Illusions occur when sensory data is misinterpreted by the brain. The illusions we perceive are proof that our minds construct our perceptions and may become confused as they try to process information. The brain works on certain assumptions of what it has or will perceive in nature. When those assumptions are broken, the brain uses what it has and constructs the best perception it can – an illusion. Some illusions are subjective (for example, the now famous blue or gold dress); different people may perceive what they see or feel differently. Most illusions tend to be optical (visual), but there are also tactile, auditory, taste, and scent illusions. There are so many types of illusions that I can only cover a small amount of them.

Visual/ Optical Illusions
An optical (visual) illusion is one in which images are perceived abnormally because of an overload of information or an underling assumption that prove false (the brain organizes sensory information in specific ways which then prove false, so the brain uses the information given and tries to fill in or construct the rest). There are three main types of illusion

  • Literal optical illusions: create images different from the objects that make them,
  • Physiological illusions: effects on the eyes and brain of excessive stimulation (brightness, tilt, color, movement)
  • Cognitive illusions: when the eyes and brain make unconscious inferences

Let’s look at three examples:

The celebrity’s illusion: plays on the strength of the fovea and the weakness of peripheral vision. The fovea is only about 2% of the visual field, the center of our vision where we see clear and crisp images. Outside of the foveal view, our actual vision is a little burly and our brain constructs a picture from that information, but when the brain is given excessive data (as in the celebrity illusion) our brain tries to compensate. This is probably compounded by the images being faces, which are very important to us and which our brain invest a lot of energy into understanding.

Forced Perspective:
perspective is a very old an important perception – it helps keep us alive by telling us how far things (like predators) are – but it’s built on certain assumptions. When our assumptions are broken, then we experience forced perspective.


Color assumptions: When we perceive colors we tend to think that they are universal, red is always red, blue is always blue, but that’s not correct. How we perceive colors depends largely on context. In the classic example below, the brown square on top center of the block is the exact same color as the “orange” square in the front center. Our brain uses light references to tell us how we should perceive the color, not what the actual color is.


Auditory Illusions
Auditory illusions can be either sounds which are not present (filling in) in the stimulus or “impossible” sounds. A simple example: you may perceive a voice coming from a dummy when watching a ventriloquist since the words seem to synchronize with the dummy mouth movements.

The Shepard tone is a well-known example of an “impossible” sound – it’s cycles between a limited set of tones, each separated by an octave, the illusion sounds like an ever raises continuously (the equivalent of the Penrose stairs illusion).

One important point to know about auditory perception is that it often depends on presumptions, which the brain can quickly learn to overcome. Here’s an example:

Taste Illusions
There are several types of taste illusions, but the classic involves the effect of color on taste. Using either a blind taste test or changing the color of white wine to read confuses even wine judges. And changing the color of sweet drinks (like a lime flavored drink to red) was suggested enough that people perceived a completely different flavor.

Olfactory/Scent Illusions

The sense of smell is very old and may not be as easy to fool as our other senses. There’s very little information available on olfactory illusions and some argument over whether they exist. The one type of illusion I can think of is when unlike molecules smell the same – for example Benzaldehyde (the smell in almonds) and cyanide. It’s difficult to call this an illusion since the brain isn’t being overloaded and no presumption is being warps, but it clearly represents an event when the brain can’t tell the difference between two molecules.

Tactile Illusions
There are several types of tactile illusions. Phantom limb syndrome is one, but since it is in effect a disorder, lets look at another type. The Cutaneous rabbit illusion can be induced by tapping two or more separate regions of the skin in rapid succession. Example: a rapid sequence of taps near the wrist, then near the elbow can create the sensation of sequential taps hopping up the arm even though no physical stimulus was applied between the two actual locations.

Web Applications

It’s always a good to have a professional graphic artists for Web development. I’ve seen unintended optical illusions on sites that distracted from the content – that hurts usability.

