An illusion is a distortion of the

sense A sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of Stimulus (physiology), stimuli. (For example, in the human body, the brain which is part of the cen ...

s, which can reveal how the

mind The mind is the set of faculties responsible for all mental phenomena. Often the term is also identified with the phenomena themselves. These faculties include thought, imagination, memory, will, and sensation. They are responsible for various m ...

normally organizes and interprets

sensory stimulation In physiology, a stimulus is a detectable change in the physical or chemical structure of an organism's internal or external environment. The ability of an organism or organ to detect external stimuli, so that an appropriate reaction can be m ...

. Although illusions distort the human perception of

reality Reality is the sum or aggregate of all that is real or existent within a system, as opposed to that which is only imaginary. The term is also used to refer to the ontological status of things, indicating their existence. In physical terms, r ...

, they are generally shared by most people. Illusions may occur with any of the human senses, but visual illusions (

optical illusion Within visual perception, an optical illusion (also called a visual illusion) is an illusion caused by the visual system and characterized by a visual perception, percept that arguably appears to differ from reality. Illusions come in a wide v ...

s) are the best-known and understood. The emphasis on visual illusions occurs because

vision Vision, Visions, or The Vision may refer to: Perception Optical perception * Visual perception, the sense of sight * Visual system, the physical mechanism of eyesight * Computer vision, a field dealing with how computers can be made to gain un ...

often dominates the other senses. For example, individuals watching a

ventriloquist Ventriloquism, or ventriloquy, is a performance act of stagecraft in which a person (a ventriloquist) creates the illusion that their voice is coming from elsewhere, usually a puppeteered prop known as a "dummy". The act of ventriloquism is v ...

will perceive the voice is coming from the dummy since they are able to see the dummy mouth the words. Some illusions are based on general assumptions the brain makes during

perception Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous system ...

. These assumptions are made using organizational principles (e.g.,

Gestalt Gestalt may refer to: Psychology * Gestalt psychology, a school of psychology * Gestalt therapy, a form of psychotherapy * Bender Visual-Motor Gestalt Test, an assessment of development disorders * Gestalt Practice, a practice of self-exploration ...

theory), an individual's capacity for

depth perception Depth perception is the ability to perceive distance to objects in the world using the visual system and visual perception. It is a major factor in perceiving the world in three dimensions. Depth perception happens primarily due to stereopsis an ...

and

motion perception Motion perception is the process of inferring the speed and direction of elements in a scene based on visual, vestibular and proprioceptive inputs. Although this process appears straightforward to most observers, it has proven to be a difficult pr ...

, and

perceptual constancy Subjective constancy or perceptual constancy is the perception of an object or quality as constant even though our sensation of the object changes. While the physical characteristics of an object may not change, in an attempt to deal with the extern ...

. Other illusions occur because of biological sensory structures within the

human body The human body is the structure of a Human, human being. It is composed of many different types of Cell (biology), cells that together create Tissue (biology), tissues and subsequently organ systems. They ensure homeostasis and the life, viabi ...

or conditions outside the body within one's physical environment. The term ''illusion'' refers to a specific form of sensory distortion. Unlike a

hallucination A hallucination is a perception in the absence of an external stimulus that has the qualities of a real perception. Hallucinations are vivid, substantial, and are perceived to be located in external objective space. Hallucination is a combinatio ...

, which is a distortion in the absence of a

stimulus A stimulus is something that causes a physiological response. It may refer to: *Stimulation **Stimulus (physiology), something external that influences an activity **Stimulus (psychology), a concept in behaviorism and perception *Stimulus (economi ...

, an illusion describes a misinterpretation of a true sensation. For example, hearing voices regardless of the environment would be a hallucination, whereas hearing voices in the sound of running water (or another auditory source) would be an illusion.

Visual

visual illusion Within visual perception, an optical illusion (also called a visual illusion) is an illusion caused by the visual system and characterized by a visual percept that arguably appears to differ from reality. Illusions come in a wide variety; thei ...

or optical illusion is characterized by visually perceived images that are deceptive or misleading. Therefore, the information gathered by the visual sense is processed to give, on the face of it, a

percept Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous system ...

that does not tally with information from other senses or physical measurement. A conventional assumption is that there are physiological illusions that occur naturally and cognitive illusions that can be demonstrated by specific visual tricks that say something more basic about how human perceptual systems work. The visual system (eye and brain) constructs a world inside our head based on what it samples from the surrounding environment. However, sometimes it tries to organize this information "it thinks best" while other times it fills in the gaps. This way in which our brain works is the basis of an illusion.

