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concerning the straightness of his path, the amount of turning, etc. In normal vision (without distorting spectacles), when one walks along such a mechanically sensed straight line, the image on the retina of the eyes undergoes a projective transformation (at least approximately) in which the apparent shapes of figures change, but in which straight lines are transformed into straight lines. Therefore, as one walks in a straight line, the mechanically derived information on the invariance of direction of movement will agree with what is implied by the corresponding optical information, abstracted from the projective transformation of lines in the field of vision. However, when one is wearing distorting spectacles, what is mechanically sensed as walking a straight line will be optically sensed as walking a curved line. Thus, there is a contradiction between what one sees and what one perceives through feeling, movement, kinesthetic sensations, etc. It seems then, that below the level of consciousness, the brain and nervous system are trying to resolve this contradiction by testing various hypotheses as to what actually constitutes a straight line.1 When a hypothesis isfound that removes the contradiction between what is seen and what is felt mechanically, then this hypothesis is, as it were, embodied directly in the structure that we perceive. Therefore, a person who is wearing distorting lenses eventually ceases to perceive an optically curved line under conditions in which he mechanically senses a straight line, but rather he comes to see and feel the same straight line (as in the work of Piaget discussed in the previous section, where the child learns to perceive an invariant correspondence between what he sees, what he hears, what he grasps, etc.).

In the discussion of the work of Ditchburn, Hubel and Wiesel, and Platt we have already seen that the optic nerve does not transmit a simple “copy” of the image on the retina of the eye, but rather that it tends to emphasize certain structural features by heightening contrasts and being sensitive to the presence or absence of lines and other such figures. From the work of Held and Gibson, however, it is clearly seen that the picture that we perceive actually contains structural features which are not even on the retina of the eye at a given moment, but which are detected with the aid of relationships observed over some period of time.

The perceived picture is therefore not just an image or reflection of our momentary sense impressions, but rather it is the outcome of a complex process leading to an ever-changing (three-dimensional) construction which is present to our awareness in a kind of “inner show.” This construction is based on the abstraction of what is invariant in the relationship between a set of movements produced actively by the percipient himself and the resulting changes in the totality of his sensual “inputs.” Such a construction functions, in effect, as a kind of “hypothesis” compatible with the observed invariant features of the person’s over-all experience with the environment in question. (For example, the perception of a straight line corresponds to a hypothesis on what is invariant in the optical, mechanical, and other changes that have been experienced in relationships with this line, as a result of the movements that have been made in a person’s perceptual contacts with it.)

1 _{Platt, for example, has suggested that the brain may find some new combination of rotations of}

the eyeball, about an axis parallel to the retina and perpendicular to it, which can consistently be coordinated with the mechanically sensed straight line.

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Not only is the process of construction dependent on the abstraction of invariant rela- tionships between movement and sense perceptions, as described above; it also depends on all that is known by the percipient. For example, if a person looks at a letter at a distance too great for clear distinct vision, he will see something very vague and indistinct in form. But if he is told what the letter is, its image will suddenly appear with comparatively great clarity. Or alternatively, he can drop a small coin on a highly patterned carpet, where he will generally find that it is lost to his sight. Then, if he catches a glint of reflected light, the coin that he knows that he has lost will suddenly stand out in his perception. Its image must have been on the retina of the eye all the time, but it did not enter the “inner show” of perception until the reflected glint contradicted the percep- tion of a carpet with nothing on it, and also suggested the lost coin that he knows about.

Gibson1 describes a great many experiments which further bring out the general properties of perception described above. He shows that in the perception of depth, or the three- dimensional character of the world, binocular vision is only one of the relevant factors. Another important factor is just the changing optical appearance of things as we move. Thus, as we walk, the image of an object that we are approaching gets larger. The closer the object is, the more rapidly does its apparent size change. In this way (as well as in many other ways, such as the placing of shadows, the relative haziness of distant objects, etc.), the brain is able to abstract information cencerning the distance of objects in the dimension along the line of sight. On the basis of such information, it is continually “constructing” the field of what is perceived in the manner that has already been described, i.e., by introducing various “hypotheses” as to what is invariant. For example, if one misjudges the distance of something, one will also misjudge its size. As one walks, one may sense that the object is not varying its apparent size in the way implied by our judgement of its distance. Suddenly, there will appear in the field of perception a different way of seeing the object, which is consistent with the new information.

