October 1964
Summary of Present Evidence for a Stimulus-Information Theory of Perception (in the fields of space-perception, event perception, and visual guidance of locomotion)
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 References to these essays must cite them explicitly as unpublished manuscripts.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.
(Evidence ought to show, first, that the information is available in ambient light and, second, that it can be registered by individuals, either by judgments or behavior).
1. The specifying of a surface in the world by a textured area in the optic array, and of “nothing” in the world by a textureless area of the array.
The existence of terrain and sky over the ages of evolution, and their appearance to men.
The observations on optical support (behavior of animals without a continuous textured array under the feet).
Observations that animals will move into a textureless patch of ambient light but not into a textured patch (deer, birds, etc.)
The placing response of animals (e.g. kittens)
The laboratory results on surfaciness and filminess as phenomenal qualities (Metzger and Katz). (Also Gibson and Waddell, AJP 1952, 263, and recently Cohen, AJP, 1957, 403).
Difficulty: The puzzles of the figure-ground phenomenon. A closed contour (pictorially) seems to induce the experience of a solid object, but optically it does not always specify it (apertures and windows). Is this difficulty confined to the pictorial mode of perception?
2a. The specifying of an edge in the world (jump in depth) by a discontinuity in the gradient of one or both of the variables of optical texture in an array: (a) density, (b) parallactic motion or “shear” or “wiping out”. (There is also discontinuity of binocular disparity for some animals, but to register this requires that they have frontal eyes and compulsory convergence).
The results of the parallax experiments (Gibson, Smith, and Flock, JEP, 1959, p.40) and certain results with visual cliff.
2b. The specifying and depth downward at an edge by a horizontal line discontinuous density or mobility in the array.
The overall results of Gibson and Walk with the “visual” cliff (Psych. Monogr. 1962).
3a. The specifying of the recession of a surface, or slant, by a continuous increase (gradient) in texture density, or motility, or disparity.
The optical tunnel results when the texture was dense (JEP, 1955, p.1) The results of Dibble, Waddell, Carroll, and the “motion perspective” film and experiments. (Problem or slant and shape).
The results in recent papers by Howard Flock.
3b. The specifying of a horizon in the world by vanishing magnitude of texture elements or infinite density, and the vanishing of parallax. The Imitation of horizon in pictures.
Examples only; no experiments.
4. The specifying of subjective movements in the environment by a continuous transformation of the total array (motion perspective), and of staying in the same place by a continuing non-transformation of the array.
The cinerama observations (illusory subjective movement).
The observed behavior of birds and fish in a floating medium.
The evidence from aircraft-landing studies (e.g., Gibson, Olum, and Rosenblatt, AJP, 1955, p. 372). Also the “contact analog” flight simulator, and the specifying of where one is going by focus of magnification.
5a. The specifying of objective motion in the environment by the perspective transformation of an edge-bounded figure in the optic array. The problem of the in depth of an object in depth (KDE).
The results with shadow-transformations in various experiments.
Some of the observations on the “law of common fate” and some of the results of Johansson.
Note that the laboratory experiments using linear motions in a window do not fit into this hypothesis, but are special cases.
5b. The specifying of rigid object-motion in the environment by some property of a perspective transformation of a bounded figure in the optic array, and of elastic (viscous) object-motion by a non-perspective transformation of the figure.
Results of Gibson and Gibson, JEP 1957, p. 129; Fieandt and Gibson, JEP, 1959, p.344, and two papers by B. W. White. Also reports by Johansson and by Reynolds.
Note that the motion of the object is specified by transformation (change) of its silhouette but that the shape of the object is specified by the invariant properties (non-change) of its silhouette. These invariants under transformation will only exist if there is transformation, not in a silhouette as a single projection. The perception of motion should not be confused with perception of the depth-shape of the object moved.
6. The specifying of one surface going behind another by the optical event of occlusion or “wiping out” of forms in the optic array. (This is related to the perception of being “hidden” or “disappearing” and to the general phenomenon of edge-depth).
Michotte’s results on the perception of “screening” and on the “tunnel phenomena”, and the seeming paradox of an invisible phenomenal object (Michotte Monograph, 1964).
Does this explain a variety of spatial behaviors such as “hiding behind” and “peering behind”, and games like “hide and seek” in children?
7a. The specifying of the approach of an environmental object by centrifugal magnification of a figure in the array without centrifugal flow of the remainder; and approach to an object by centrifugal magnification of the figure with accompanying flow of the rest of the array.
. 7b. The specifying of imminent collision with an environmental object or surface by optical looming (the terminal magnification of any figure in the array approaching the limit of magnification).
Results with the shadow-apparatus yielding approach of an apparent object. Schiff’s thesis and the condensed manuscript on “impending collision” on experiments with chicks and fiddler crabs (and other animals).
Observations on controlled locomotion in animals (e.g. pursuit and flight) and on the avoidance of obstacles. Gibson in Brit. J. Psychol., 1958, 49, p.182.
Note. In all the above cases the information that specifies a constant in the external world is a mathematical constant over time in a (normally) changing stimulus array. The information that specifies changein the world ( or change in the position of the observer in the world) is change or motion in the stimulus array. There are two kinds of information therefore; for the “permanencies” of the environment, and for the “changes”, both objective and subjective. Perceptual constancy is supposed to depend on stimulus-invariants. Event-perception is supposed to depend on certain local transformations of the ambient array.
The only constancies referred to in the above summary are constancies of “layout” in the environment and of shape and size of the objects. Other types of perceptual constancy occur, of course, and the theory of stimulus-invariants may also be applied to them. See for example the study by Rhinegold and Hess (J. Comp. Physiol., 1957, 417) on the drinking response of chicks, which suggests those of the invariant optical properties specifying water that are responded to by chicks (color, luster, and ripples seemed to be important).