In order to answer this question we have to go back to the concepts of sensation and perception. Let us start with sensation. Our bodies, and for that matter most living things, have special cells called sensory receptors. They can detect light, sound, temperature, pressure, odor, taste, pain, pressure, balance, etc. Most of these receptors are on or near the surface of the body such as our eyes, ears, tongue, etc., but some are internal. Thus, we sometimes experience a sense of pain with a toothache, headache, stomachache, etc.
Each sensory receptor detects its own special form of energy and transmits a signal to the brain. The reception of the signal in the brain represents sensation. How the brain interprets these signals and makes them meaningful is called perception. Most of the time the interpretation of the received energy is consistent with it. Sometimes, however, our interpretation is incorrect. These misinterpretations are called illusions. When we witness an illusion, we perceive something that does not correspond to what is actually out there—what exists in the real world. Illusions fool us. They convince us of things that are not true. Dictionary definitions of illusion usually state that an illusion is a sensory perception that causes a false or distorted impression, or a misrepresentation of a “real” sensory stimulus. While these definitions are correct, psychologists have also included a number of phenomena where there is no necessary incorrect interpretation of a stimulus but rather that there are two or more quite different interpretations from a single stimulus, but never more than one at a given moment. We will give examples of such illusions when we discuss visual illusions called figure-ground illusions and ambiguous figures. In these cases two or more different interpretations of a figure can be seen, but never at the same time.
The interesting thing is that we often seem to enjoy being fooled in this way! Magicians use illusions all the time. In fact, magicians are sometimes referred to as illusionists. Famous magicians, like the great Harry Houdini, admit that what they do is create illusions. They do not do the impossible, they just seem to do it.
Illusions are different from both hallucinations and delusions. Illusions are misperceptions that are perceived by most people, and are based on a specific stimulus received under certain conditions. Some experiments with animals indicate that several species of mammals and birds are “fooled” by illusions in much the same way we are.
Hallucinations are usually seen by only one individual. Most often they are experienced by people who suffer from specific kinds of mental illness, or who are influenced by drugs or extreme amounts of alcohol. Hallucinations are false perceptions that occur in the absence of appropriate external stimuli, whereas illusions are misinterpretations of external stimuli that are, in fact, present.
Delusions are different from both illusions and hallucinations. They are beliefs, not perceptions. Like hallucinations, they tend to be found in people who are mentally ill. A person may have delusions of grandeur (believing that he or she is a very important or famous person) or delusions of persecution (believing someone or something is out to harm them) when the facts clearly do not support these beliefs.
Many common perceptions involve illusions although people are not aware of it. That is, much of what we perceive does not correspond to the stimulation of our sense organs. Thus, for example, we do not see a person who is walking away from us as getting smaller and smaller, even though the image in our eyes rapidly decreases in size. We also get the illusion of depth in paintings, stereoscopes and holographs, even though these are presented to us on two-dimensional surfaces. Another good example of an illusion which we simply take for granted is the motion picture. Actually there are two illusions involved when we go to see a movie. The first is that there is really nothing moving as we experience the film. That is not quite correct. What is moving is a series of still photographs on a reel of film. Each is exposed for only a very short time and our eyes and brain to not see the separate still shots but see figures on the screen moving quite naturally. The second part of the movie illusion is the sound. When an actor speaks we fully accept that the words are coming from his or her mouth. The fact is that the sounds are actually coming from speakers well off to the side of the screen and possibly even in back of us. Yet as the actor walks across the scene we accept that the words are coming from his or her mouth from a different spot on the screen—a misperception, and therefore an illusion.
Perception may also be distorted in other ways. One such distortion results from what is called selective perception. Selective perception is a result of personal factors on perception. What a person perceives often reflects that person’s past learning and present state of mind, as well as what is actually “out there.” A Republican and a Democrat who listen to the same political speech will “hear” and remember different things. If you ask them about it afterwards, it may be hard to believe they listened to the same speech.
