VII. Read the text “Illusions” and say what is the difference between illusion and hallucination?

Illusions

Illusion is a misinterpretation of a “real” sensory stimulus – an interpretation that contradicts objective “reality”. For example, a child who perceives tree branches at night as if they are goblins may be said to be having an illusion. An illusion is distinguished from a hallucination, an experience that originates without an external source of stimulation.

Illusions are special perceptional experiences in which information arising from “real” external stimuli leads to an incorrect perception, or false impression, of the object or event from which the stimulation comes. Some of these false impressions may arise:

• from factors beyond an individual’s control (such as the characteristic behavior of light waves that makes a pencil in a glass of water seem bent);

• from inadequate information (as under conditions of poor illumination);

• from the functional and structural characteristics of the sensory apparatus (distortions in the shape of the lens in the eyes).

Such visual illusions are experienced by every sighted person.

Another group of illusions results from misinterpretations one makes of seemingly adequate sensory cues. In such illusions, sensory impressions contradict the “facts of reality”.

In these instances the perceiver makes an error in processing sensory information. The error arises within the central nervous system (brain and spinal cord); this may result from diverse sensory information, psychologically meaningful distorting influences, or previous expectations.

For example, the driver who sees his own headlights reflected in the window of a shop may experience the illusion that another vehicle is coming toward him even though he knows there is no road there.

Numerous optical illusions are produced by the refraction (bending) of light as it passes through one substance to another in which the speed of light is significantly different.

For example, rainbows result from refraction. As the sun’s rays pass through rain, the droplets separate (refract) the white light into its component colors.

Another illusion that depends on atmospheric conditions is the mirage, in which the vision of a pool of water is created by light passing the layers of hot air above the heated surface of a highway. In effect, cooler layers of air refract the sun’s rays at different angles than do less dense strata of heated air, giving the appearance of water where there is none.

Visual illusions include a variety of contrast color phenomena. A successive contrast occurs when after one has stared at red surface, a green surface looks much brighter.

As one enters a dark room from bright sunshine, the room at first seems quite dark by contrast. If a gray piece of paper is placed on a black background, it looks whiter than it did before; if placed on a white background, it looks darker.

Some illusions depend on perceiver characteristics such as brain function. When an observer is confronted with a visual assortment of dots, for example, the brain may group the dots that “belong together”. There groupings are made on the basis of such things as observed similarity (f.e. red versus black dots), proximity, common direction of movement, perceptional set (the way one is expecting to see things grouped), and extrapolation (one’s estimate of what will happen based on an extension of what is now happening).

Time-induced error is an example of a Gestalt illusion that occurs over brief time intervals. Two images of the same line, will appear to differ in length if they are flashed quickly one after the other.

Closure is the illusion of seeing an incomplete stimulus as though it were whole. Thus, man unconsciously tends to complete (close) a triangle or a square with a gap in one of its sides.

In watching movies, closure occurs to fill the intervals between what are really rapidly projected still pictures, giving the illusion of uninterrupted motion.

The “figure and ground” illusion is commonly experienced when one gazes at the illustration of a white vase the outline of which is created by two profiles. At any moment one will be able to see either the white vase (in the central area) as “figure” on the black profiles on each side (in which case the white is seen as “ground”). The fluctuations of figure and ground may occur even when one fails deliberately to shift attention, appearing without conscious effect. Seeing one aspect apparently excludes seeing the other; younger people perceive these changes more readily than do their elders.