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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Taste is an enigma, a fusion of experiences for our brain made up of taste, smell, site, sound, and texture. A series of sensory intakes, yet one single sensation, a reverse Synesthesia, a crossmodal perception. A classic experiment demonstrates the weakness of “taste” when confused by other senses.

French researchers conducted a series of experiments where they colored a white wine red with an odorless dye. They then asked a panel of wine connoisseurs to describe its taste of the wine. The results: connoisseurs described the wine as if it were a red wine descriptors instead of a white wine. This experiment has been conducted in various ways with different flavors and colors of drink. The colors we see play a significant role in how we perceive drink.

The Process
Taste buds contain the taste receptor cells (aka gustatory cells). The receptors are located around the small papillae structures (taste buds) on the upper surface of the tongue, soft palate, upper esophagus, the cheek and epiglottis. The papillae structures detect five elements of taste perception: salty, sour, bitter, sweet and umami.

We each have somewhere between 2,000 and 10,000 taste buds, each consisting of 50 to 100 specialized sensory cells that can be stimulated by tastants; taste-provoking chemical molecules that are dissolved in ingested liquids (saliva). When stimulated sensory cells trigger signals that are transferred to the ends of nerve fibers, then send impulses along cranial nerves to the brainstem’s taste regions, which are relayed to the thalamus and the cerebral cortex – we perceive taste.

That’s a very simple explanation of the process. But as said before, our perception of taste requires more. Our sense of smell is closely aligned with our sense of taste – every child knows to hold his or her nose when ask to taste something bad. Our sense of smell can discriminate between 1,000 to 4,000 odors. The interface of taste and smell greatly enhances our perceptions of food and drink. But research shows that texture and sound also enhance taste, and as shown in the wine experiment, the appearance of foods and drinks can make people “see” flavors before they taste them – a point that every good chef is well aware of.

Web Applications
And again, the only possible application I can think of is virtual reality.

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Smell (Olfaction)

Our sense of smell is probably our “oldest” sensory system – it has a very long evolutionary history.

The basic process is called olfaction, and a simplified breakdown of the process looks like this:

  1. Chemical molecules floating in the air enter the nose
  2. The molecules are dissolve in mucous within a membrane called the olfactory epithelium (this is an important safety step since the olfactory tract is a direct path to the brain)
  3. The olfactory receptor cells interact with particular chemicals
  4. The olfactory tract transmits signals to several brain areas such: the olfactory cortex, hippocampus, amygdala, and hypothalamus

It’s that “interact with particular chemicals” that we haven’t completely figured out. No one knows exactly what causes olfactory receptors to react. Until recently the science suggested that a chemical molecule’s shape fit into specific receptors (there are about 400 different types, as compared to four for vision – 3 cones and one rod, and five for taste). But new evidence has been uncovered that suggest a quantum connection, and it can be demonstrated by looking at the smell of almond.

The Quantum factor
Benzaldehyde (the smell in almonds) and Cyanide smell the same to us, but the molecules are completely different in size and shape – which creates some doubt about the concept of molecule’s shape fitting into specific receptors as the soul method for detecting a scent. So what do Benzaldehyde and Cyanide have in common? Both molecules vibrate at the same frequency. This conjecture has been demonstrated repeatedly with other chemicals (see Luca Turin’s TED Talk, The science of scent).

The conclusion is astounding, our olfactory system is listening to molecules … we are hearing smells … our olfactory system is a nanoscale spectroscope that may also be detecting electron tunneling … we are detecting quantum state information. Reference notes on Quantum perception.

Emotions, memory and smell
Smells often bring back memories associated with an object or specific event. The reason has to do with how the brain processes scent information. When the olfactory transmits signals to the brain those signals are not only processed by the olfactory cortex, but also the hippocampus, amygdala, and hypothalamus, and the limbic system, which is involved with emotional behavior and memory.

Smell also has an important part in how we taste food -I’ll talk about that tomorrow.

The ancient Greeks knew that scent was a factor in sexual attraction for some animals, then during the 20th century pheromones were discovered in moths. We now know that many insects and animals use pheromones, but science has been unable to detect pheromones in humans.

Additional interesting facts about scent

  • Humans have about 400 types of olfactory receptors (and about 40 million total. In comparison, a German Shepherd dog as about 2 billion).
  • Smells can vary from person to person when there are issues with olfactory receptors types (compare just three eye cone cells in anomaly causing color blindness).
  • Humans are, for some reason, very sensitive to the smell of green bell pepper – capable to detecting the smell at only 0.5 parts per trillion.
  • Anosmia is the loss of the sense of smell. It’s usually caused by a nasal condition, virus, brain injury, or age. In addition, some people are born without a sense of smell.

Web Application
We are again left with only the possibility of future applications in virtual reality, but designers are already pushing ahead in other areas. One obvious example is that any good chief knows that the visual appearance of food enhances the enjoyment of a meal. Additional thoughts can be found in Jinsop Lee’s TED Talk, Design for all 5 senses.

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