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Cognitive Illusions

The way you look at an object can affect how you see it. Sometimes there are two images in the same picture, but you can only see one at a time so your brain chooses one (when it deals with too much information).

Version of the image first created
by Swedish artist Oscar Reutersvärd 

Cognitive “illusions” rely on stored knowledge about the world (depth, rabbits, women) and are also under some degree of conscious control (we can generally reverse the perception at will).

Cognitive illusions.
Instead of demonstrating a physiological base they interact with different levels of perceptual processing, in-built assumptions or 'knowledge' are misdirected. Cognitive illusions are commonly divided into ambiguous illusions, distorting illusions, paradox illusions, or fiction illusions. They often exploit the predictive hypotheses of early visual processing. Stereograms are based on a cognitive visual illusion.

Ambiguous illusions are pictures or objects that offer significant changes in appearance. Perception will 'switch' between the alternates as they are considered in turn as available data does not confirm a single view. The Necker cube is a well known example, the motion parallax due to movement is being misinterpreted, even in the face of other sensory data. Another popular is the Rubin vase.

Paradox illusions offer objects that are paradoxical or impossible, such as the Penrose triangle or impossible staircases seen, for example, in the work of M. C. Escher. The impossible triangle is an illusion dependent on a cognitive misunderstanding that adjacent edges must join. They occur as a byproduct of perceptual learning.

Distorting illusions are the most common, these illusions offer distortions of size, length, or curvature. They were simple to discover and are easily repeatable. Many are physiological illusions, such as the Café wall illusion which exploits the early visual system encoding for edges.
Other distortions, such as converging line illusions, are more difficult to place as physiological or cognitive as the depth-cue challenges they offer are not easily placed. All pictures that have perspective cues are in effect illusions. Visual judgments as to size are controlled by perspective or other depth-cues and can easily be wrongly set.

Fiction illusions are the perception of objects that are genuinely not there to all but a single observer, such as those induced by schizophrenia or hallucinogenic drugs.


Cognitive Illusions - Examples

Ambiguous Figures

Ambiguous figures demonstrates our ability to shift between figure and ground which provides the basis for the two interpretations of these figures.
They exemplify the fact that sometimes the same perceptual input can lead to very different representations. The mind was actively involved in interpreting the input.

The Reversible Figures


Look at the red dot. Is it located in the upper right front
 or the upper right rear? To help you see the different options,
the front wall of the cube is colored gray.

This reversible figure is called the Schroder Staircase
Are you seeing the stairs from below or above?

Optical Illusion

Are you looking at this cylinder from left to right or right to left?


Rubin's Vase

Rabbit or a Duck?

A young lady or an old woman?

Click on 3 images above to enlarge.

Octavio Ocampo Mouth of flower

Octavio Ocampo Shiva

Octavio Ocampo Calvary

Octavio Ocampo Silver Threads

Octavio Ocampo

Octavio Ocampo

There are 9 people in this picture, which is a work
by the Mexican artist Octavio Ocampo


For more, please visit Official Website of Octavio Ocampo

                 Copyright 2005 by

Mysterious Figures: Dark Angel

                                                                          Copyright 2005 by

Killer Whale?


Tessellations in art can be mainly linked to M.C. Escher, a Dutch artist, born in 1898. After being a graphic artist, he traveled to Spain and did sketches of the art he saw at the Alhambra, a Morrish temple. He became interested in tilings and started to incorporate geometric designs into his art. Escher created hundreds, maybe thousands, of tessellating shapes in the forms of fish, dogs, crabs, and other beasts.


Completion Figures

Completion figures are figures which the mind rather unambiguously interprets in a particular way despite the fact that the input is incomplete relative to what is typically "seen". Illusory contours may be partly accounted for by low level contrast effects, partly by more cognitive processes inferring the existence of occluding objects.

Triangle Completion - Seeing what is not there!

These two Kanizsa figures shown above illustrate the mind's willingness to see an equilateral triangle despite the fact that no border information about the center triangle is in the picture.

