An illusion is a false perception, a perception that does not fit an objective description of a stimulus situation. An illusion is usually associated with a particular sense. Consequently, there are optical illusions, auditory illusions, and so forth. Illusions tend to be remarkably stable. They affect most normal observers in the same way. For example, for almost all of us the Moon is perceived to be larger when low and near the horizon than when it is high and overhead.
It is important to distinguish the concept of an illusion from a delusion and a hallucination. A delusion is a false belief. If Ray, a schizophrenic mental patient, believes that he has an eye with X-ray vision on the back of his head, this is a delusion.
A hallucination is a perception created by the individual. It has no relationship to reality at all. If Ray sees and hears an invisible companion that nobody else can see or hear, this is a hallucination. Illusions are thought to be normal and experienced by most of us. Delusions and hallucinations are thought to be abnormal and experienced in an idiosyncratic fashion. Illusions teach us that perceptions are. to some extent, created by the brain and nervous system, that we are not passive observers of our world. Let’s return to figureground perception. We perceive the relationship between a figure and its associated ground as being a fact about the world itself. But is it? The vase-faces illusion can be perceived in two different ways. It can be seen as
a vase. Or it can be seen as two profiles facing each other. When seen as a vase, this becomes figure and tends to stand forward a little in perception. The faces disappear and become absorbed into a receding ground. When seen as two laces, these become figure, and both tend to stand forward a little in perception. The vase disappears and becomes absorbed into a receding ground. These two different perceptual alternations wiU take place for most observers on a predictable basis. Also, it is impossible to simultaneously perceive both organizations. All of this suggests that figure and ground are organizing tendencies linked to perception, not facts about the external world.
How can the vase-faces illusion be explained? Here is one approach. The vasefaces drawing is said to be ambiguous, meaning that it can be perceived in more than one way. The process of attention, characterized by a tendency to focus on some stimuli and ignore others, determines that one organization will be temporarily favored over another. Let us say that the first organization favored is the vase. The region of the brain being stimulated by the vase organization becomes satiated (“overfilled”; with the vase organization. It spontaneously rejects it for a second organization, one that is briefly refreshing. The satiation hypothesis suggests that the brain tends to reject excessive stimulation of one kind and tends to seek novel stimulation of another kind. Ambiguity, attention, and satiation are factors that all work together to produce the fluctuations in perception that take place when one experiences the vase-faces illusion.
Returning to the Moon illusion, why does the Moon appear larger on the horizon than when it’s overhead? The illusion is a variation of the Ponzo illusion, an illusion associated with linear perspective. Parallel lines, like those associated with railroad tracks or the sides of a roadway, appear to converge as they approach the horizon. At the horizon itself they meet, and this is called the vanishing point. If in a drawing two objects of the same size are simultaneously placed so that the first object is far from the horizon and the second one is near the horizon, the second object wiU be perceived as being larger than the first one. This is because, in a drawing, the retinal size of both objects is the same. However, the second object seems to be larger than it is in terms of comparisons we automatically make with other objects near the horizon.
Note that in everyday perception the Ponzo illusion does not occur. This is because the retinal size of an object near the horizon is smaller than that of an object closer to you. When the size of an image projected on the retina shrinks with distance, the apparent size of the object remains the same. This is a perceptual phenomenon called size constancy. For example, an approaching friend first seen when twenty feet away and then when closer to you appears to be the same size. However, in the case of the Moon illusion, the size of the Moon''s image projected on your retina is about the same size when it is near the horizon and when it’s “far” from it (when it’s overhead). As the Moon orbits our planet, its actual distance from the Earth doesn’t change significantly. Consequently, the conditions of the Ponzo illusion are met.
What we learn from illusions is that the world appears to us the way it does not only because it actually is the way it is. We also interpret sensory information, transforming it into a constructed perceptual, or psychological, world. And it is our perception of the world that determines much of our behavior.