J. J. Gibson (1966) Introduction

Gibson, J. J. (1966). "Introduction" (pp. 1-6). In The Senses Considered as Perceptual Systems. Boston: Houghton Mifflin.

Introduction

What are the senses?

It has always been assumed that the senses were channels of sensation. To consider them as systems for perception, as this book proposes to do, may sound strange. But the fact is that there are two different meanings of the verb to sense, first, to detect something, and second, to have a sensation. When the senses are considered as perceptual systems the first meaning of the term is being used.

In the second meaning of the term there is a vast difference between sensations and perceptions. In 1785, Thomas Reid wrote:

The external senses have a double providence; to make us feel, and to make us perceive. They furnish us with a variety of sensations, some pleasant, others painful, and others indifferent; at the same time they give us a conception, and an invincible belief of the existence of external objects. This conception of external objects is the work of nature. The belief of their existence, which our senses give, is the work of nature; so likewise is the sensation that accompanies it. This conception and belief which nature produces by means of the senses we call perception. The feeling which goes along with the perception, we call sensation. The perception and its corresponding sensation are produced at the same time. In our experience we never find them disjoined. Hence we are led to consider them as one thing, to give them one name, and to confound their different attributes. It becomes very difficult to separate them in thought, to attend to each by itself, and to attribute nothing to it which belongs to the other (Essays on the Intellectual Powers of Man, II, p. 17).

That province of the senses which is to "furnish us with a variety of sensations" is by no means the same as that which is to "make us perceive." Reid was right. The part of this passage that might be objected [p.2] to is the suggestion that perception of objects must depend on "conception and belief." It will here be suggested that the senses can obtain information about objects in the world without the intervention of an intellectual process -or at least that they can do so when they operate as perceptual systems.

In this book I will distinguish the input to the nervous system that evokes conscious sensation from the input that evokes perception. I will not even speak of the ingoing impulses in nerves as "sensory," so as not to imply that all inputs arouse sense impressions. For it is surely a fact that detecting something can sometimes occur without the accompaniment of sense impressions. An example is the visual detection of one thing behind another, which is described in Chapter 10. There will be many examples of the principle that stimulus information can determine perception without having to enter consciousness in the form of sensation.

The reader should make allowance for the double meaning of the verb to sense. The detecting of stimulus information without any awareness of what sense organ has been excited, or of the quality of the receptor, can be described as "sensationless perception." But this does not mean that perception can occur without stimulation of receptors; it only means that organs of perception are sometimes stimulated in such a way that they are not specified in consciousness. Perception cannot be "extrasensory," if that means without any input; it can only be so if that means without awareness of the visual, auditory, or other quality of the input. An example of this is the "obstacle sense" of the blind, which is felt as "facial vision" but is actually auditory echo detection. The blind man "senses" the wall in front of him without realizing what sense has been stimulated. In short, there can be sensationless perception, but not informationless perception.

The seemingly paradoxical assertion will be made that perception is not based on sensation. That is, it is not based on having sensations, as in the second meaning, but it is surely based on detecting information, as in the first meaning.

There are two different levels of sensitivity. It will be evident in Chapter 2 that the so-called sense organs are of at least two different sorts: the passive receptors that respond each to its appropriate form of energy, and the active perceptual organs, better called systems, that can search out the information in stimulus energy. The receptors have measurable thresholds below which they are not excited; the organs and systems do not have fixed thresholds except as they depend on receptors.

Similarly, there are different levels of stimulation. The stimulus energy of optics, mechanics, and chemistry is coordinate with receptors, but the stimulus information to be described is coordinate with perceptual systems. Stimulus energy varies along simple dimensions like intensity and [p.3] frequency, but stimulus information varies along innumerable complex dimensions, not all amenable to physical measurement.

