlusion if all environmental surfaces were transparent.
2. The ambient optic array at a point of observation consists of visual solid angles that are the projections of faces and facets of illuminated surfaces.
3. At a stationary point of observation some of the surfaces of an environment are projected while others are unprojected (occluded). That is to say some things are unhidden and others hidden, some visible and other invisible, some revealed and some concealed. (Other synonyms for occluded are covered and screened. cp. Michotte.)
4. Any fixed surface that is occluded at a given point of observation can be disoccluded by a change in the point of observation (a locomotion). The converse is also true.
5. In the case of a detached object (as distinguished from a fixed surface) the revealed face (“front”) can become the concealed face (“back”), and the converse, by a rotation of the object without any displacement of the point of observation. When the object is displaced instead of rotated, there exists a change in the occlusion of the background of the object, and this is the way we ordinarily see the “motion” of an object.
6. All occlusion is caused by what I will call an occluding edge. This does not necessarily imply an edge in the sense of a sharp dihedral angle, an apex, since a curved surface can occlude another surface at the tangent to the curve (cf memorandum on the classification of types of surface layout). The sharp edges of a surface-layout and the occluding edges of it are not the same (although both will yield abrupt contours in an optic array).
7. An occluding edge entails two surfaces, one that occludes and one that is occluded. Hence the perception of an occluding edge involves the perception of two such surfaces, one superposed on another. There is what Koffka called “double representation” (Principles, p. 178), that is, two surfaces at different distances but in the same direction-from-here. This duality occurs only on one side of the edge, the occluding side.
8. An occluding edge implies depth at the edge, that is, a step or increase of distance on the non-occluding side. Hence the perception of an edge implies this perception of depth.
9. Inasmuch as observers are mobile, occlusion at an edge changes in time. What is perceived ordinarily is an event, either one of progressive concealing (covering) or revealing (uncovering). These events can be isolated and controlled in a psychophysical experiment (e.g. Kaplan).
10. Just as one surface is perceived to extend behind another surface at an occluding edge, so the object being concealed is perceived as persisting after it has been concealed and the object being revealed is perceived to have preexisted before it was revealed. Koffka’s “double representation” and Michotte’s “tunnel effect” are thus consistent with one another. The detection of “superposition” in space and the perception of “object-permanence” in time are not separate problems of perception or cognition but are related problems of “amodal” perception, connected with occlusion and disocclusion.
References
Gibson, Kaplan, Reynolds, and Wheeler, The change from visible to in visible. A study in optical transitions, Perception & Psychophysics, 1969.
Motion picture film, same title.
Kaplan, A psychophysical study of occlusion, Perception & Psychophysics, 1969.
Koffka, Principle of Gestalt Psychology. 1935.