April 1972
The Exploring, Selecting, and Enhancing of Optical Stimulus Information.
A New Classification of Ocular Adjustments
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 References to these essays must cite them explicitly as unpublished manuscripts.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.
(See Also: A Reconsideration of Eye Movements and Eye Postures based on Ecological Optics. January 1969)
A. Exploring the Available Information
1. Successive sampling of the ambient optic array by head-turning.
a. The eyes are stabilized during head-turning by compensatory eye-turning and are thus anchored to the external array. (When the head turns too far for compensation, the eyes jerk ahead to a new stabilization.)
b. Successive samples of the ambient optic array overlap, and the structure of a subsequent sample coincides in part with that of a previous sample. In this way the ambient environment is perceived.
2. Successive foveating of details of the array-sample by eye-turning.
a. The function of rapid (saccadic) eye movements is to point the eye toward significant items of the environment insofar as these are projected by details of structure in the array-sample. Otherwise stated, the retinal fovea is applied to the details of a retinal image, using that term to mean a panoramic potential image of the environment. A completely panoramic “image” of the environment would be equivalent in some ways to the ambient optic array (but see below on accommodation).
b. The result of a sequence of eye-turnings is the perception of a scene, not of the sequence; the result of a sequence of head-turnings is a perception of the environment, not of the sequential visual fields.
c. The sequence of foveations (fixations) skips around the ordinary field of view in zigzag movements and the temporal order of these fixations does not seem to be important for the perception of the scene. Only when the layout of items is conventional, as in writing, is the sequence important, i.e., only when the sequence of foveations is compulsory.
3. The maintaining of foveation of a changing optic array (pursuit fixation).
a. With a stationary observer and a moving object in the world, the eye can foveate the corresponding in the otherwise frozen array as it sweeps across. This is sometimes called “tracking”.
b. With a moving observer in the stationary world, and hence a centrifugally flowing optic array, the eye can fix on one of the flowing details.
B. Selecting Details in the Ambient Array
1. Foveating or fixating is best thought of as an act of orienting the eyes, together with the head and the body, not as a response to a stimulus, or a reflex, as we have been taught. Considering the panoramic retinal image with its details, fixating is a centering of the retina behind the image on a significant detail of the image, the center being the most acute part of the retina. It is a symmetricalizing or balancing of the input, a maximizing of something, a controlled adjustment, not a triggered reaction. The laboratory response of the eye turning in darkness to the flash of a single spot of light is an unrepresentative situation.
2. Presumably a detail of the optic array is only fixated (or at least the fixation is only maintained) if it conveys information about the environment, or constitutes information about it.
3. Among many informative details of an optic array, the one detail that conveys information on accordance with the “set” of the observer is selected for fixation. This means that those objects in the environment with affordance that are relative for the temporary need of the observer get looked at.
4. The object or item in the environment that is being fixated is said to be temporarily “clear” in experience and other objects or items in the periphery are said to be temporarily “unclear”, the more so as they are farther out from the center of the field. But the fact of the matter is that some information about an object in the periphery can be reported while other information cannot; one detail may be visible whereas another may not. “Clarity” is a vague term. What the fovea of the retina is especially sensitive to is information, not stimulation, and the center of the visual field corresponding to the fovea is not so much a center of “clear vision” as it is of determinate perception.
C. Conjugate Foveation of the Same Detail by both Eyes
In men, unlike the horse, the exploratory saccadic eye movements of the two eyes are conjugated, like the front wheels of an automobile, and whatever detail of the array is foveated by one eye is also foveated by the other. This dual pickup of the same information yields, first of all, a double assurance of perception, and then, insofar as the binocular system can register the disparity of the two arrays, an extra kind of information for the perception of occluding edges, for depth, and for controlling precisely the use of both hands.
D. The Enhancing of Optical Texture at the Retina
In addition to the adjustments of the extrinsic eye-muscles already described, there are two kinds of adjustment of intrinsic eye muscles; the accommodating of the lens and the closing of the pupil.
1. Accommodation. The theory of information pickup asserts that the purpose of vision is to register as many of the features of the structure of ambient light as possible, including the fine structure. In order to register fine detail projected from both very distant things at one extreme and very small things at another, the lens of a chambered eye needs to focus differentially. Blur of the retinal image, so-called, is normally eliminated at the fovea by neural control of the shape of the lens but we have no idea of how this is accomplished. It cannot be a response to a stimulus. But when accommodation is successful the perception of a fixated surface or edge is said to be “clear” or “sharp”. Accommodation accompanies foveation, but each has its own special kind of phenomenal “clarity”.
2. Adjustment of the Pupil. The optic array is defined in terms of its relational structure independent of its luminance. But the illumination of a medium may be very high or very low. With high intensity ambient light the structure of the array is hard to pick up. Sensations of dazzle or glare become obtrusive and interfere with visual perception; phenomenally they are said to “veil” the objects and surfaces of the world. Too much stimulus energy gets in the way of the stimulus information. Similarly, when ambient light is of low intensity the structure of the array becomes hard to pick up with a mosaic of photoreceptors. Darkness is said to “obscure” the objects and surfaces, and “acuity” of several sorts becomes poor. The effect of pupillary closing is to minimize high-intensity stimulation and of opening is to maximize low-intensity stimulation, so as to optimize the pickup of information.
For very low intensities of ambient light the rod retina comes into play. Visual attention and visual “clearness” are quite different with night vision since, for one thing, the fovea is night-blind.