The sandlotscience web site is devoted to visual illusions and these are often the ones most people think of when the subject comes up. We will discuss these later. However, we will give you a few examples of illusions involving senses other than vision.
One of the oldest known illusion related to touch was described by Aristotle more than 2,000 years ago. If you cross two adjacent fingers and then touch an object such as a pen, with both crossed finger tips at the same time it will feel as though you are touching two pens, not one.
An illusion involving temperature can be seen if you touch a piece of silverware at a dinner setting and touch the table cloth. You think that the silverware is cooler than the cloth when, in fact, they are both at room temperature. This is because metal conducts heat away from your finger more rapidly that cloth does.
Some illusions can actually be dangerous to us. Our sense of equilibrium or balance is located in the inner ear but it works closely with our visual world. When the pilot of an aircraft is flying at night or in a cloud and has no visual reference points it is possible for the pilot to become disoriented. He or she cannot tell whether the plane is gaining or loosing altitude, or turning left or right. This is called vertigo. It is an illusion, and pilots are trained to never rely on their sense of position but to respond entirely to the plane’s instruments.
One final non-visual illusion we will describe is called the size-weight illusion. You can demonstrate it to yourself and your friends.
Take a small attaché case, a small overnight bag, and a large suit case. After you have determined each one’s weight, fill the attaché case with some heavy material such as books. After you have determined its weight, put books in both the overnight case and the large suit case so that all three weigh the same amount. Ask someone to lift each case and tell you which is the heaviest. We virtually guarantee that the small attaché case will be judged to be much heavier than the others, even though they both weigh exactly the same amount. You can to the same thing with cans holding 8 ounces, 16 ounces and 32 ounces filled with dirt or pebbles so that their weights are all the same.
In part, this illusion is explained by our expectations. When we see the largest container we prepare our muscles to lift something heavy. When it is only filled to a portion of its capacity, it goes up easily. On the other hand, we don’t expect to have a particularly difficult time lifting the small container, and we are surprised by its weight. Even though you prepared each container yourself, the effect is so strong that you too may have difficulty accepting that they all weigh the same.
The Sandlotscience home page is devoted to optical illusions. Many different kinds are recognized and grouped together. At the beginning of this paper we indicated that some illusions are not misinterpretations of sensory stimuli but rather developing two or more interpretations of a single figure. What this web site calls “ambiguities” present examples of such illusions. The lines and shapes of an image have two different meanings depending on which perception you have at the moment. Thus, the picture “All is Vanity” can either be seen as a woman looking into a mirror, or a skull. The back of the woman’s head and her face as seen in the mirror become the skull’s eyes. In the case of the old woman/young woman faces called “My Wife and My Mother-in-law” also has two interpretations. What is the young woman’s chin is the tip of the old woman’s nose. Once you see both interpretations, you cannot see only one of them. The other will keep “popping” into your vision from time to time. In many cases the ambiguity is simply whether you see the image as facing right or left as in the figure by Joseph Jastrow which can be seen as a rabbit facing right and a duck facing left.
A similar group of visual illusions is called figure-ground illusions. This group is generally similar to ambiguous figures but the two different interpretations depend on whether you are focusing on the printed part of the figure or its background. Usually when we look at a printed page or a computer screen, the figure is much smaller than the background. It is usually darker and usually has a more precisely defined shape. When the figure and its background are more evenly balanced it is possible to look at the figure by itself, and then see the background as a different but recognizable image. Again, one cannot hold one interpretation without the other intruding. A classic example here is the well known Ruben’s Vase which can either be seen as a vase or the profiles of two people looking at one another.
While somewhat different, figures included in the page on Impossible Objects tend to have two different meanings. In the Devil’s Fork or Trident shared lines at one point may suggest two prongs at one end of the figure yet their role changes as one moves to the opposite end when three prongs appear. As the page notes, we can see such figures, but they cannot be constructed except as they are perceived from a given angle. Again, this is not a false perception, but the meaning of a given line changes in a given context.