When the senses are considered as channels of sensation (and this is how the physiologist, the psychologist, and the philosopher have considered them), one is thinking of the passive receptors and the energies that stimulate them, the sensitive elements in the eyes, ears, nose, mouth, and skin. The experimenters in physiology and psychology have been establishing the conditions and limits at this level of stimulation for more than a century. A vast literature of sensory physiology has developed and a great deal is known about the receptors. It is a highly respected branch of science. But all this exact knowledge of sensation is vaguely unsatisfactory since it does not explain how animals and men accomplish sense perception.

It can be shown that the easily measured variables of stimulus energy, the intensity of light, sound, odor, and touch, for example, vary from place to place and from time to time as the individual goes about his business in the environment. The stimulation of receptors and the presumed sensations therefore, are variable and changing in the extreme, unless they are experimentally controlled in a laboratory. The unanswered question of sense perception is how an observer, animal or human, can obtain constant perceptions in everyday life on the basis of these continually changing sensations. For the fact is that animals and men do perceive and respond to the permanent properties of the environment as well as to the changes in it.

Besides the changes in stimuli from place to place and from time to time, it can also be shown that certain higher-order variables of stimulus energy -ratios and proportions for example- do not change. They remain invariant with movements of the observer and with changes in the intensity of stimulation. The description of such stimulus invariants is a main concern of the chapters to follow. And it will be shown that these invariants of the energy flux at the receptors of an organism correspond to the permanent properties of the environment. They constitute, therefore, information about the permanent environment.

The active observer gets invariant perceptions despite varying sensations. He perceives a constant object by vision despite changing sensations of light; he perceives a constant object by feel despite changing sensations of pressure; he perceives the same source of sound despite changing sensations of loudness in his ears. The hypothesis is that constant perception depends on the ability of the individual to detect the invariants, and that he ordinarily pays no attention whatever to the flux of changing sensations.

The ways in which animals and men pick up information by looking, listening, sniffing, tasting, and touching are the subject of this book. [p.4] These five perceptual systems overlap one another; they are not mutually exclusive. They often focus on the same information -that is, the same information can be picked up by a combination of perceptual systems working together as well as by one perceptual system working alone. The eyes, ears, nose, mouth, and skin can orient, explore, and investigate. When thus active they are neither passive senses nor channels of sensory quality, but ways of paying attention to whatever is constant in the changing stimulation. In exploratory looking, tasting, and touching the sense impressions are incidental symptoms of the exploration, and what gets isolated is information about the object looked at, tasted, or touched. The movements of the eyes, the mouth, and the hands, in fact, seem to keep changing the input at the receptive level, the input of sensation, just so as to isolate over time the invariants of the input at the level of the perceptual system.

The Senses and the Sensory Nerves

What about the input of the sensory nerves? We have been taught that the impulses in these fiber bundles comprised the messages of sense and that they were the only possible basis for perception. This doctrine is so generally accepted that to challenge it seems to fly in the face of physiology. There is said to be a receptor mosaic for each sense connecting with the central nervous system and projecting the pattern of excited receptors to the brain. But let us note that if the perceptual organs are normally exploratory, as they are, this anatomical projection of receptors is quite simply irrelevant for the process of normal perception. It is not false, for it explains afterimages, as well as many of the curiosities of subjective sensory experience that occur when stimuli are imposed on a passive observer by an experimenter. Experiments on sensation are usually of this sort. But the neural input of the mobile eyes in the mobile head of a mobile animal for example, cannot be thought of as the anatomical pattern of the nerve cells that are excited in the fiber bundle. This anatomical pattern changes from moment to moment. Neurophysiologists in the past have been reluctant to face up to this difficulty in explaining perception, for they know more about the anatomy of the eyes, ears, and skin than they do about the physiology of looking, listening, and touching.

What might be a physiological or functional equivalent of the external information, if it cannot be anatomical? How could invariants get into the nervous system? The same incoming nerve fiber makes a different contribution to the pickup of information from one moment to the next. The pattern of excited receptors is of no account; what counts is the external pattern that is temporarily occupied by excited receptors as the eyes roam over the world, or as the skin moves over an object. The [p.5] individual sensory units have to function vicariously, to borrow a term from Lashley, a neuropsychologist.

