An illusion is a distortion of the senses, revealing how the brain normally organizes and interprets sensory stimulation. While illusions distort reality, they are generally shared by most people.[1] Illusions may occur with more of the human senses than vision, but visual illusions, optical illusions, are the most well known and understood. The emphasis on visual illusions occurs because vision often dominates the other senses. For example, individuals watching a ventriloquist will perceive the voice is coming from the dummy since they are able to see the dummy mouth the words.[2] Some illusions are based on general assumptions the brain makes during perception. These assumptions are made using organizational principles, like Gestalt, an individual's ability of depth perception and motion perception, and perceptual constancy. Other illusions occur because of biological sensory structures within the human body or conditions outside of the body within one’s physical environment.
The term illusion refers to a specific form of sensory distortion. Unlike a hallucination, which is a distortion in the absence of a stimulus, 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 other auditory source) would be an illusion.
Mimes are known for a repertoire of illusions that are created by physical means. The mime artist creates an illusion of acting upon or being acted upon by an unseen object. These illusions exploit the audience's assumptions about the physical world. Well known examples include "walls", "climbing stairs", "leaning", "descending ladders", "pulling and pushing" etc.
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An optical illusion is always characterized by visually perceived images that, at least in common sense terms, are deceptive or misleading. Therefore, the information gathered by the eye is processed by the brain to give, on the face of it, a percept that does not tally with a physical measurement of the stimulus source. 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 human brain constructs a world inside our head based on what it samples from the surrounding environment. However sometimes it tries to organise this information it thinks best while other times it fills in the gaps.[3] This way in which our brain works is the basis of an illusion.
An auditory illusion is an illusion of hearing, the sound equivalent of an optical illusion: the listener hears either sounds 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.
Examples of tactile illusions include phantom limb, the thermal grill illusion, the cutaneous rabbit illusion 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. Interestingly, the brain areas activated during illusory tactile perception are similar to those activated during actual tactile stimulation.[4] Tactile illusions can also be elicited through haptic technology.[5] These "illusory" tactile objects can be used to create "virtual objects".[6]
Illusions can occur with the other senses including that of taste and smell. It was discovered that even if some portion of the taste receptor on the tongue became damaged that illusory taste could be produced by tactile stimulation.[7] Evidence of olfactory (smell) illusions occurred when positive or negative verbal labels were given prior to olfactory stimulation.[8]
Some illusions occur as 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.
Just like many other words often used in a different sense in spirituality the word "illusion" is used to denote different aspects in Hindu Philosophy (Maya). Many Monist philosophies clearly demarcate illusion from truth and falsehood. As per Hindu advaita philosophy, Illusion is something which is not true and not false. Whereas in general usage it is common to assume that illusion is false, Hindu philosophy makes a distinction between Maya (illusion) and falsehood. In terms of this philosophy maya is true in itself but it is not true in comparison with the truth. As per this philosophy, illusion is not the opposite of truth or reality. Based on these assumptions Vedas declare that the world as humans normally see is illusion (Maya). It does not mean the world is not real. The world is only so much real as the image of a person in a mirror. The world is not real/true when compared to the reality. But the world is also not false. Falsehood is something which does not exist. if we apply this philosophy to the above example, the illusion is not actually illusion but is false. This is because in general usage people tend to consider lllusion to be the same as falsehood. As per adishankar's a guru of monist teachings the world we think is not true but is an illusion (not true not false). The truth of the world is something which can only be experienced by removing the identity (ego).