Optical Illusion

Do you see the letter "E" or just black lines?

Paradox Illusions: Impossible Figures and Objects

Impossible Triangle


Impossible Triangle


Version by Penrose

Another version of the
Impossible Triangle

Another version of the
Impossible Triangle


More Impossible Objects


Impossible stairs

Impossible wheels - by M.C. Escher


Devil's Pitchfork.
This 2-D picture represents a 3d object that cannot possibly exist.

Instructions from hell

Near the ground, you can count five different elephant feet.
If you cover up the ground, you can only see four limbs extending from the torso.


M.C. Escher was very clever at representing impossible objects.
Click on the images above to enlarge.

Follow the stairs on the castle terrace.
They appear to be going up forever!
Image by M.C. Escher.

M.C. Escher "Drawing Hands", 1948 (lithography)

Stereograms - 3D illusions

Stereograms are 3D images hidden within another picture. In order to view the 3-D images, simply stare at the picture until the image starts to take shape.

Image Courtesy of Studio V2
3-D Roses. Can you see the hidden image?

Image courtesy of and copyright Gary W. Priester.  Source:

Zen Spiral. Can you see the hidden image?

Distorting Illusions

 Geometric illusions

Geometric illusions are examples of how our mind attempts to find orderly representations out of sometimes ambiguous and disorderly 2d images. The images transmitted from our retina to our brain are imperfect representation of reality (for example 2d images cannot accurately represent 3d space). Our visual system is capable of performing complex processing of information received from the eyes in order to extract meaningful perceptions. Sometimes, however, this process can lead to faulty perceptions.

The Café Wall Illusion.  Are the lines crooked are straight? If you stare at a single cube, do the adjacent lines appear to slide past each other?

Some visual stimuli cannot be perceived in a way that accords with what we can measure (with a ruler or similar) e.g. The Café Wall Illusion Even if we know that the lines of mortar are all straight, we see them as sloping. Illusions are cases where we find significant differences between perceived and measured reality (a very broad definition).

Müller-Lyer Illusion


The center connecting line is seen as shorter
in the top figure that in the lower figure.

The figure above shows both figures superimposed on one another in order to demonstrate in yet another way that the center line is of equal length in both figures.

Optical Illusion

Which line is longer, AB or BC?
They are the same length.


Ebbinghaus Illusion

You probably perceive the middle circle as smaller in the figure on the left than the circle in the center of the second figure. They are actually the same size.


Ebbinghaus Illusion



Perspective, Depth and Distance

Your eyes judge distance based on the size of objects and where the objects are positioned. For example, if you don’t know the size of two objects, you may see one as smaller because it is farther away. In reality, the objects are the same size. An easy way to think about this is by using train tracks. Train tracks appear to get smaller as they get farther away, but as you move along them, you see they are the same. Lines that appear to come together in the distance make you have a distorted perception of distance.

Perspective is an illusion that makes parallel road lines
appear to come together in the distance. 


Version of Ponzo Illusion.
Perspective illusion. Both yellow lines are the same length.


Moon Illusion

The variation in the apparent size of the Moon (smaller when overhead, larger when near the horizon) is another natural illusion; it is not an optical phenomenon, but rather a cognitive or perceptual illusion.

Image based on photo by R. Berdan

Moon Illusion.  The moon illusion is one of the most famous of all illusions. Stated simply, the full moon, when just above the horizon, appears much larger than when it is overhead. Yet the moon, a quarter of a million miles away from the earth, always subtends the same angle wherever it is in the sky, roughly 0.5 degrees.

Explanation: Is the Moon larger when near the horizon? No -- as shown above, the Moon appears to be very nearly the same size no matter its location on the sky. Oddly, the cause or causes for the common Moon Illusion are still being debated. Two leading explanations both hinge on the illusion that foreground objects make a horizon Moon seem farther in the distance. The historically most popular explanation then holds that the mind interprets more distant objects as wider, while a more recent explanation adds that the distance illusion may actually make the eye focus differently. Either way, the angular diameter of the Moon is always about 0.5 degrees. In the above time-lapse sequence taken near the end of last year, the Moon was briefly re-imaged every 2.5 minutes, with the last exposure of longer duration to bring up a magnificent panorama of the city of Seattle.


