ambient light away from the pole that coincides with the direction of
locomotion is less familiar than the flow of a current, but more predictable
(Gibson, Olum, and Rosenblatt, 1955). This panoramic streaming is an
unfailing index of locomotion with respect to the earth (see Figure 9.3).
and the absence of streaming is an absolute index of non-locomotion. The
way to keep from drifting with the tide of water or air is to cancel the
optical flow. If it expands ahead and contracts behind, one is "gaining
Figure 9.3 The flow of the optic array during locomotion in a terrestrial environment. When a bird moves parallel to the earth, the texture of the lower hemisphere of the optic array flows under its eyes in the manner shown. The flow is centrifugal ahead and centripetal behind - i.e., there are focuses of expansion and contraction at the two poles of the line of locomotion. The greatest velocity of backward flow corresponds to the nearest bit of the earth and the other velocities decrease outward from this perpendicular in all directions, vanishing at the horizon. The vectors in this di- agram represent angular velocities. The flow pattern contains a great deal of information.
162 / THE SENSES CONSIDERED AS PERCEPTUAL SYSTEMS
ground"; if it contracts ahead and expands behind, one is "losing ground." A visual system does not have to be sensitive to fine detail in order to register this How pattern, especially if the system is highly panoramic. The seeing of small landmarks is not required; only the overall streaming of gross structure need be detected. A relatively small number of directional photoreceptors each accepting a diHerent cone of the total array might serve. Some insects and crustaceans possess no more.
The How pattern of ambient light should be distinguished from the rotation of ambient light around the animal. The former specifies trans- location of the animal from one place to another; the latter specifies turn- ing of the animal in the same place. Actually a rotation of the array relative to the animal is always caused by a rotation of the animal relative to the array, except in a certain experimental circumstance never found in life. The animal takes rotation as information for being turned, as when, for example, he is whirled around by an eddy in the medium. This was illustrated in Figure 2.1.
Consequently, if an animal is put on a small platform at the center of a textured cylindrical environment which is then artificially rotated, he shows what are called optokinetic reflexes, (or compulsory optokinetic nystagmus). Their function again, is to keep the animal anchored to the optical environment for as much of the time as possible. In this experiment there is no stimulus information from the statocyst organs, only informa- tion from the eyes. We shall return to this experiment when describing compensatory reactions.
• GUIDING LOCOMOTION. It is useful to be able to see whether or not one is moving; it is better to be able to see vaguely where one is going; it is best to be able to see exactly where one is going. Visually guided locomo- tion is a matter of going to a specific goal in the environment, and this requires that the goal be identified in the array of ambient light. The form in the array on which the focus of expansion is centered corresponds to the place toward which the animal is proceeding. If it is the right form - if it specifies prey, or a mate, or home - all he has to do is magnify it in order to reach the object. He governs the muscles of locomotion so as to enlarge the form, to make it loom up. The same rule of visual approach holds true for swimming, Hying, or running: keep the focus of centrifugal
How centered on the part of the pattern of the optic array that specifies the attractive thing or the inviting place. The controlling rules for steering among obstacles, for avoiding collision, and for stopping without collision are all related to this rule for aiming locomotion. The cybernetics of pur- suit, evasive Hight, and landing depends on the pickup of certain invariants in the optical flow pattern (Gibson, 1958). The human aircraft pilot must learn to detect them, but the bird and the bee, with their panoramic eyes, seem to find it easy to do so.
• COMPLEX FEEDBACK.When the forms of an optic array at one station- point are supplemented with all possible transformations to other station- points, and when sequence over time is added to order in space and all motions are considered, it is obvious that the specifying capacity of this super-stimulus is unlimited (Gibson, 1961). It is an inexhaustible reservoir of potential information about the world and about the individual's be- havior in it. Vision can develop into the dominant system in our species, the "queen of the senses," because the opportunities for perception are so vast. When
sequence
is combined withscene,
vision makes possible the achieving of geographical orientation, the feats of navigation over time, and the cognitive mapping of the environment. It also makes possible the control of skilled movements and the coordination of hand with eye in primates. In man, the ability to control the movements of the hand by vision has led to picture-making and even to ideographic or phonetic writing, from which a new level of cognition emerges. There is visual feedback at all levels of activity: upright posture, locomotion, homing, and the control of vehicles; manipulation, tool-using, mechanical problem- solving, and graphic representation. As the informative feedback becomes differentiated the skill can become learned.The foregoing ninefold classification of the uses of vision is not ex- haustive. It is offered to illustrate the opportunities for development of visual organs and their neuromuscular systems, and the possibilities for exploiting the information in light. The classification should also loosen our human and historical fixation on the retinal image as the stimulus for seeing. We now turn to the evolution of visual organs.