Auditory

An auditory illusion is an illusion of

hearing Hearing, or auditory perception, is the ability to perceive sounds In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psycholog ...

, the auditory equivalent of a visual illusion: 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). One example of an auditory illusion is a

Shepard tone A Shepard tone, named after Roger Shepard, is a sound consisting of a superposition of sine waves separated by octaves. When played with the bass pitch of the tone moving upward or downward, it is referred to as the ''Shepard scale''. This cre ...

Tactile

Examples of tactile illusions include

phantom limb A phantom limb is the sensation that an amputated or missing limb is still attached. Approximately 80 to 100% of individuals with an amputation experience sensations in their amputated limb. However, only a small percentage will experience painf ...

, the

thermal grill illusion The thermal grill illusion is a sensory illusion originally demonstrated in 1896 by the Swedish physician Torsten Thunberg. The illusion is created by an interlaced grill of bars which are warm, e.g. , and cool, e.g. . When someone presses a hand ...

, the

cutaneous rabbit illusion The cutaneous rabbit illusion (also known as cutaneous saltation and sometimes the cutaneous rabbit effect or CRE) is a tactile illusion evoked by tapping two or more separate regions of the skin in rapid succession. The illusion is most readily ev ...

and a curious illusion that occurs when the crossed index and middle fingers are run along the bridge of the nose with one finger on each side, resulting in the perception of two separate noses. The brain areas activated during illusory tactile perception are similar to those activated during actual tactile stimulation. Tactile illusions can also be elicited through haptic technology. These "illusory" tactile objects can be used to create "virtual objects".

Temporal

temporal illusion The study of time perception or chronoception is a field within psychology, cognitive linguistics and neuroscience that refers to the subjective experience, or sense, of time, which is measured by someone's own perception of the duration of the ind ...

is a distortion in the perception of time, which occurs when the time interval between two or more events is very narrow (typically less than a second). In such cases, a person may momentarily perceive time as slowing down, stopping, speeding up, or running backward.

Intersensory

Illusions can occur with the other senses including those involved in food perception. Both sound and touch have been shown to modulate the perceived staleness and crispness of food products. It was also discovered that even if some portion of the taste receptor on the tongue became damaged that illusory taste could be produced by tactile stimulation. Evidence of

olfactory The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. In humans, it ...

(smell) illusions occurred when positive or negative verbal labels were given prior to olfactory stimulation. The McGurk effect shows that what we hear is influenced by what we see as we hear the person speaking; when the auditory component of one sound is paired with the visual component of another sound, leading to the perception of a third sound.

Disorders

Some illusions occur as a result of an illness or a disorder. While these types of illusions are not shared with everyone, they are typical of each condition. For example, migraine sufferers often report

fortification illusions Scintillating scotoma is a common visual aura that was first described by 19th-century physician Hubert Airy (1838–1903). Originating from the brain, it may precede a migraine headache, but can also occur acephalgically (without headache), a ...

Neuroscience

Perception Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous system ...

is linked to specific brain activity and so can be elicited by brain stimulation. The (illusory) percepts that can be evoked range from simple

phosphene A phosphene is the phenomenon of seeing light without light entering the eye. The word ''phosphene'' comes from the Greek words ''phos'' (light) and ''phainein'' (to show). Phosphenes that are induced by movement or sound may be associated wit ...

s (detections of lights in the

visual field The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments". Or simply, visual field can be defined as the entire area that can be seen when an eye is fixed straight at a point ...

) to high-level percepts. In a single-case study on a patient undergoing presurgical evaluation for epilepsy treatment, electrical stimulation at the left temporo-parietal junction evoked the percept of a nearby (illusory) person who "closely 'shadowed' changes in the patient's body position and posture".

References

External links

Universal Veiling Techniques
by J.R. Block.
Optical illusions and visual phenomena
by Michael Bach
Auditory illusions
- touch illusions, forces and the geometry of objects, by Gabriel Robles-De-La-Torre.
Silencing awareness of visual change by motion
{{Authority control Reality Perception Deception Articles containing video clips