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Cognitive Psychology

Sensory Phenomena

Sensory Selectivity

Our sensory receptors respond to a limited range of stimuli. Let’s consider vision. The electromagnetic spectrum ranges in wave-length from a billionth of a meter up to more than a thousand meters. Of this spectrum we only see the tiny portion between 400 and 700 billionths of a meter. Our eyes are completely insensitive to everything else. It is beyond our conception to imagine what an X ray would look like or what color it would be. There is a similar selectivity for all the senses. We can hear only a small segment of the frequency range of vibration, we can smell only a few of the molecules that exist, and so on.

An extreme in sensory selectivity is perhaps reached by the wood tick. The female of this species, once mated, climbs onto a branch or twig and stays there, oblivious to all stimuli, until she detects the smell of butyric acid, a component of sweat, which causes her to release her grip. When this happens, hopefully she lands on an animal passing below on which she deposits her eggs. The biologist von Uexkull (1934) was able to keep a dormant tick in his laboratory for 17 years. When, after all this time, she was exposed to the odor of butyric acid, she came to life. It was not so much that the tick was dormant as that no stimuli for her had occurred for 17 years.

Sensory Thresholds

A stimulus must be above a certain threshold if it is to be perceived. It must also be below an upper limit if it is not to destroy the corresponding sense organ. Thus, our senses are really like walls to the world into which are punched very tiny windows or pinholes. If the small black space represents the window, you can see that the white area of the wall is much greater.

After Effects: Positive

Brief stimulation results in a sensation that persists for a fraction of a second after the actual physical stimulus is no longer present. There is an easy way of seeing visual positive after effects. Look straight ahead in a dark room and slowly move a lighted cigarette from side to side at arm’s length. You will see a trail of light produced by the glow of the cigarette. This trail of light, called Bidwell’s Ghost , is a positive after effect; the cigarette does not really leave a trail of light behind it. What you see is a sensory memory from the past.

After Effects: Negative

Prolonged stimulation results in a sensation that is precisely the opposite from the original sensation. This called negative after effect. If you stare fixedly at the black cross for about a minute and then move your eyes to the center of the gray square, you will see a negative after effects, a white cross.

A similar phenomenon can be demonstrated with colors. If you stare fixedly at a red patch of color for a few seconds and then look away at a white surface, you will see a green patch of color, the after effect would have been red.

Contrast Effects

Contrast effects are related to sensory after effects in that they demonstrate sensations produced by the sensory receptor rather than by physical reality. Look at the two patches of gray. Both are exactly the same shade of gray.

Successive contrast refers to the case where an earlier sensation influences a later one. John Locke (1690) introduced the classic demonstration of this phenomenon. Take three empty glasses. Fill one with hot water, one with ice water and the third with lukewarm water. Put your right hand in hot water and left hand in the ice water. After a few seconds you will note that the hot water seems less hot and the cold water seems less cold. This is because of the adaptation of the temperature receptors in your skin. Now plunge both hands into lukewarm water. It will feel cold to your right hand and hot to your left hand. The very same stimulus (the lukewarm water) is producing opposite sensations.

Perceptual Corrections

Several profound perceptual corrections automatically leads us to perceive what we know we should be perceiving rather than something corresponding to our raw sensations. Our perception of external reality is determined not only by reality but also by our organs of perception.

Stability of the Visual World

One of the most basic and easy ways to demonstrate perceptual corrections concerns the stability of our visual world. Why is it that we can see objects moving in the world, but we can move our heads or eyes without the world seeming to move? In both cases- objects actually moving and moving one’s head- patterns of light move across retina in exactly the same way. Obviously, only real movement is sensed as such, but why is this? The differentiation is accomplished by means of a feedback network that allows us to compensate automatically for head and eye movements. Some animals do not have such a compensation mechanism. The housefly’s world moves every time that its head moves. It has no way of discriminating that sort of movement of objects in its visual field. That is why its movement are so jumpy and erratic. You can get a fly’s view of the world by mechanically moving your own eye. Just press lightly on the outside of your eye. In the course of evolution it was adaptive to develop a compensation system for voluntary eye movements.

