OPTICAL ILLUSION - 10 Cool Optical Illusions

10 Cool Optical Illusions

A Selection of Fun and Fascinating Illusions

By Kendra Cherry, About.com Guide

Optical illusions can be fun and fascinating, but they can also tell us a great deal of information about how the brain and perceptual system function. There are countless optical illusions out there, but here is a sampling of some of the most fun and interesting.

1.   Hermann Grid Illusion

What Do You See?

The Hermann grid was first discovered by a physiologist named Ludimar Hermann in 1870. When the viewer looks at the grid, the white dots and the center of each 'corridor' seem to shift between white and gray. When the viewer focused his or her attention on a specific dot, it is obvious that it is white. But as soon as attention is shifted away, the dot shifts to a gray color.

How Does the Hermann Grid Illusion Work?

So why do people see gray where there should be white? Why do we see something so different from reality?

Researchers have traditionally used what is known as lateral inhibition to explain why people see these gray areas. This phenomena demonstrates a very important principle of perception: we don't always see what's really there. Our perceptions depend upon how our visual system responds to environmental stimuli and how our brain then interprets this information.

However, there is evidence suggesting that this explanation is likely inaccurate. The fact that the illusion is not dependent upon size, can be seen with contrast reversal and can be negated by slightly distorting the lines have been cited as reasons why the classic theory is wrong. One possible explanation that has been proposed is known as the S1 simple-cell theory.

2. The Spinning Dancer Illusion

What Do You See?

In this image, you see the silhouette of a woman spinning. Which direction is she turning? You may be surprised to learn that it is possible to see her spinning both clockwise and counterclockwise. How? While it may be very difficult, you can probably get her to switch directions spontaneously. Try looking at the figure and then blink; she may appear to change directions immediately after you blink. Another strategy is to focus on a specific part of the figure.

How Does the Spinning Dancer Illusion Work?

After it was initially created by Nobuyuki Kayahara, the illusion was mistakenly referred to as a scientific personality test of right brain/left brain dominance by numerous websites and blogs. In reality, the spinning dancer illusion is related to bistable perception in which an ambiguous 2-dimensional figure can be seen in from two different perspectives. Because there is no third dimension, our brains try to construct space around the figure. Similar illusions include the Necker Cube and the Reversible Face/Vase Illusion.

In a New York Times column, Thomas C. Toppino, chair of the department of psychology at Villanova University suggested, "What's happening here to cause the flip is something happening entirely within the visual system. If we can understand why it is these figures reverse then we're in a position to understand something pretty fundamental to how the visual system contributes to the conscious experience."

3.   Zöllner illusion

What Do You See?

The Zöllner illusion is another commonly demonstrated optical illusion. First discovered in 1860 by a German astrophysicist named Johann Karl Friedrich Zöllner, this illusion presents a series of oblique lines crossed with overlapping short lines. The oblique lines look as if they are crooked and will diverge. In reality, all of the oblique lines are parallel.

How Does It Work?

Much like the Muller-Lyer and Herring illusions, this optical illusion demonstrates how the background of an image can distort the appearance of straight lines. Several different explanations for the Zöllner illusion have been suggested. First, the angle of the short lines compared to the longer lines creates an impression of depth. One of the lines appears to be nearer to us; the other farther away. Another possible explanation is that the brain attempts to increase the angles between the long and short lines. This results in a distortion as the brain tries to bend the lines away and towards each other.

Interestingly, if the color of the lines are switched to green and the background to red, the effect completely disappears as long as the two colors are of equal brightness.

4.   The Ames Room Illusion

How Does the Ames Room Illusion Work?

The effect works by utilizing a distorted room to create the illusion of a dramatic disparity in size. While the room appears square-shaped from the viewers perspective, it is actually has a trapezoidal shape. The woman on the right hand side of the image above is actually standing in a corner that is much further away than the woman on the left.

The illusion leads the viewer to believe that the two individuals are standing in the same depth of field, when in reality the subject is standing much closer. The woman on the left in the image above appears at a much greater visual angle, but the fact that she appears to be at the same depth of field as the figure on the right makes the closer individual look much larger.

The effect can be observed in a number of films, including The Lord of the Rings trilogy. Note the early scenes in The Fellowship of the Ring where the effect is prominently used to make Gandalf appear larger than the hobbits.

5.   The Ponzo Illusion

What Do You See?

In the image above illustrating the Ponzo illusion, the two yellow lines are the exact same size. Because they are placed over parallel lines that seem to converge in the distance, the top yellow line actually appears to be longer than the bottom one.

