Optical Illusions

How They Work and What They Reveal About the Brain

Hermann Grid Illusion

Hermann Grid Illusion
Understanding the Hermann Grid Illusion. Public Domain Image

What is an optical illusion? Optical illusions, more appropriately known as visual illusions, involves visual deception. Due to the arrangement of images, effect of colors, impact of light source or other variable, a wide range of misleading visual effects can be seen.

If you've ever struggled to see the hidden image in a single-image stereogram, you may have discovered that not everyone experience visual illusions in the same way. For some illusions, some people simply are not able to see the effect.

While optical illusions can be fun and interesting, they also reveal a great deal about the working of the brain. Learn more about some of the most famous optical illusion and discover exactly how and why these visual illusions occur.

In the Hermann Grid Illusion, the white dots at the center of each square seem to shift from white to gray.

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.1

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.



1 Bach, M. (n.d.). Grid illusions. http://www.michaelbach.de/ot/lum_herGrid/index.html

2 Shiller, P. (n.d.) The neural control of vision. http://web.mit.edu/bcs/schillerlab/research/A-Vision/A15-2.htm

The Spinning Dancer Illusion

spinning dancer illusion
Understanding the Spinning Dancer Illusion The Spinning Dancer illusion can appear to spin both clockwise and counterclockwise.. Nobuyuki Kayahara

The spinning dancer illusion shows an ambiguous silhouette that appears to abruptly change direction. Learn more about how this illusion works.

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 perception1 in which an ambiguous 2-dimensional figure can be seen in from two different perspectives.2 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 column3, 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."


1 The spinning dancer and the brain. http://greengabbro.net/2007/10/20/the-spinning-dancer-and-the-brain/

2 Bach, M. (n.d). Silhouette illusion. http://www.michaelbach.de/ot/sze_silhouette/index.html

3 Parker-Pope, T. (2008, April 28). The truth about the spinning dancer. The New York Times, http://well.blogs.nytimes.com/2008/04/28/the-truth-about-the-spinning-dancer/index.html

Zöllner illusion

Zollner Illusion
Understanding the Zöllner illusion. Fibonacci

In the Zöllner illusion, straight lines appear to move even though they are static.

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.

The Ames Room Illusion

the ames room illusion
Understanding the Ames Room Illusion The Ames Room Illusion. Image courtesy Mosso

In the Ames room illusion, two people standing in a room appear to be of dramatically different sizes, even though they are the same size.

What Do You See?

The image above was captured by a visitor to the "Ames room" in Vilette science museum in Paris, France and uploaded to Flickr, a photo-sharing website. In the room, the individual on the left appears to be very tall, while the person on the right looks very small. In reality, both people are of approximately the same height and size.

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.

You can see more examples of Ames rooms in these YouTube videos.


Ryan, C. (1997). Exploring Perception. Brooks/Cole Publishing: Pacific Grove, CA.

The Ponzo Illusion

ponzo illusion
Understanding the Ponzo Illusion The Ponzo Illusion. Image from Wikimedia Commons

In the Ponzo illusion, two identically-sized lines appear to be different sizes when placed over parallel lines that seem to converge as they recede into the distance.

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.

The Kanizsa Triangle Illusion

Kanizsa Triangle
Understanding the Kanizsa Triangle In the Kanizsa Triangle, illusory contours create the illusion of an equilateral triangle.. Image from the Wikimedia Commons

The Kanizsa Triangle is an optical illusion in which a triangle is perceived even though it is not actually there.

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.


Kanizsa, G. (1955). Margini quasi-percettivi in campi con stimolazione omogenea. Rivista di Psicologia 49(1): 7–30

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