The Blind Spot on the Retina

Another sort of compensation we make if for blind spot on our retina where the axons of the nerve cells collect to form the optic nerve. At this collection point there are no receptor cells. Thus, any stimulus falling on this point of the retina cannot be seen. Focus your left eye on the X . Now gradually move the book closer to your eyes. At some point the dot to the left of the X will vanish. It is now on the blind spot of the retina where there are no receptor cells. Since you know it is there, you compensate for it. Now focus on the X and move the book toward and away from your eyes. There will be no point at which a gap appears in the bar to the left of the X. This is because the gap is being automatically closed. Since most of the image of the bar falls onto visual receptors, this information is being used to fill in the small gap. That is why we are not aware of the blind spot in everyday life.

Experiments with Distorting Lenses

Studies of the effects of wearing distorting lenses contribute to the view of perception presented by cognitive psychology. Stratton (1897) designed glasses that reversed the visual field from right to left and from top to bottom. As might be imagined, when he put on these glasses, tasks involving vision became tremendously difficult. Pouring a glass of water became a major operation, since the water seemed to flow upwards. Also the world lost its stability. Head and eye movement made the world swing violently. Since the glasses inverted right and left, the built-in correction malfunctioned. They now exacerbated rather than corrected for the effects of head and eye movements. The thing that is really surprising is that Stratton adapted to his new world rather quickly. After a week he was able to get around fairly well, and his visual world no longer seemed strange. This adaptation would be difficult to explain, if the brain were receiving anything remotely resembling copies of the images that fall on the retina.

An even more striking experiment involving distorting lenses was done by Ivo Kohler (1962) . He designed glasses with lenses that were blue on the left half and yellow on the right half of each lens. Whenever subjects put on these glasses, leftward eye movements would cause the world to appear bluish while the rightward eye movements, it would assume a yellowish tinge. Kohler wanted to see exactly how blue or yellow the world looked after his subjects had worn these glasses for a long time. To do this, he designed an apparatus that presented a uniform gray visual field to the subject, whose task was to adjust a knob that optically added blue or yellow to the field until it appeared gray. On the first day of the experiment, without the goggles, subjects made no adjustments, since the field was gray to start with. With goggles, when they looked left while viewing the field, it appeared blue. So they had to add yellow to bring it to the desired neutral gray color. When they looked right, they had to add blue to bring it to desired neutral gray color. Kohler has his subjects wear these glasses continually for 60 days.

At the end of the time the subjects said they did not really notice the glasses, which at first had been disorienting and rather annoying. What bothered them was taking off the glasses! Tests with the visual-field apparatus revealed an amazing finding. Subjects without glasses no longer saw the gray field being gray. When they looked left, it looked yellow to them, and when they looked right , it looked blue. Tested with the glasses the field looked gray no matter which way they looked. Clearly, the subjects had somehow compensated for the glasses. Whenever they looked to the left through the blue glass they subjectively added yellow to cancel the effect of the blue lens. On the other hand, an opposite strategy was used when they looked to the right. The result was that the world looked quiet normal with the glasses. Proprioceptive cues from moving the eyes has become a stimulus in a learned feedback loop involving color perception. This loop must be similar to the one that keeps our world from jumping when we move our eyes. The surprising thing is that such a basic biological process as color vision could be affected by experience or learning . To be learned or conditioned, something must be a response. If we can condition it, perception must be an active response rather than just a passive input.

Conditioning Perceptual Responses

An experiment of Hefferline and Perera (1963) illustrates a more direct conditioning of perception. In this experiment a small involuntary muscle twitch by the subject was detected with an electromyography. Whenever such a twitch was detected, a tone came on. The subject’s task was to press a key whenever the tone was heard. Gradually the tone was made fainter and fainter and eventually it was omitted. Subjects not only continued to press the key at the appropriate time, but they also reported that they still heard the tone. The subjects knew that a faint tone followed each muscle twitch. Accordingly they “heard” what they knew were supposed to hear even after the experimenters had tricked them by turning off the tone altogether. Obviously the copy theory cannot explain this, since there was no reality to be perceived.

