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The Senses

HelpLink : PerceptionLink : OmnivisionLink : IllusionTours
Research : Frank Tong
Experience : Visual illusionsExperience : Sight (Vision)
Tool : Definition of Perception

Better Optical Illusions

Adelson’s Checkerboard

One of the most common optical illusions is the Full Moon, which looks so enormous when it first comes up over the horizon. This is clearly a misinterpretation on the brain's part, because the Moon is 385 000 km from Earth and therefore always covers the same number of degrees of arc on the retina—about 0.5—whether it is at the horizon or directly overhead.

Since the image formed on the retina is thus always the same, it is the presence of the horizon close to the disc of the Moon that makes us perceive it as larger. To prove this to yourself, cut a hole in a piece of paper, then close one eye, hold the paper up to the other, and look through the hole at the rising Moon while blocking your view of the horizon. The Moon will look much smaller—so much smaller that if you open your other eye, which can see the horizon as well as the Moon, your two eyes will each see a different-sized Moon!

From experience, we know that a cloud, an airplane, or a tree will seem smaller if it is near the horizon than if it is overhead. This rule applies to all the objects that we deal with on Earth. Our visual apparatus seems to have been shaped by evolution on the basis of this reality, so that we are at a loss when we have to interpret an object like the Moon, which is so far away that it occupies the same amount of area on our retina regardless of whether it is near to or far from the horizon. When the Moon is near the horizon, the brain may tell itself that if this object's image doesn't become smaller near the horizon, then it must be really big, and the brain then makes us perceive the Moon accordingly.

Another way to explain this is to say that to the brain, the default distance to an object is always less than the distance to the horizon. Of course, the brain applies this rule to terrestrial objects, since these are the only ones it knows, except for the Moon and the Sun. The distance to these two heavenly bodies does not vary whether they are at the horizon or the zenith, which confuses our visual systems. Some refinements to this explanation have been proposed, and the psychological mechanisms that cause this illusion with the Moon are still being debated.

Link : Experiment in Perception: The Ponzo Illusion and the Moon.Link : The Moon Illusion ExplainedLink : The Moon Illusion, An Unsolved Mystery.

Mechanisms that cause optical illusions have been located in various places along the nervous system's visual pathways. Some of these mechanisms arise in the retina, but most of them result from the way that the images captured with the eyes are reconstructed by the visual cortex (see box below).

Contrary to what we intuitively believe, the information presented by our senses does not actually correspond to reality directly. With vision, for example, the image striking the retina contains vastly more information than is actually transmitted to the brain by the optic nerve. This makes sense, when you consider that the 125 million photoreceptors in each retina converge onto 100 times fewer ganglion cells.

To compensate for this massive loss of information and provide us with visual perceptions that are rich in contrast, colour, and movement, the brain introduces abstract parameters that often fill in or amplify the fragments of reality that it is given to work with. The brain's powers to interpret visual information in this way are so great that it sometimes creates an impression of coherence where there is none—in other words, an optical illusion.

In geometric optical illusions, there is generally an "inducing element" that causes the misinterpretation and a "test element" that is the subject of it. For example, in Zöllner's illusion (right), the small vertical and horizontal lines are the inducing element and the long diagonal lines are the test element.

Experience : Poggendorff IllusionExperience : Café Wall Illusion

In the size-relationship illusion, the proximity of a test element to larger inducing elements causes the size of the test element to be underestimated. The opposite occurs with smaller inducing elements, which cause the size of a test element to be overestimated. The result is that though two test elements are identical, they can look different to us, because of the context effect.

The presence of lines suggesting perspective can also create size illusions. Given two objects of equal size, if one of them looks farther away because of perspective, we will perceive it as being larger.

Lines that converge toward the horizon are an indicator of distance that the brain has been relying on to estimate distances ever since it first began evolving. This is also the explanation for the illusion that makes the full moon look so big when it is rising (see sidebar).

Link : Context and Expectations; Categorization and Selectivity


In Zöllner's illusion, the long lines are parallel even though they look as if they would intersect one another if they were extended. (If you don't believe it, place your mouse cursor over the picture.) The reason for this illusion is that the brain tries to bring the angles between the short lines and the long ones closer to 90°, thus "tilting" the lines toward one another.


Place your mouse cursor over this picture, and you'll see that the two central circles are actually the same size.


The effective of perspective is strengthened here by the checkerboard pattern, which your brain uses to estimate the size of the two vertical lines. Place your mouse cursor over this picture, and you'll see that these two lines are actually the same height.

Two incompatible viewpoints cleverly combined in one drawing.

Young woman or old woman? The young woman's chin is the old woman's nose, and the old woman's eye is the young woman's ear.


Artistic optical illusions are works of art in which the artist has deliberately introduced elements designed to lead to unusual interpretations or to depict objects that could not exist in physical reality.

Some artistic optical illusions are constructed by combining two different drawings that lead to incompatible interpretations.





Other artistic optical illusions involve ambiguity, so that a drawing can be visually interpreted in at least two ways that are mutually exclusive. Once an observer has identified the markers for the various possible interpretations, he or she can move among them at will. These kinds of illusions in which the observer goes back and forth between two interpretations of the same image are similar to illusions in which the figure and the background are interchangeable.

Motion illusions are another major category of optical illusions .

Some images can give the illusion that their elements are moving when you move yourself slightly relative to them. In the image here, for example, if you stare at the centre dot, then move your head in toward the screen, the two circles will start to seem as if they are turning in opposite directions.


For other motion illusions, you don't even have to move. The particular arrangement of the graphic elements in the picture suffices on its own to create the appearance of movement as you look at it. That is what happens in the figure here, because the pattern makes it hard for your eye to determine the contours of the circle in the centre.

In the image below, the illusion that some of the wheels are turning occurs only in your peripheral vision: as soon as you look straight at one of the wheels, it holds still, but the wheels that are peripheral to it keep turning. Though this illusion has not been fully explained, we do know that the order in which the four areas of differing colour and brightness are placed is decisive. More specifically, the illusory movement seems to occur from black areas to adjacent areas that are dark but brighter than the black ones (here, the blue areas), or from white areas to adjacent areas that are coloured but not so bright as the white ones (here, the yellow areas).

Source: Akiyoshi Kitaoka, Department of Psychology, Ritsumeikan University, Kyoto, Japan

Link : Phenomenal Characteristics of the peripheral drift illusionLink : Akiyoshi's illusion pagesExperience : “Rotating Snake” Illusion Experience : “Stepping feet” illusion 1
Experience : Pinna-Brelstaff IllusionExperience : Motion Aftereffect (Waterfall Illusion)Experience : Reverse Spoke IllusionLink : Movement Perception

In other circumstances, we can perceive motion when we are simply shown two or more stationary images with a short enough time interval between them. One familiar example of this kind of illusion is the beta effect. In its simplest form, it occurs when an observer who has no reference markers is alternately shown two points of light that are slightly separated from each other (when one point goes dark, the other lights up).

Link : Motion Perception

Geometric illusions do not arise from the retina, because they appear almost as clearly when the inducing element is placed in front of one eye and the test element in front of the other. This indicates that these illusions arise at the point where the information from the two eyes converges for the first time, beyond the lateral geniculate nucleus, in the visual cortex.

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