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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Saccadic eye movements

A saccade (sakad′ik – French, twitch, jerk) is a quick, simultaneous movement of both eyes between two or more phases of fixation in the same direction. Saccadic eye movements are extremely fast voluntary movements of the eyes, allowing them to accurately “refix” on an object in the visual field, and change retinal foci from one point to another. Some Saccadic eye movements can be involuntary.

Saccades are one of the fastest movements produced by the human body with peak angular speeds of the up to 900°/s. An unexpected stimulus can commence a saccade in about 200 milliseconds (ms), and last from about 20–200 ms, depending on their amplitude. 20–30 ms is typical movement for language reading.

We do not look at the world with fixed steadiness, although our brain tells us otherwise. Our eyes move around, locating interesting parts of the scene and building a mental map in three-dimensions. Our eyes saccade, or jerk/twitch quickly, stop, scan, and then move again. The fovea (the high-resolution portion of vision, 1-2 degrees of vision) is one of the main reasons for Saccadic eye movements – we must move our eyes to resolve objects in our minds.

Saccadic masking
One of the most interesting points about Saccadic eye movements involves what we don’t perceive as our eyes move. One would think that no information is passed through the optic nerve to the brain while the eyes move in saccade, that is at least our perception experience, but that’s not correct. Saccadic masking or saccadic suppression begins just before your eyes move and keeps us from experience a blurred or smeared image. You can experience the saccadic masking effect with a very simple experiment: look in a mirror, look at your left eye, then change your gaze to look at your right eye – you won’t perceive any movement of your eyes, which is evidence that the optic nerve has momentarily ceased transmitting or that the brain just refuses to process the transmission.

Spatial updating and Trans-saccadic perception
One of the continually amazing things about perception is that our brain often perceives information that isn’t there. Spatial updating occurs when you see an object just before a saccade, and allows you to “make another saccade back to that image, even if it is no longer visible.” The brain somehow “takes into account the intervening eye movement by temporarily recording a copy of the command for the eye movement” and compares it to the remembered target image.

Trans-saccadic memory is the process of retaining information across a saccade. Neurologist think that perceptual memory is updated during saccades so information gathered across fixations can be compared and produced, creating what researches believe is a type of visual working memory.

Saccadic Dysfunction
There are a series of disorders that can produce abnormal eye movements. One is Nystagmus (also known as “dancing eyes”) a condition of involuntary eye movement (side to side, up and down, and other) that may reduce or limit vision.

Web Development Application
The understanding of saccadic eye movements has had a remarkable impact on Web usability in the form of eye tracking studies. By employing technology that monitors eye movements that can pinpoint precisely where a user is looking on a page, usability testers can study and better understand how people interact with text or online documents.

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Tracking moving objects

There’s an old magic trick call the vanishing ball in which the magician throws a ball into the air several times and on the last time it disappears in midair. Here’s an example:

In reality, the ball doesn’t disappear, the magician palms it on the last throw, but up to two thirds of people claim to see the ball move up, and then disappear, on the last throw. This illusion takes advantage of our brain science and expectation, and it’s based on a very old evolutionary strategy that can best be as a hunting adaptation.

Predicting the future
A bird flying 30 mph can travel 44 feet in one second. When we see that bird our eyes take about 1/10th of a second to process the information (for the light hitting the retina to travel along the optic nerve and then be processed by the visual cortex). During that tenth of a second the bird will travel 4.4 feet, so our vision is always a bit off reality, the actual position of the bird should be invisible to us, 4.4 feet further along its flight path than what we see. Yet a primitive hunter could throw a rock and hit that bird – that’s because our minds construct a reality to predict where the bird will be. We can thank our evolution as hunters for this type of vision/perception


Web Development Application
Our eyes evolved to scan the environment and process massive amounts of visual data – as much as two billion pieces of information each second – but that’s image information, not text, and we can’t turn our evolution off. Reading text is really not natural to us, and when we read our brains are still in a very old evolutionary hunting mode. The point: we don’t see as much as we believe we do – we construct much of reality (and reading) in our minds. It is essential to understand that when people read that they don’t read every word (eye scan studies bear this out. See example below), so we have to construct headlines and writing for scanners.


We experience reading as a smooth experience, but this eyescan shows our eyes actually stop on words (dots) and that’s the only time we see with sharp focus.

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The limits of vision

Our eyes can do amazing things, but we need to keep in mind that they can do nothing without the visual cortex, which takes up about 30 % of the brain’s capacity and processes as many as two billion pieces of information each second. It is the combination of sensor (the eye) and processor (the visual cortex) that create what we call vision. Let’s just list some of the capabilities of vision:

The earth surface curves (the horizon) out of sight at a distance of about 3.1 miles (5 km) but our vision can perceive far beyond the horizon. The farthest star we can see with our naked eye is V762 Cas (brightness magnitude 5.8 ) in Cassiopeia at 16.308 light-years distance (9.5868623^16 miles), but if the night is clear and you know where to look, you can see the Andromeda Galaxy (M31), that’s 2.537 million light-years away.

The shortest moment a person can see a light source is based on Bloch’s Law, which defines the visual threshold that is reached when both illumination and time reach a constant. Simply said, there’s a balance point between the intensity and duration in a flash of light. As extremely birth light might appear the same when shown for a nanosecond as a dim light shown for a tenth of a second.

One study shows that fighter pilots (people with very good eyesight) can observe an image flashed on a monitor for only 1/250th of a second. Most people can see a flickering light source as steady illumination at a rate of 50 to 60 times a second (or hertz).

The visual resolution of the human eye is about 1 arc minute (1/60 of a degree). At a distance of 20″, that’s about 170 dpi (pixels) – a dot pitch of around 0.14 mm. To translate this to something familiar, a 30″ monitor with a 16:9 aspect ratio would be sized around 26″ x 15″ and would need a resolution of 4400 x 2600 pixels to realize 170 dpi.

But there’s also angular resolution (the ability to distinguish two similar points that are close to each other). A simple example is car head lights, at a great distance they appear as one light, but at some point you are able to tell that there are two lights.

Angular resolution is measured in arc minutes (1/60th of a degree) and seconds (1/3600th of degree) of field of view. The angular resolution average human eye is one arc minute. For example, a one third of a millimeter wide line seen at arm’s length is 1 arc minute.

Foveal viewpoint
The fovea is the part of human eye responsible for sharp central vision (the only part of the retina that permits 100% visual acuity) The fovea is small, only about two degrees, and is necessary for visual detail like reading or tracking moving objects (hunting). Objects outside of the fovea are not in focus, even though our brains tell us they are.


Over the next few days I’ll concentrate solely on vision.

Web Development Application:

There are many applications for understanding vision. For example, it is important to keep the limits of the fovea in mind when writing because most readers scan before they read, and only see a limited amount of the page.

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