Perceptual Constancies

Perceptual constancies enable us to perceive what we know we should be seeing rather than anything resembling the pattern of light falling on the retina. That is, the percept resembles our idea of what reality must be more closely than it resembles the sensation that reality has engendered. We see a thing as maintaining its proper size, shape and color regardless of the angle or distance from which we view it. The perceptual constancies allow this. As with the other mechanism we have been discussing, the constancies can also be brought into play by various perceptual illusions. In the case of such illusions we are correcting sensory impressions that should not be corrected.

Size Constancy

The size of the retinal image of an object varies directly with the distance of the object. The more distant any given object is, the smaller will be its size on the retina. A six-foot-tall man standing 50 feet away subtends a smaller retinal angle than your thumb held a few inches in front of your eye.

However, the man is perceived as being his true height. In this case we know quiet well that a man can’t be an inch high. So we do not see him that way. It never occurs to us that we are being confronted by a person only an inch tall. I think you will agree that your perception of distant objects is automatic. We do not consciously compute a correction equation. The correction is “wired into” our perceptual apparatus.

Correct perception of the size of distant objects does not even depend on prior knowledge of exactly how large the object really is. A number of experiments show this. Let’s say that we show a subject a 36-inch rod, 24 inches in front of his eyes. The person’s task is to pick out of a set of rods 12 feet away the one that is exactly the same size as the 24-inches rod. Normal subjects can perform this sort of task with virtually no errors. If they are choosing on the basis of retinal size, they would choose an 18-foot rod! How do they do this? Apparently they do it by an unconscious estimate of distance based on cues or hints regarding linear perspective (the convergence of parallel lines), aerial perspective (distant objects are hazy), interposition (an object apparently obscured by another object is perceived as more distant), and so on. Removing those cues by having the subject match luminous rods in a completely dark room destroys size constancy. With environmental cues the subject in our hypothetical experiment will correctly pick a 36-inch rod, while with no such cues the 18-foot rod will probably be picked.

Other Constancies

When you see a half-open door, you perceive it as being rectangular. It never occurs to you that someone has invented a rhomboidal door, even though the pattern on your retina is not at all rectangular. This represent the operation of space constancy-the tendency to perceive objects as being the shape we know them to be regardless of the perspective from which we view them. A related phenomenon is seen with brightness constancy. The pages of this book look white to you even if you happen to be reading it in deep shade. Unless all context is cut off and you do not know that what you are seeing is white paper, it is very difficult for you not to see the page as being its true brightness. Color constancy is similar. It is very difficult to see the leaves of a tree as any other color than some shade of green even though local conditions of light may make their actual color blue or yellow or purple.

Illusions Involving Constancy

The constancies are very basic and automatic but they develop gradually in children and thus seem to be based upon some degree of learning. Turnbull (1961) reports on what happened when pygmies who had spent their whole lives in dense forest were first taken into an area consisting of open grasslands. A herd of buffalo was seen in the distance. The pygmies took them to be insects because of their small size. They refused to believe that such tiny creatures could be buffalo. As the herd was approached, the size of the buffalo naturally increased. The pygmies could only attribute this to witchcraft. They had had any experience in seeing distant objects, since the forest restricted their line of sight to a few feet. As as consequence, they had not developed size constancy. Just the opposite experience can also occur. Edgar Allan Poe’s short story, “The Sphinx,” concerns a man looking out a window and seeing a horrible creature of monstrous size crawling slowly along the distant horizon. Suddenly he realizes that in reality he is observing a tiny insect crawling along the window pane a few inches in front of his eyes. The apocalypse is transferred into a mistake in size constancy.

Castenada (1971) reports a similar encounter with the “guardian of the other world.” The guardian was perceived as a huge and grotesque animal seen approaching him from a distance. It really turned out to be a gnat a few inches in front of his eyes. With a little effort and imagination we can achieve this sort of effect consciously. The same is true of shape constancy. We can almost see the door as being really rhomboidal. Other constancies, such as those involving brightness or color, are much more difficult to overcome.

