ILLUSION

Dr. Debjani Datta, Principal

Edited by: Anis Ahmed

 

Illusion, a misrepresentation of a “real” sensory stimulus—that is, an interpretation that contradicts objective “reality” as defined by general agreement. An illusion is a distortion of the senses, which can reveal how the human brain normally organizes and interprets sensory stimulation. Illusion refers to a specific form of sensory distortion. An illusion is distinguished from a hallucination, an experience that seems to originate without an external source of stimulation. Illusions are special perceptual 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 , from inadequate information, or from the functional and structural characteristics of the sensory apparatus.

Types of illusion: Different types of illusion are described below:-

Optical: An optical illusion or visual illusion is characterized by visually perceived images that are deceptive or misleading. Optical Illusions can use color, light and patterns to create images that can be deceptive or misleading to brains. The information gathered by the eye is processed by the brain, creating a perception that in reality, does not match the true image. Perception refers to the interpretation of what we take in through our eyes. 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. A ray of light passing from one transparent medium (air) to another (water) is bent as it emerges. Thus, the pencil standing in water seems broken at the surface where the air and water meet; in the same way, a partially submerged log in the water of a swamp gives the illusion of being bent.

Another illusion that depends on atmospheric conditions is a mirage, in which, for example, the vision of a pool of water is created by light passing through layers of 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; nearby objects may even appear to be reflected in it.

Some illusions are related to characteristics of the perceiver, namely the functioning of the brain and the senses, rather than to physical phenomena that distort a stimulus. Many common visual illusions are perceptual: they result from the brain’s processing of ambiguous or unusual visual information. Other illusions result from the aftereffects of sensory stimulation or from conflicting sensory information.

When an observer is confronted with a visual assortment of dots, the brain may group the dots that “belong together.” These groupings are made on the basis of such things as observed similarity (e.g., red versus black dots), proximity, common direction of movement, perceptual 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).

Some of the examples of such illusion are:

 

Hermann Grid Illusion

In the Hermann Grid Illusion, the white dots at the center of each square seem to shift from white to gray.

Figure-ground illusion

 The “figure-ground” illusion is commonly experienced when one gazes at the illustration of a white vase, the outline of which is created by two black profiles. At any moment, one will be able to see either the white vase (in the centre area) as “figure” or the black profiles on each side (in which case the white is seen as “ground”). The fluctuations of figure and ground may occur even without conscious effort. Seeing one aspect usually excludes seeing the other.

 

Auditory : An auditory illusion is an illusion of hearing, the listener hears either sound which are not present in the stimulus, or "impossible" sounds. In short, audio illusions highlight areas where the human ear and brain, as organic, makeshift tools, differ from perfect audio receptors (for better or for worse). In auditory illusions, the human brain thinks that it can hear something that is either not “there” or exists in a very different form to how it is perceived. Normally, we rely on being able to translate the world around us into accurate sense perceptions. But in certain circumstances, the relationship between stimuli and senses breaks down, leading to some interesting results.

The way we visualize the world has an important impact on the things that we hear. This is neatly illustrated by one of the most famous auditory illusions: the McGurk Effect. Named after one of its discoverers (the British psychologist Harry McGurk), the effect is particularly common in conversation. It turns out that the way words are formed by the speaker is as important in how they are perceived as the sound that they make. So, if you shape your mouth in two different ways, but say the same word, viewers may well hear two separate sounds.

 

Another  example of an auditory illusion is a Shepard tone.

 

Tactile: A tactile illusion is an illusion that affects the sense of touch. Tactile illusions are found when the perception of a quality of an object through the sense of touch does not seem to be in agreement with the physical stimulus. They can arise in numerous circumstances and can provide insights into the mechanisms subserving haptic sensations. Examples of tactile illusions include phantom limb, the thermal grill illusion, the cutaneous rabbit illusion and a curious illusion.

One of the oldest tactile illusions is the Aristotle illusion. It is easy to perform. Cross your fingers, then touch a small spherical object such as a dried pea, and it feels like you are touching two peas. This also works if you touch your nose. This is an example of what is called “perceptual disjunction”.

Yet another example of vision influencing touch occurs in patients with phantom limbs. After amputation of an arm, the vast majority of patients continue to feel vividly the presence of the missing arm, a phenomenon termed phantom limb in the late 1800s by physician and author Silas Weir Mitchell. Many people report that their phantom limb is frozen, paralyzed in a constant or fixed position, and that this experience is sometimes painful.

Temporal: A temporal illusion is a distortion in the perception of time that occurs for various reasons, such as due to different kinds of stress. In such cases, a person may momentarily perceive time as slowing down, stopping, speeding up, or even running backwards, as the timing and temporal order of events are misperceived.

In a wonderful book, “Brain Bugs”, the author, Dean Buonomano, writes on temporal illusions.  

“On the much shorter scale of hundredths of a second (tens of million seconds) to a few seconds, the brain needs to keep track of time effectively to understand speech, appreciate music, or perform the highly coordinated movements necessary for catching a ball or playing the violin. Although timing on this scale is often automatic, it is critical to our ability to communicate with and navigate the world.

One example is the stopped clock illusion. If you have an analog clock with a second hand (one that “ticks,” not one with smooth and continuous motion), on occasion you may have shifted your gaze to the second hand, and for a brief moment thought to yourself “damn, the clock stopped,” but by the time you finished your thought you realized you were mistaken. As we first look at the second hand it seems to remain motionless for longer than we expect a second to last; it is as if time dilated or stood still for a moment, and for this reason is sometimes referred to as chronostasis. The illusion is related to shifts in attention, motion, and our internal expectations. Additionally, the physical characteristics of what we are looking at influence our estimates of duration, and generally the more the physical characteristics of an object engage our attention, the longer it seems to last. For example, when people are asked to judge the duration that pictures of faces are shown on a computer screen, people judge faces of angry people to have lasted longer than pictures of smiling people.

 

Illusions of psychiatric significance

Illusions called pseudohallucinations occur at times when feelings of anxiety or fear are projected on external objects, as when a child perceives threatening faces or monsters in shadows at night or sees goblins in trees.