August 1967
What is Perceived? Notes for a Reclassification of the
Visible Properties of the Environment
J. J. Gibson, Cornell University
This memo has been published in E.S. Reed & R. Jones (Eds.) (1982). Reasons for Realism. Chapter 4.9, Part I. pp. 401 – 403. Hillsdale, NJ: Lawrence Erlbaum Associates, Inc.
The abstract analysis of the world by mathematics and physics rests on the concepts of space and time. The study of sense perception by psychologists has conformed to this analysis. Mass is exemplified by what we call “objects,” and energy by what we call “light.” So it is assumed that we perceive space, time, and objects, and that we sense light (or color). This way of thinking about perception has a long history. The perceptible qualities of objects were classified in accordance with it. John Locke’s list consisted of the primary qualities of position, shape, size, duration, and solidity, to which was added the secondary quality of color and other non-visual secondary qualities (sound, taste, smell, and warmness or coldness). Psychology is still trying to explain the perception of the position of an object in space, along with its shape, size, and so on, and to understand the sensations of color. The explanations are tortured and success is not in sight. But a direct explanation of the perception of the properties of the visible environment may be possible if these properties are taken from concepts of ecology instead of from mathematics and physics. (Perhaps they are ultimately “reducible” to the latter, but the psychologist cannot wait on such a reduction.) What might such a list be? Here is a preliminary classification.
Spatial Properties (We do not visually perceive “space,” but we do perceive the following persisting, i.e., relatively invariant, properties of the world.)
1. Surface layout. This includes surface slant, corners (dihedral angles), curvature, and the edges of surfaces which occlude other surfaces. It includes the recession of the ground (distance from “here”). Finally, it includes objects, with the position of each object in the layout, the dimensions of the object (size) and the proportions of its cases (shape). The persistence or permanence of hidden objects is discussed below.
2. Substance or composition. This includes the solidity, liquidity, or viscosity of the substance, the color of the surface (reflectance and differential reflectance), and the texture of the surface, (its small-scale layout).
3. Lighting or illumination. This includes cast shadows, attached shadows (“shading”), and direction of illumination on a surface.
Spatio-Temporal Properties (We do not perceive “time” as such, but we do perceive changes or varying properties of the world, which are spatio-temporal.)
1. Motions of rigid objects. This includes displacements and rotations relative to the ground.
2. Deformations of elastic objects. This includes the flow of viscous or fluid surfaces.
3. Progressive occlusion and disocclusion, that is, the optical covering and uncovering of surfaces (objects) by edges. An object that is thus covered is specified as persisting. An object may also recede “into the distance” and persist.
4. The ending and beginning of the solid state, that is, the melting, decomposition, dissolution, etc. of an object or (occasionally) the opposite.
5. The onset and cessation of illumination. This includes many kinds of transients, both naturally and artificially produced (e.g. “flashes” of light).
6. Animate motions and deformations. This includes the whole realm of events in the animate and social environment, e.g. expressive movements and social signals.
7. Events in general. For events with sufficiently abrupt onset and cessation it is possible to measure duration (“time”). For multiple events of this sort it is possible to measure frequency (“rate”). But, in general, the centimeter-gram-second system of physics can only be applied at present to uninteresting events in the visible environment.
Perceptual research needs a program newer than the one formulated in Newton’s Principles and Locke’s Essay Concerning Human Understanding.
The Visual Detection of the Self
Proprioception accompanies perception; we proprioceive visually as well as perceive visually, and this kind of detection is also spatio-temporal. We can detect the following characteristics or variables of the self as the center of the environment. (The information is multiple but we are considering only visual information.)
1. The temporal posture of the body. The upright or inclined posture of the head is “visible.”
2. The locomotion of the body. The direction and speed of locomotion through the environment is “visible” (by means of motion perspective and the changing occlusion of surfaces at edges). This extends to locomotion in vehicles.
3. The movement of the limbs. Gross movements and fine manipulations are “visible.” Both movements of the hands and of the tools grasped are thus registered (pointing, reaching and grasping, but also raking, pounding and trace-making). Note that the term perception is reserved for the environment, and detection or registration is applied to the self. Note also that the visual registration of body movement may be either obtained or imposed, i.e., from an active or a passive movement (Senses Considered, ch. 2). Hence not all movements are self-produced, and not all inputs should be considered as “feedback” or “reafferent,” as seems to be widely assumed. The problem of the registering of information is distinct from the problem of the purposive control of movement.