Illusion

Key takeaways

Illusion belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Illusion to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Illusion from memory before moving on to harder problems.

Reference excerpt

An illusion is a distortion of the senses, which can reveal how the mind normally organizes and interprets sensory stimulation. Although illusions distort the human perception of reality, they are generally shared by most people. Illusions may occur with any of the human senses, but visual illusions (optical illusions) are the best-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 as 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. These assumptions are made using organizational principles (e.g., Gestalt theory), an individual's capacity for depth perception and motion perception, and perceptual constancy. Other illusions occur due to biological sensory structures within the human body or conditions outside 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 another auditory source) would be an illusion. So, it should not be wrong to consider that illusions are just "misinterpretations" of how our brain perceives something that exists (unlike a hallucination where a stimulus is absent).

Visual

A visual illusion 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 create a percept that does not tally with information from other senses or physical measurements. The visual system, which includes the eyes (namely the retinas) and the central nervous system (namely the brain's visual cortex), constructs reality through both perceptual and cognitive neural pathways. Visual illusions are (at least in part) thought to be caused by excessive competing stimuli. Each stimulus follows a dedicated neural path in the early stages of visual processing, and intense/repetitive activity or interaction with active adjoining channels (perceptual neural circuits, usually at the same level) causes a physiological imbalance that alters perception. During low-level visual processing, the retinal circuit arranges the information in the photoreceptors, by creating initial visual percepts from the patterns of light which fall on the retina. The Hermann grid illusion and Mach bands are two illusions that are widely considered to be caused by a biological phenomenon named lateral inhibition, where the receptor signal in the retina's receptive fields from light and dark areas compete with one another. The assembly of visual elements into a collective percept, that distinguishes objects from backgrounds, takes part during intermediate-level visual processing. Many common visual illusions are a consequence of the percept constructed during this processing stage, as the elements first captured during low-level processing might easily be interpreted to form an image that differs from objective reality. An example is when two objects of the same size are placed on a certain background which conditions us to believe that one object might be larger than the other, and when the background is removed or replaced our perception immediately changes to the correct scenario (effectively concluding that both objects have equal dimensions). High-level visual processing consolidates information gathered from various sources to apply cognitive influences that create a conscious visual experience. Thus, allowing us to recognize the complex identity of different elements, and the disparate relations between them through cognitive processes. Visual illusions are also often a product of this processing stage, and it is during this stage that we might ultimately become conscious of any optical illusion. There are two crucial properties of our visual system related mostly to high-level visual processing, referred to as selectivity and invariance (which we have consistently attempted to replicate in image recognition computer algorithms). Selectivity refers to the identification of particular features that are relevant to recognize a specific element or object, while abstracting from other features that are not fundamental to performing the same recognition (e.g. when we see the shape of a house, certain contours that are essential for us to recognize it while other contours or image properties are not, such as color). On the other hand, invariance refers to the ability to be indifferent to small variations of a given feature, effectively identifying all those variations as simply being different versions of the same feature (e.g. we can recognize a given handwritten letter of the alphabet, written by different people with distinct styles of calligraphy). The whole process that constructs our visual experience is extremely complex (with multiple qualities that are unmatched by any computer or digital system). It is organized by many sequential and parallel sub-processes, each of which is essential in building our conscious image of the world. Our whole visual system seeks to simplify and categorize the unstructured low-level visual information, through both selectivity and invariance. Thus, while trying to organize an image by "filling in the gaps" through assumptions, we become vulnerable to misinterpretation.

Auditory

An auditory illusion is an illusion of hearing, the auditory equivalent of a visual 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.

Tactile

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Worked examples

Example 1 — a first encounter with Illusion

Start with the simplest possible case. Write down what Illusion claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Illusion before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Illusion ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Illusion

In research: Illusion appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Illusion in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Illusion is common in secondary-school and first-year university syllabi. It links to neighbouring topics Deception, Illusions, Perception, so understanding it makes those chapters shorter.
In everyday life: Look for Illusion outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Illusion in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Illusion means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Illusion out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Illusion in simple terms?

An illusion is a distortion of the senses, which can reveal how the mind normally organizes and interprets sensory stimulation. Although illusions distort the human perception of reality, they are generally shared by most people.

Why does Illusion matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Illusion?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Illusion.