Art as a Disintegration of the Localization Constancy

The cubist painters(a style of painting developed in the early 20th century) often depicted objects as viewed from several simultaneous perspectives. It is as if they did not combine momentary views of the world into one final integration from one stable viewing point. In normal vision world with momentary eye fixations and then automatically integrate them into a total picture. Cubist objects arise from omitting the final integration. When you look at a visual scene, you seem to see it all at once. This is not really the case. This person’s subjective experience would have been one of seeing the picture in a single glance. During any one fixation, however, only a small area around the fixation point would be clear. The rest of the picture would be in the peripheral field of vision and would actually be blurred. The fact that the picture seems subjectively to be seen all at once cannot be attributed to mere persistence of the clear sensations arising from each fixation. This would actually result in a completely blurred image because the persisting images would be superimposed on one another in a chaotic fashion (Hochberg 1972). What is perceived is thus, in fact, a reconstructed theory in the brain rather than an image based directly on retinal sensations.

The paintings of the abstract expressionists resemble the chaotic series of retinal sensation upon which perception is ultimately based. Ehrenzweig says that these paintings represent a complete disintegration of the “thing facade” that we impose on the world. Objects disappear and we are left with only the sort of patternless chaos that is produced by splashing paint on a canvas more or less at random. In a certain sense we could argue that the paintings of the abstract expressionists represent the chaos of sensation as it exists on our retina at any second in we reconstruct the perceived world of objects much as a sculptor constructs his statue from memory.

Perceptual Structuring

Gestalt Principles

Gestalt psychologists have devised a number of experiments which demonstrate that we have built-in ways of structuring or organizing our perceptions. One of the most basic laws governing perception is that we automatically structure any input into figure and ground. Although we use environmental cues to do this, figure and ground are something we impose on the world rather than vice versa. For example, if you stare at Figure 7 you will see that it looks like two faces for a while, then suddenly shifts to looking like a vase, then back to two faces, and so on. Obviously, reality (the picture) is not changing. The only thing left to explain the variance in perception is your visual system.

It automatically, and quiet unconsciously, structures its input into figure and ground. Since in this case the environmental cues are ambiguous, figure and ground are not constant. Because the world does not come to us automatically structured into figure and ground or objects and backgrounds, there must be a set of implicit rules that our perceptual organs follow to so divide it. The Gestalt psychologists were interested in determining those rules. One of them is the principle of “closure.” In figure 8A you see a square. In so doing you are closing up small gaps between each dot and also the larger gap on the left.

If someone asked you what you had seen in a day or so you would probably remember having seen just a square unless you had made a special effort to do otherwise. Another perceptual rule is the principal of “similarity.” In figure 8B you are likely to see columns of circle and squares, this is because similar percepts tend to be perceived as groups. In Figure 8C you perceive three sets of circles rather than just a number of unrelated circles. This is a demonstration of the principle of proximity. Stimuli that are close together are perceived as being grouped.


It might be argued that our senses give us a limited and distorted view of reality but that they do, after all, give us some idea of external objects that compose reality. It can be said, however, that things or objects are created as much by our mind as by reality. The theorist Ernest Schachtel (1957) has dealt with what he calls “objectification,” the tendency to perceive the world as structured into independent objects. We take it for granted that the environment is, in fact, so structured and forget that our organs of perception contribute as much if not more than the world to this view. Schachtel points out that structuring reality in terms of objects is only one of the modes of perception. He calls it “allocentric” or object-centered perception. With this type of perception the organs of perception structure the world into objects. Schachtel terms the other mode of perception “autocentric” or subject-centered. Here there is little or no onbjectification. In the adult the higher senses – vision and hearing – operate allocentrically while the lower senses – taste, smell , body awareness – Schachtel, the human infant originally apprehends the world autocentrically through all of its senses. Only gradually, over the course of time, does the child begin to structure his world in an allocentric way.

Perception in the autocentric mode is objectless. Think of taste. You do not taste an object in the sense that you see and object. Rather, there is just the taste. Of course, you may later consciously infer that the taste came from a particular source. But the point is that this inference is carried out automatically and unconsciously with an allocentric sense such as vision.