How Does the Ponzo Illusion Work?

The Ponzo illusion was first demonstrated in 1913 by an Italian psychologist named Mario Ponzo. The reason the top horizontal line looks longer is because we interpret the scene using linear perspective. Since the vertical parallel lines seem to grow closer as they move further away, we interpret the top line as being further off in the distance. An object in the distance would need to be longer in order for it to appear the same size as a near object, so the top "far" line is seen as being longer than the bottom "near" line, even though they are the same size.

6.   The Kanizsa Triangle Illusion

The Kanizsa Triangle illusion was first described in 1955 by an Italian psychologist named Gaetano Kanizsa. In the illusion, a white equilateral triangle can be seen in the image even though there is not actually a triangle there. The effect is caused by illusory or subject contours.

Gestalt psychologists use this illusion to describe the law of closure, one of the gestalt laws of perceptual organization. According to this principle, objects that are grouped together tend to be seen as being part of a whole. We tend to ignore gaps and perceive the contour lines in order to make the image appear as a cohesive whole.

7.   The Müller-Lyer Illusion

In the Müller-Lyer illusion, two lines of the same length appear to be of different lengths.

What Do You See?

In the image above, which line appears the longest? For most people, the line with the fins of the arrow protruding outward appears to be the longest while the line with the arrow fins pointing inwards appears shorter. In reality, the shafts of both lines are exactly the same length.

First discovered in 1889 by F.C. Müller-Lyer, the illusion has become of the subject of considerable interest and different theories have emerged to explain the phenomenon.

How Does It Work?

Optical illusions can be fun and interesting but they also serve as an important tool for researchers. By looking at how we perceive these illusions, we can learn more about how the brain and perceptual process work. However, experts do not always agree on exactly what causes optical illusions, as is the case with the Müller-Lyer illusion.

According to psychologist Richard Gregory, this illusion occurs because of a misapplication of size constancy scaling. In most cases, size constancy allows us to perceive objects in a stable way by taking distance into account. In the three dimensional world, this principle allows us to perceive a tall person as tall whether they are standing next to us or off in the distance. When we apply this same principle to two-dimensional objects, Gregory suggests, errors can result.

Other researchers contend that Gregory's explanation does not sufficiently explain this illusion. For example, other versions of the Müller-Lyer illusion utilize two circles at the end of the shaft. While there are no depth cues, the illusion still occurs. It has also been demonstrated that the illusion can even occur when viewing three-dimensional objects.

An alternative explanation proposed by R. H. Day suggests that the Müller-Lyer illusion occurs because of conflicting cues. Our ability to perceive the length of the lines depends upon the actual length of the line itself and the overall length of the figure. Since the total length of one figure is longer than the length of the lines themselves, it causes the line with the outward facing fins to be seen as longer.

Researchers from the University of London suggest that the illusion demonstrates how the brain reflexively judges information about length and size before anything else. "Many visual illusions might be so effective because they tap into how the human brain reflexively processes information. If an illusion can capture attention in this way, then this suggests that the brain processes these visual clues rapidly and unconsciously. This also suggests that perhaps optical illusions represent what our brains like to see," explained researcher Dr. Michael Proulx.

8.  The Moon Illusion

Have you ever noticed how the moon looks bigger when it is on the horizon than it does when it is high in the sky? This phenomenon is known as the moon illusion. While the moon illusion is well known through human history and culture, researchers are still debating explanations for why it happens.

Possible Explanations for the Moon Illusion

• Apparent distance theory: According to this possible explanation for the moon illusion, depth perception plays an important role in how we see the moon at the horizon versus high in the sky. This theory is centered on the idea that when you view the moon at the horizon, you are seeing it in the presence of depth cues such as trees, mountains, and other scenery. When the moon has moved higher into the sky, those depth cues disappear. Because of this, the apparent distance theory suggests, we tend to see the moon as further away on the horizon than we see it when it elevated in the sky.
  
  Researchers have found evidence supporting the apparent distance explanation. In one experiment, participants perceived the moon as farther away and 1.3 times larger when it was viewed over natural terrain. Experimenters then masked off the terrain by having participants view the moon through a hole in a piece of cardboard, which caused the moon illusion to vanish.


• Angular size-contrast theory: This explanation focuses instead on the visual angle of the moon in comparison to surrounding objects. When the moon is on the horizon and surrounded by smaller objects, it appears larger. At its zenith, the moon appears much smaller because it is surrounded by the large expanse of the sky.