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“Mere color, unspoiled by meaning, and unallied with definite form, can speak to the soul in a thousand different ways.”
Oscar Wilde

There are so many things to say about our color perception. For example, the Human eye is capable of seeing between 7 and 10 million colors. Question: why is that massive range of perception so important?

We can define color vision as the ability of our eyes and brain to distinguish objects based on the wavelengths (or frequencies) of the light they reflect, emit, or transmit. We can point out that the typical human eye is only capable of perceiving light at wavelengths between 390 and 750 nanometers (the “visible” spectrum for Humans), and that our perception of colors is a somewhat subjective process – we all see the same illuminated object or light source a bit differently.

cone_cellWe can delve into the neurological process of three types of cone cells in the retina (up to 7 million) gathering information about visible wavelengths of light that correspond to short-wavelength, medium-wavelength and long-wavelength (red, green, blue). We can even frame the definition of vision as the process of perceiving color:

“‘Vision,’ by common usage, suggest a process; but it is now known that it is built out of many processes, subdivided into at least 32 areas. Before they eye’s input gets to the cortex it goes through two vision systems. One sees motion, and the other color. They largely come together in the primary visual cortex. Nonetheless, the overall visual system continues to stream continues this division into higher areas of the cortex. Vision goes into what a stream that identifies things and into a where stream that locates their positions. And even this description is a gross simplification.” Sagan & Skoyles, Up From Dragons

But the perception of color is much more complex …

What we see as color is, in a sense, an absence of colors. White light from the sun hits an object – let’s say a blue object – and all of the color in the white light (the full spectrum) is absorbed by the object with the exception of blue (and perhaps ultra violet and infrared). The blue is reflected to our eyes. The cones in our retina are stimulated at a specific frequency and sent to our brains for interpretation. We see blue – unless, of course we’re color blind.

But all color is perceived in context, as explained in Neuroscientist, Dr. Beau Lotto’s TED Talk,

Dr. Lotto sums our experience up like this, “The light that falls on your eye (sensory information) is meaningless, because it could mean literally anything. … There’s no inherent meaning of information, it’s what we do with that information that matters.”
Dr. Lotto also provides an example of why we evolved to see colors – pure survival. The example he gives in his TED Talk involves seeing a predator in the jungle. But there are alternative hypotheses.

Theoretical neurobiologist Mark Changizi has speculated that the reason Humans see is because it gives us an advantage in sensing emotions or health on the skin of others. Other neuroscientists suggest that our ability to see many shades of green help us differentiate between and choose plants to eat versus avoid poisonous ones.

No matter how we see the world, there always seems to be another view (as followers of biocentrism might say). Here are just a few interesting facts that we know about color:

  • Color Blindness: Men have a higher chance of being color blind than women. 1 out of 12 as opposed to 1 out of 255. The most common type of color blindness is the disability to tell the difference between red and green.
  • Tetrachromats: About 1.36% of the world’s population (only women) have a fourth type of cone cell in their retinas, (resulting in Tetrachromacy) giving them true four-color vision allowing them to see more than 100 million colors.
  • Depression & Color: Research published by Dr. Emanuel Bubl demonstrates that the retinas of depressed patients were less sensitive to contrast – making colors appear duller.
  • Shades of green: Human cones in the eye are more sensitive to green frequencies than any other. Humans are omnivores, so that not only can differentiating between shades of green plant help us find edible plants and avoid poisonous ones.
  • Ultraviolet colors: The Human eye is capable of seeing ultraviolet when the lens is Removed – some people are born with Aphakia – the absence of the lens on the eye. The great impressionist painter Claude Monet developed cataracts in his old age and after struggling to paint (with his colors washed out) he decided to, at age 82, have the lens of his left eye completely removed – the operation allowed him to see familiar colors, but it also let him see, and paint in ultraviolet (colors we cannot see).

Water Lily Pond by Claude Mone – circa 1926 – includes ultraviolet colors

Web Development Application

Understanding color in for development can be very helpful, underestimating its importance can be crippling, for example, not understanding the limits of users with color blindness. The best bet for avoiding issues is to have a knowledgeable graphic artist on your team.

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