In the page dealing with Distorted Illusions we come closer to false perceptions. For most of the examples these distortions occur because there are conflicting or competing images. Thus, in the Perspective Illusion, the two red balls seem to be quite different in size when in fact they are identical. The illusion is caused because of other information about depth perception. The lampposts get smaller toward the back of the picture and what we see as parallel lines converging in the distance (as with railroad tracks) suggests depth. We know that objects far away from us seem smaller than they would if they were closer. Since the higher of the red balls seems to be further away as suggested by the other cues, the fact that it is physically the same size seems impossible to us and we see it as larger making the entire picture satisfactory to our senses. Something similar happens when we look at the Top Hat figure. The top of the hat seems to be much taller than the width of the brim yet the height and width of the figure are the same. As the page says, the vertical part breaks the horizontal part in half. In addition, there are physical cues. Our eyes move horizontally much more easily that when we look up and down. The different muscle tensions suggest that the distance our eyes must cover is greater for the top of the hat than its brim. In addition to these examples of the real stimulus being perceived incorrectly there are many examples of perfect squares, circles and straight lines being distorted when they are placed on backgrounds of competing lines. Our eyes are simply not able to ignore the background lines and the “target” square or circle appears distorted even when their geometry is perfect.
Another false perception can be seen in what the page calls Kanzia figures. Here we see complete objects in the background of black circles with small openings. The figures do not exist, but our brains make the space more meaningful if we complete a whole picture.
In the page of Typographical Illusions, the ambigrams give us a good example of how the mind is able to ignore what it chooses to ignore to make something more meaningful. When the artist, John Langdon, cleverly creates letters which look the same upside down as right side up and has to dot the “I” in “Ambiguity” we see it correctly from one perspective but conveniently ignore the dot below the figure when it doesn’t suit us for a meaningful perception.
A final example of a class of illusions we will deal with is after effects. In the page on after images on this site there are several examples of what happens when you stare at a figure for a period of time. Certain light receptors in the back of the eye (the retina) are more sensitive to certain light wave lengths (i.e. colors) than others. When you fixate your eyes on a particular color these receptors become “fatigued.” When you shift your gaze to a neutral source of light they tend to under respond while those receptors which were not sensitize by the prior stimulus respond appropriately. Without the balance of both kinds of receptors, those which were not stimulated dominate in the visual part of your brain and you see the opposite color. Thus, in the example of the flag, the green is seen as red, the yellow as blue, and the black as white producing an image of the American flag.
After images are not limited to the role of cells in the retina. There are motion aftereffects as well. After viewing one direction of motion for several minutes, people experience illusory motion in the opposite direction. This is sometimes known as the waterfall illusion since it is often seen by people staring at waterfalls. As your eyes follow the falling water, the muscles controlling them are pulled down. When you shift your gaze to a stationary object, a tree for example, the muscles pull back and the tree seems to go up for a brief time. You can see the motion after effect in the page on After Effects in what is called the Andrus Spiral Illusion . When the spiral spins, your eye muscles are drawn toward the center. When you shift your gaze to the target image to the right, the muscles pull out and the target image seems to expand. You can experience motion after effect in some television programs. Many programs have a “crawl” along the bottom of the screen providing you with information. The next time you see a crawl try to not read the words but fix your eyes on the center of the screen. When you shift your gaze to a stationary object it should seem to move to the right for a second or so as your eye muscled pull back.
People tend to be fascinated by illusions, but as we noted earlier, not everyone responds to every illusion. There is no simple explanation for this variability in perception. There is no evidence that these differences are related to intelligence or personality, although experience with them often helps one see new versions of the same kind of illusion. Thus, you or your friends should not be upset if an illusion is not apparent to you. There are many of them and we can enjoy all that we can perceive.