See Explanation.  Clicking on the picture will download
 the highest resolution version available.

Moonrise Over Seattle. Credit & Copyright: Shay Stephens
Sky watchers have known this for thousands of years: moons hanging low in the sky look unnaturally big. Cameras don't see it, but our eyes do.

How does this illusion come about? Since the moon always subtends an angle of 0.5 degrees, the image on the retina must always be the same. Clearly the problem is one of interpretation. One simple experiment shows this to be so. A full moon just above the horizon will not appear so large to the human eye if a piece of paper is held up to that eye with a hole in it, so that only the moon can be seen through the hole and not the horizon. If the other eye is open at the same time, viewing both the moon and the horizon, the two eyes will each see different sized moons!

The explanation is believed to be as follows. We 'know' that a cloud that is overhead will be larger than when it moves towards the horizon. And an airplane that is a mere speck on the horizon becomes large when it is overhead. And we are all familiar with standing under a tree which seems enormous, yet at a couple of hundred paces seems insignificant. It would seem that so much of our world is interpreted this way that we are ill-equipped to cope with an object like the moon, that subtends the same angle at the eye, whatever position it occupies in the sky. And so our brain 'interprets' the image that it 'sees', and tells us that the moon is larger than it really is.

People have thought that the thicker atmosphere along the horizon could act as a magnifying glass enlarging the image of the full Moon when it is on the horizon. That could not be the case as there is not enough atmosphere around the Earth to cause a dramatic lens effect. Anyway, according to the laws of physics, if the atmosphere was really refracting the image of the Moon, it would appear smaller!

Regardless of its elevation, the distance between an observer (at the center of the horizontal line) and the moon remains constant (unfilled circles). However, a moon perceived as growing closer as its elevation increases (filled circles), must appear as growing smaller. Source:

Some scientists have proposed that the Moon Illusion effect depends on our perception of the sky as a flat-topped dome the rim of which appears further away than the top of the dome. The effect of this error in perspective is for the Moon overhead to appear smaller than the horizon Moon. The diagram (fig. 2) opposite shows the apparent location of the Moon at various points as it travels across the sky. This is the diagram commonly seen in books promoting this hypothesis… But the diagram can be misleading! The hemispherical flat-topped dome in the picture is not of proven relevance to the effect and ought to be omitted as it falsely suggests a mental process of "projecting" the moon onto that dome.

Others have proposed that the Moon Illusion had to do with the fact that the eye-brain system is designed to work on the horizontal plane, not the vertical plane. On the horizon we process the Moon image in the optimal orientation giving us its true apparent size. Tipping our head back to view the high Moon, we see a non-optimal image. The illusion is not that the horizon Moon is larger, but that the overhead Moon is smaller in size than it "ought" to be. Others have argued that comparisons with buildings and other objects on the horizon are responsible for the differences between the Moon’s apparent size when looking horizontally and looking vertically (this explanation is contradicted by the fact that the Moon Illusion also occurs over open water).

Finally, here is an explanation that is sufficiently satisfactory. The effect of this illusion is due mainly to the fact that our brain interprets the sky as being farther away near the horizon, and closer near the zenith (directly overhead, see fig. 3 opposite). This isn’t surprising; look at the sky on a cloudy day and the clouds overhead may be a few kilometers above you, but near the horizon they might be hundreds of kilometers away. The Moon, when it’s on the horizon, is interpreted by your brain as being farther away. Since it’s the same apparent size as when it’s high up, your brain figures it must be physically bigger (as illustrated in fig. 4 further below). Otherwise, the distance would make it look smaller. This effect is the well-known Ponzo Illusion (fig. 5.a). Actually, the Moon Illusion effect is the result of a mix of Ebbinghaus size illusion (fig. 5.b) plus Ponzo illusion (see resulting fig. 6).


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