While these are just two of the most prominent theories, there have been many different explanations proposed over the years and no true consensus exists. Part of the reason is that there are a number of factors that appear to influence the occurrence of this optical phenomenon, including:

• Color: When the moon appears red (due to smoke or dust in the air), it appears larger.


• Atmospheric perspective: When it is hazy or smoky outside, the moon appears larger on the horizon.


• Visual factors: Convergence of the eyes when viewing things on the horizon also causes objects to appear larger.

As with other visual phenomena, it is possible that no single variable can adequately explain the moon illusion. Instead, it is possible that many different factors might play a role.

9.   The Lilac Chaser Illusion

The lilac chaser is a type of visual illusion that was first discovered by vision expert Jeremy Hinton in 2005. In order to view the illusion, start by clicking here to open the image in a new window. Stare at the black center cross for a minimum of 30 seconds and see what happens. Want to learn more? Continue reading to discover how this fascinating illusion works and what it reveals about the brain and perception.

What Do You See?

In the lilac chaser illusion, the viewer sees a series of lilac colored blurry dots arranged in a circle around a focal point. As the viewer stares as the focal point, a few different things are observed. At first, there will appear to be a space running around the circle of lilac discs. After about 10 to 20 seconds, the view will then see a green disc moving around the circle instead of the space. With longer observation, the lilac discs will disappear altogether and the viewer will only see the green disc moving around in a circle.

How Does the Lilac Chaser Illusion Work?

According to its inventor Jeremy Hinton, "the illusion illustrates Troxler fading, complementary colours, negative after-effects, and is capable of showing colours outside the display gamut." What exactly does this mean? Let's break it down a bit further.

• Why do the lilac discs appear to move around the circle?
  This is an example of what is known as apparent movement or beta movement. When we see something in one spot and then again in a slightly different spot, we tend to perceive movement. Motion pictures and neon signs operate work based upon this principle.


• Why do we begin to see green discs in place of the gray spaces?
  This is an example of a negative afterimage effect. When a color is presented in the visual field for an extended period of time, an afterimage results. In the case of this illusion, we see a green afterimage in place of the lilac discs. We generally don't notice afterimages because we move our eyes frequently enough that they rarely occur in day-to-day experience.


• Why do all of the lilac discs eventually disappear?
  This is an example of what is known as Troxler fading, which occurs when blurry objects that are located in the periphery of our visual field disappear while we have our eyes fixated on a certain spot.


• Why does the green disc appear to fly around in a circle?
  After fixating on the center cross for about 30 seconds or so and the lilac discs have disappeared, it seems as if the green disc is now flying around the circle by itself. This can be explained by a Gestalt effect known as the phi phenomenon. The sequential movement of the retinal afterimage (aka, the green disc) causes the illusion of movement.

10.   The Negative Photo Illusion

Did you think that you needed a darkroom to process a negative photo into a full-color image? In this fun optical illusion, you can see how your visual system and brain are actually able to briefly create a color image from a negative photo.

How to Perform the Illusion

1. Stare at the dots located at the center of the woman's face below for about 30 seconds to a minute.


2. Then turn your eyes immediately to the center x of the white image on the right.


3. Blink quickly several times.

What do you see? If you've followed the directions correctly, you should see an image of a woman in full-color. If you are having trouble seeing the effect, try staring at the negative image a bit longer or adjusting how far you are sitting from your computer monitor.

Explanations

How does this fascinating visual illusion work?

What you are experiencing is known as a negative afterimage. This happens when the photoreceptors, primarily the cone cells, in your eyes become overstimulated and fatigued causing them to lose sensitivity. In normal everyday life, you don't notice this because tiny movements of your eyes keep the cone cells located at the back of your eyes from becoming overstimulated.

If, however, you look at a large image, the tiny movements in your eyes aren't enough to reduce overstimulation. As a result, you experience what is known as a negative afterimage. As you shift your eyes to the white side of the image, the overstimulated cells continue to send out only a weak signal, so the affected colors remain muted. However, the surrounding photoreceptors are still fresh and so they send out strong signals that are the same as if we were looking at the opposite colors. The brain then interprets these signals as the opposite colors, essentially creating a full-color image from a negative photo.

According to the opponent process theory of color vision, our perception of color is controlled by two opposing systems: a magenta-green system and a blue-yellow system. For example, the color red serves as an antagonistic to the color green so that when you stare too long at a magenta image you will then see a green afterimage. The magenta color fatigues the magenta photoreceptors so that they produce a weaker signal. Since magenta's opposing color is green, we then interpret the afterimage as green.