We see then that what actually appears in the field of perception, at least when one is viewing something relatively static, is a structure, order, and arrangement of things regarded as invariant in their sizes, shapes, and spatial relationships. This construction in the “inner show” is such that the assumption that it is invariant explains not only its present optical appearance but also the alterations in its appearance that have been experi- enced as a result of past movements, as well as all that we know or think we know about it. At each moment such a construction has a tentative character, in the sense that it may be subject to changes, if what it implies leads to contradictions in later experiences at- tending subsequent movements, probings, tests, etc. Here we see an essential role of the active movements of the percipient, for it is through these that the current “hypotheses” in the “inner show” of perception are always being tested, corrected, modified, etc.

Thus far we have been considering only the case in which a percipient moves in a relatively static environment. If movements are taking place in his environment as well, then there is the additional problem of knowing which of the observed changes are due to

1 _{J.G.Gibson and E.J.Gibson, Journal of Experimental Psychology, 54, 129 (1957).}

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the movements of the observer and which are due to movements of what is in the environment. This problem is dealt with, in effect, by the capacity to abstract a higher order type of invariant, i.e., a relatively invariant state of movement.

Generally speaking, as a person moves in his environment his brain begins (largely unconsciously) to note those features which do not change significantly as a result of these movements. These are treated as a distant and relatively fixed background, against which other movements can be perceived. The closer objects do, of course, change their apparent sizes, shapes, etc., appreciably in a systematic way as a person walks, moves his head, etc. It seems that the brain has developed the ability to be sensitive to such apparent movements and changes in the nearby environment, especially when they are coordinated with movements produced by the percipient himself. This permits the elimination of the self-produced movements in the field of what is perceived, so that the construction of the “inner show” corresponds to a generally static world, in which the percipient himself is seen to be moving. Therefore, as a person walks around a room, he does not feel the room to be moving, whirling around, and changing its shape, etc. Rather, he perceives the room as fixed and himself as moving, in such a way as to explain all the variations in what he has perceived. But if, for example, he has suffered damage to the delicate balancing mechanism in the inner ear, he can no longer coordinate his mechanical perceptions with his optical perceptions. He may then suffer vertigo, and feel that the world is moving around him. The difference between these two modes of perception is very striking to anyone who has ever experienced it.

On the basis of the elimination of the movement of the percipient, the brain is then able to go to the next level of abstraction, in which it senses the movement of some part of the field of vision against a background that is perceived as fixed. The simplest case arises when a given object merely suffers a dislocation in space and perhaps also a rotation. In this case one is able to perceive the object as actually having a constant size and shape, despite the fact that its image on the retina is changing all the time. This perception is inextricably bound up with the ability to see such an object as possessing a certain state of motion, rather than as a series of “still” pictures of the object in question, each in a slightly different position. It is almost as if the brain were able to establish a co-moving reference frame, in which a moving object could be seen to have a constant shape. In this way, the brain seems to include in its construction process the ability to abstract a certain state of movement, which under the assumption of an object of a given shape is compatible with the changes that have been perceived in the appearance of the object over some period of time.

Of course, there will then be further kinds of changes which cannot be explained in this way (e.g., an object may actually grow in size, change its form, etc.). These will have to be perceived in terms of more subtle internal changes in the object in question.

The problem of how movement is perceived is far from being fully solved. Yet it is already clear that such perception cannot be based merely on “sense impressions” at a given moment. Rather, the “inner show” that we perceive embodies certain structural features, based not only on abstractions from immediate sensations, but also on a series of abstractions over a more or less extended set of earlier perceptions. Indeed, without such a series of abstractions we could not be able to see a world having some well-defined order, organization, structure, etc. Even a static environment is effectively presented in the “inner show” as a tentative and hypothetical structure, which when assumed to be invariant, will be compatible with the changing experiences that the percipient has had

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with this environment, in movements that he himself has produced. And an environment which is itself changing is presented in the “inner show” as a structure expressed in terms of invariant states of movement of parts of the environment which account for earlier changing experiences that are not explained by the movements of the percipient.

There may also arise an ambiguity in the attribution of movements to the observer or to various parts of the environment. Thus, if a person is sitting in a train that is not moving, and watches another moving train through the window, he may find that he perceives himself as moving, and that he even gets some of the physical (kinesthetic) sensations of movement. But when he fails to feel the expected shaking and vibration of the train, he begins to look more carefully, and can soon see in the environment certain further clues, suggesting that the other train is moving and that he is at rest. Suddenly his mode of perception of the world changes. This is a striking demonstration of how our perceptions of the world are a construction in the “inner show,” based on the search for a hypothesis that is compatible with all that we have experienced in connection with a certain situation. So we do not perceive just what is before our eyes. We perceive it organized and structured through abstractions of what kind of invariant state of affairs (which may include invariant states of movement) will explain immediate experience and a wide range of earlier experiences that led up to it.