A second difference between allocentric and autocentric modes is that the latter is more closely related to feelings of pleasure or pain and to reflex- like responses. Compare, for example, seeing a rotten object as compared to smelling or tasting it. In the latter case there is an immediate feeling of unpleasantness and a reflexive withdrawal. Vision and hearing are thus more neutral and abstract . Another example would be touching a hot stove as to merely hearing about it. In the latter case, you have to imagine the pain, since it is not an immediate consequence of the sensation.

Another difference between the two modes concerns what Schachtel calls felt-organ localization. In the allocentric sense, such as hearing and seeing, we project the object outwards. We hear or see something where we know it is, not where it is sensed. We do not see things on our retina or feel sounds in our ears unless the sensations are extremely intense. But we feel touches, smells and tastes right at the receptors. All of our sensory receptors receive some sort of stimulation, translate this into firing of nerve cells, and finally set up a pattern of nervous activity in the brain. With the allocentric senses we automatically construct models of what we know is causing the sensations and locate it where we know it is coming from. This is the basis for Schachtel’s idea of objectification. There is a built-in inferential leap that leads us to postulate objects and to structure sensory patterns into objects.

Perception as Categorization

We have seen that to perceive is actually to structure or objectify raw sensory input. Another way of putting this is to say that it involves categorization. This can go to rather striking extremes. Schachtel calls this tendency secondary autocentricity. He says that people, especially adults, tend not really to look at things in their own right but merely to classify them in terms of a label or a use. In other words we do not use the world in the fresh, vivid way that a child does. We tend to name or label things and file them away without really looking at them or enjoying them. The remark attributed to Ronald Reagan, “When you have seen one tree, you have seen them all,” is a good example of this tendency in most of us. The idea that perception is categorization is supported by an experiment conducted by Bruner and Postman (1949). Subjects were briefly shown playing cards, such as those illustrated in Figure 9. They merely had to repost what cards they saw2. Look at these cards in the figure.

Do you see any reason why this may be an interesting experiment? Chances are that it took some time to notice-if you noticed at all-that one of the cards is anomalous. It is the black six of hearts. As you might predict from your own experience, subjecsts, even with a long exposure period, “saw” a six of spades when they were shown this card. Unless they were specifically warned ahead of time that some of the cards might not be exactly what they seemed to be, nobody saw a black heart for what it really was. In seeing something, we sample a little bit of the available information, make our categorization and let it go at that.

Everyone has had the experience of seeing something in the twilight that upon closer examination turned out to be something totally different than what it initially seemed. This soon became an issue for experimentation. Subjects were tachistoscopically shown unfocused pictures. The picture was brought into clearer focus on each successive presentation. After each presentation, the subject was encouraged to guess what the picture was. It turned out that, if subjects made early incorrect guesses, it prevented them from correctly recognizing the picture even when it was focused enough so that no-guessing subjects had no difficulty at all in recognizing it. The early hypothesis or structure imposed on the picture actually prevented or delayed recognition.

This sort of effect can be explained with Jean Piaget’s notion of the schema (1950). For Piaget perception of the world involves an assimilation of stimuli to internal schema, models or categories. We assimilate inputs to pre-existing schema, if at all possible, and then pay relatively little attention to them. In other words an input somehow activates a pre-existing internal representation. This means in a very real sense that perceiving is remembering. Only if an input is quiet different from our expectation do we have to accommodate our schema to the world. Then we really tend to focus on the input and build a new schema to accommodate or fit it.

Summary and Conclusion

With all the empirical evidence cognitive psychology gives we can only repeat :

  • What we perceive is not an exact copy of external reality but neither is it wholly created by our senses. Organs of perception, as well as reality, determine what we perceive.

  • Common sense tells us that our senses to bring as much information as serve to bring as much information as possible into consciousness. But it turns out that it is more profitable to take just the opposite view point. Our sensory receptors really serve to reduce, filter and exclude as much data as possible.

  • It might be argued that our senses give us a limited and distorted view of reality, but that they do after all give us some idea of the external objects that compose reality. It can be said, however, that things or objects are created as much by our mind as by reality.

Anil is an internationally certified NLP Master Practitioner and Gestalt Therapist. He has conducted NLP Training in Mumbai, and across 6 other countries. The NLP practitioner course is conducted twice every year. To get your NLP certification


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