The answers to these questions are not yet clear, but I am suggesting new directions in which we may look for them. Instead of looking to the brain alone for an explanation of constant perception, it should be sought in the neural loops of an active perceptual system that includes the adjustments of the perceptual organ. Instead of supposing that the brain constructs or computes the objective information from a kaleidoscopic inflow of sensations we may suppose that the orienting of the organs of perception is governed by the brain so that the whole system of input and output resonates to the external information.

If this formula is correct, the input of the sensory nerves is not the basis of perception as we have been taught for centuries, but only the basis for passive sense impressions. These are not the data of perception, not the raw material out of which perception is fashioned by the brain. The active senses cannot be simply the initiators of signals in nerve fibers or messages to the brain; instead they are analogous to tentacles and feelers. And the function of the brain when looped with its perceptual organs is not to decode signals, nor to interpret messages, nor to accept images. These old analogies no longer apply. The function of the brain is not even to organize the sensory input or to process the data, in modern terminology. The perceptual systems, including the nerve centers at various levels up to the brain, are ways of seeking and extracting information about the environment from the flowing array of ambient energy.

The Improvement of Perception with Learning

The elementary colors, sounds, smells, tastes, and pressures that were supposed to be the only data of sense (and that are indeed obtained when a passive observer is stimulated by carefully measured applications of energy in a laboratory) have been thought of as an inborn repertory of experience on which a baby's later perception is founded. Learning to perceive then, had to be some such process as the associating of memories with these bare impressions, or the interpreting of them, or the classifying of them, or the organizing of them. Theories of perception have been concerned with operations of this sort.

If the senses are perceptual systems, however, the infant does not have sensations at birth but starts at once to pick up information from the world. His detection equipment cannot be exactly oriented at first, and his attention is imprecise; nevertheless, he looks at things, and touches and mouths them, and listens to events. As he grows he learns to use his perceptual systems more skillfully, and his attention becomes educated to the subtleties of stimulus information. He does learn to perceive [p.6] but he does not have to learn to convert sense data into perception.

On the assumption that the senses are channels of sensation the process of learning has been thought of by stimulus-response psychologists as an attaching of new responses to a fixed set of possible inputs. On the assumption that the senses are perceptual systems, however, the emphasis is shifted to the discovery of new stimulus invariants, new properties of the world to which the child's repertory of responses can be applied. This is perceptual learning as distinguished from performatory learning. Both kinds of learning occur in the child, but perceptual learning is the more in need of study because it is the more neglected. In Chapter 3, and especially in Chapter 12, this idea will be followed up.

The Facts to be Surveyed

The plan of this book is to put together the existing knowledge of the senses in the framework of the theory that has just been outlined -to survey the senses in the interest of understanding perception. Sensory physiology and psychology are factual disciplines, not theoretical ones, and the solid body of facts written down in journals and books would fill a large library. It is hard for the curious student of perception to weigh this mass of evidence for it is highly specialized and much of it makes dull reading.

Few modern attempts have been made to integrate all this information within the covers of a book. The only two are Pieron The Sensations (1952), and Geldard, The Human Senses (1953). The handbooks of physiology and experimental psychology do not attempt an overview, and the popularizations that have been written are misleading. The investigators of the senses tend to stay close to that sense in which they specialize.

As to the books on perception, there are many, by psychologists and philosophers, but they are not much grounded in biology of sensitivity. They take sensations for granted, leaving them to the specialists, and are mainly concerned with theories of perception, the problem of perceptual constancy, the ways in which perceiving depends on the personality of the perceiver, and the reasons we have such an invincible belief in external objects when we have no right to it [according to the usual assumed theories].

The question, then, is, how do the senses work? Since the senses are being considered as perceptual systems, the question is not how the receptors work, or how the nerve cells work, or where the impulses go, but how the systems work as a whole. We are interested in the useful senses, the organs by which an organism can take account of its environment and cope with objective facts.