Results of the kind described above led Gibson1 to suggest a new concept of what constitutes perception. He emphasizes the need to drop the idea that perception consists of passively gathering sense impressions, which are organized and structured through principles supplied only by the observer. Indeed, the isolated sense impression is seen to be an extremely high level abstraction, which does not play any significant part in the actual process of perception. Instead, we are sensitive directly to structure of our environment itself. In the last analysis the observer therefore does not supply the structure of his perceptions, so much as he abstracts it. Or as Gibson himself puts this point, the structure of our environment is the stimulus that gives rise to what we perceive (i.e., to the construction in the “inner show” that is presented in our awareness). With regard to optical perception, for example, Gibson points out that through each region of space there passes an infinity of rays of light, going in all directions. These rays of light implicitly contain all the information about the structure of the world that we can obtain from vision.1 But an eye fixed in a certain position cannot abstract this information. It must move in many ways, and at least some part of these movements must be produced by the observer himself, because (as was first brought out by Held and his co-workers) structural information is abstracted mainly from invariant relationships between the outgoing nervous excitations that give rise to these movements and the corresponding ingoing nervous excitations that result from them.

Gibson raises a related set of questions regarding the role of time in perception. A typi- cal question is, for example; “When does a particular stimulus come to an end?” The old- er way of looking at this problem is to refer to what is called the “specious present.” That is, it is found that there is an interval of time, of the order of a tenth of a second, which

1 _{J.G.Gibson, American Psychologist, 15, 694 (1960).}

1 _{The same principle applies to the radio telescope, which is in contact (as it were) with the}

structure of the whole universe, through a similar set of radio waves.

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is “speciously” experienced as a single moment, in the sense that people do not seem to be able clearly to discriminate changes that take place in times less than this. From this notion it would follow that all our perceptions can in principle be uniquely ordered in time, within an accuracy of a tenth of a second or so. Nevertheless, Gibson raises questions which suggest that it is a source of confusion to try to understand the essential features of the process of perception by referring it in this way to such a time order.

To see why Gibson questions the simple time order of perceptions described above, let us recall that we do not perceive momentary sensations, to any appreciable extent. Rather, we perceive an over-all structure that is abstracted from these, a structure evidently built up over some period of time. We have already seen in connection with optical perception, for example, that clues obtained over some time may come together at a given moment and give rise to a new structure of what is perceived. It evidently makes no sense to say that this new structure is based only on the very last clue to be received. Rather, it is based on the whole set of clues. This means that a given stimulus to our perceptions is not restricted to the smallest time interval that can be discriminated. Rather, it may be said that some stimuli take place over much longer intervals.

In music the property of stimuli is much more clearly seen. As one is listening to a tune, the notes heard earlier continue to reverberate in the mind, while each new note comes in. One may suddenly understand (i.e., perceive the over-all structure) of a piece of music at a certain moment in this process. But evidently the very last note to be received is not the sole basis of such an understanding. Rather, it is the whole structure of tones reverberating in the mind. These tones have manifold relationships, which are not restricted to their time order. To grasp these relationships is essential to the understanding of the music. The ef- fort to regard the essential content of the music in terms of its time order could then lead to too narrow a way of looking at the problem, which would tend to produce confusion.

In a similar way one can consider the problem of how one perceives rhythm. At any moment there is only one beat to be heard. But one beat is not a rhythm. Evidently it is the reverberation of a whole set of beats in the mind, all in a certain relationship that constitutes the perception of rhythm. The stimulus that constitutes a rhythm cannot then refer only to a single moment of time. So it seems important to realize that the essential features of perception will not always be understood by stringing out what is perceived in a time order.

Indeed, in many cases it is not possible to assign a unique moment of time to a given feature of what is perceived. While listening to a piece of music one may be appreciating a rhythm that is based on many seconds, a theme that may require a minute or more to be apprehended, and we may be looking at a stop-watch, seeing the movements of the hand that perhaps indicates some fraction of a second. When one says “now,” what does one mean by this? Does it refer to the perception of a certain position of the indicator on the