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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Balance – Equilibrioception

Every step we take dances on the edge of disaster – one miscalculation at the moment when we are shifting weight from one foot to the other, and we fall. Balance is a sixth sense and a crossmodal perception. It has nothing to do with hearing, but clearly begins with sensors in the inner ear. We call it Equilibrioception.

Our sense of balance depends on the integration of three sensory systems:

  • Vision: seeing helps us determine our body’s position in reference to the world (gravity). Note: some blind people have issues with balance
  • Proprioception: (see related post) uses the skeletal systems (the muscles and joints and their sensors) to determine the position of the body
  • Vestibular system: The section of the inner ear composed of semicircular canal system, which indicate rotational movements; and the otoliths, which indicate linear acceleration.

The vestibular apparatus (shown below) includes the utricle, saccule, and three semicircular canals (Anterior, Horizontal, and Posterior). The utricle and saccule detect gravity (information in a vertical orientation) and linear movement. As we move our heads fluid moves through the canals and tells us the relative position of our head and its movement. The otoliths act as a kind of accelerometer, helping determine the speed of the body or heads movement. The vestibular system then sends signals to the neural structures that control eye movements, and to the muscles that keep an animal upright.


Human balance perception is not quite terrestrial, that is, we certainly have some perceptual systems or strategies that other land animals do not. For example: a human can stand in a bus holding onto a pole and have little or no issues with balance, but a horse standing in a horse trailer has significant problems with the movement of the vehicle, even though it has four legs (four points of stability). We almost certainly owe this extra bit of balance expertise to our ancestors of the trees.

Web Applications
Virtual reality comes to mind again. It should be noted that professional level flight simulators use hydraulic mechanisms to provide a sense of pitch and acceleration by moving entire simulator rooms. Without some feedback of this sort, virtual reality will always be semi-virtual.

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I could try to explain how hearing works, but I could not do better than this video by Crash Course:

Quick facts

  • The ear’s malleus, incus and stapes (aka the hammer, anvil and stirrup) are the smallest bones in the human body – all three can fit together on a penny.
  • Your internal ear is the size of a pea but contains more than 20,000 sensor “hairs.”
  • Hearing can be damaged permanently from a single incident of exposure to extremely loud noise.
  • The ear continues to hear sounds, even while you sleep.
  • A healthy young person can hear all sound frequencies from approximately 20 to 20,000 hertz.
  • The maximum range of human hearing is about 15 to about 18,000 cycles per second.
  • Humans can hear an impulse about 20 microseconds long, although some studies suggest that some musicians can hear even shorter impulses.
  • Some blind people have learned how to “see” using a form of echolocation.

Sound direction
The pinna, the outer part of the ear and catches sound waves, but it has another critical function. This structure of the pinna (curves and position) helps determine the direction of a sound. Sound coming from behind bounces off the pinna differently than that coming from in front, which alters the pattern of sound waves. The brain recognizes the pattern differences and determines direction of the sound. In addition, the distance between your ears can help you determine direction, and it all happens very quickly:

  • Sound travels at the speed of 1,116 feet per second (761.2 mph) at sea level,
  • The average human head is 6 to 7 inches wide,
  • So hearing can detect the difference of half a thousandth of a second (about 500 microseconds)

Hearing loss
We need to remember that in nature the loudest sound a Human is likely to hear is thunder; our distant ancestors didn’t go to rock concerts, work in loud factories or set off fireworks. So it should not be a surprise that about 20 percent of Americans, 48 million, report some degree of hearing loss. Noise-induced hearing loss (NIHL) may happen slowly or suddenly (depending on the decibel level of sounds exposed to). The issue is aggravated by the fact that you usually can’t tell when your hearing is being damaged by loud sounds.

The effects of gunfire or explosions are immediately evident, but exposer to everyday noises, such as listening to very loud music often seem innocuous when they are not. Researchers estimated that up to 17% of teenagers have NIHL in one or both ears.


Tinnitus (ringing/buzzing in ears) affects ~10% of Americans and about 90% of cases occur with an underlying hearing loss. Tinnitus is cause by genetic factors, noise, trauma, Ototoxic medications, infections, and aging.

Web Applications
I could find no usability information on safe sound levels for Web based video or audio. I’ve often noticed that many videos on the Web start at full volume. I’d suggest 50% is a much better percentage – then let the listener decide if they want it louder.

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