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The Senses

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Visual illusions
Definition of Perception
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Tool Module : OpticsOptics
Experience Module : The Blind Spot  The Blind Spot

Better Optical Illusions


Perceptual constancy is our tendency to see familiar objects as having a constant shape, size, and colour, regardless of any changes in perspective, distance, or lighting that they undergo. Our perception of these objects under such changing conditions is much closer to the general image of them that we have memorized than to the actual stimulus reaching our retinas. Thus, perceptual constancy is what lets you recognize a plate of vegetables, for instance, regardless of whether you are looking down on it at your own table, or noticing it on someone else's table in front of you in a dark restaurant, or seeing it in side view on a huge billboard several dozen metres away from you as you drive past it in broad daylight.

French psychologist Jean Piaget showed that this perceptual constancy is far from innate and is learned in childhood. As children learn how to grasp objects, they form an idea of the objects' size and distance, and as children move around, they learn to recognize objects from various angles and under various light conditions.









Adelson’s Checkerboard


The human visual system analyzes the interactions between visible electromagnetic waves and the objects in our environment, extracts information about the world from them, and makes visual perception possible. Visual perception is considered a dynamic process that goes far beyond simply replicating the visual information provided by the retina.

To perceive is to create a figure or shape that does not necessarily appear as such in the real world but that we can represent mentally so that we can recognize it under various conditions (for instance, when it is partly hidden). Hence, by studying the way that the brain fills in missing or ambiguous visual information, we can learn a lot about the way that we perceive the world. Optical illusions provide fertile ground for such study, because they involve ambiguous images that force the brain to make decisions that tell us about how we perceive things.

Most optical illusions result from processes in the cortex, but some do originate in the retina. One such illusion is the Hermann grid shown here, in which gray spots appear at the intersections of the rows and columns created by the squares, because of a phenomenon called lateral retinal inhibition. If you stare directly at one of these intersections, however, the gray spot disappears, and it looks white, because then you are using the cells of the fovea, which do much less correction for an area's surroundings.

Experience : An illusion by Victor VasarelyLink : TRICKS OF THE EYE, WISDOM OF THE BRAIN


Here is another example of an illusion of retinal origin. Look at the bird for about 20 seconds, then look at the cage: the bird's silhouette will appear inside it, in red. The reason this time is the cones, the colour-sensitive receptors in the retina. As you stare at the bird, the green cones covering its shape on your retina gradually become desensitized, so that the other cones begin to dominate. When you then look at the cage, with its white background, a red bird appears, because the white minus the green creates a reddish light. This image that persists when you stop looking at an object is called a residual image.



Despite appearances, these two circles are the same shade of grey (place your mouse cursor over either image to prove it).


These two circles are the same colour, even though the one on the blue background seems reddish and the one on the red background seems bluish (if you don't believe it, place your mouse cursor over the picture). Thus the background effect also influences colour perception.


Because the human visual system interprets lighting as coming from above, we perceive the spheres whose upper part is brighter as emerging from this image, and the spheres whose upper part is darker as receding into it.

Context strongly affects our perceptions. A person of average height will seem like a dwarf if placed in the middle of a basketball team, and like a giant if placed in the middle of a kindergarten class. In short, an object's surroundings always influence our perception of that object.

The same phenomenon also applies to the apparent brightness of a surface, which depends not only on its own luminance but also on the luminance of the areas surrounding it. As shown here, given two circles that are the same shade of grey, the one placed on a dark background will look lighter and the one placed on a light background will look darker. One explanation often offered for this phenomenon is that the brain thinks of the circle on the dark background as a light disc in a dimly lit location, and the circle on the light background as a dark disc in a brightly lit location. This would suffice to make the two circles appear to be different shades of grey even though they are not.

Another way to explain this phenomenon is to think of our perception as being adapted to past surrounding stimuli, so that they constitute a sort of baseline from which subsequent stimuli are perceived—a bit like the profound silence you perceive when the refrigerator suddenly stops running. What you were perceiving as silence before the refrigerator stopped was actually the steady humming that you have gotten used to.

Link : Lightness Perception and Lightness IllusionsExperience : Hermann-Grid and Scintillating GridExperience : “Induced Grating”Experience : “Contrast Constancy”Link : An empirical explanation of color contrastExperience : how many colours can you see?
Experience : Neon Colour SpreadingLink : Color constancyLink : Color Constancy Link : Context EffectsExperience : Context

The lighting of a scene is an important factor that the visual system considers to help it identify objects. As soon as you interpret a visual object as possibly being three-dimensional, your visual system immediately tries to determine where the light is coming from, then uses this information to decode the object's properties.

Link: The influence of depicted illumination on brightness

The most disturbing optical illusions often combine several phenomena all working in the same direction to accentuate the error of interpretation that the visual system is making. In Adelson's checkerboard, below, you would swear that squares A and B were black and white, but they're actually the same shade of grey. Here's how it works.

First of all, there is an illusion of context (see above). Square A is surrounded by light squares, which make you perceive it as darker. Conversely, square B is surrounded by dark squares, which make you perceive it as lighter. Meanwhile, your eyes automatically interpret the darker area to the left of the cylinder as a shadow, because the gradations of green on the cylinder suggest that a light source to its right is making it cast a shadow to its left. On the basis of past experience, your visual system assumes that square B would actually be brighter in full light, since even in "shadow" it seems brighter than the squares surrounding it.

This illusion ultimately works because every square is surrounded by a well defined "X" structure consisting of four other squares, which strongly indicates to your visual system that the square at the centre must be interpreted as a change in the colour of the surface itself, and not a change caused by differences in light or shadow.

Far from demonstrating the faults in the human visual system, these phenomena actually reveal the powerful mechanisms of discernment that let us isolate and identify objects among the myriad confusing shapes in the real world.

Experience : The simultaneous contrast effect Experience : The Criss-Cross illusion Experience : Adelson's  “Corrugated Plaid” Experience : Contrast Gain Control Experience : The Impossible Steps Experience : White's illusion Experience : interactive movies based on a paper by Edward H. Adelson Experience : L' échiquier d'Adelson - la preuve - démonstration

There are many illusions in which you perceive a figure standing out from its background even though this subjective image has no lines defining its boundaries and displays no differences in brightness that would let you isolate it from its surroundings.

Once again, the visual system is not passive. It automatically extends line segments into parts of the drawing where they are missing, so that your mind imagines that an object has been placed over the abstract shapes and lines in the drawing. The perception of such contourless figures thus reflects some innate properties of the way the visual system is wired.

Experience : Blur & picture contentExperience : No sex… Blur & figure-ground reversalExperience : Blotted letters - a Gestalt phenomenonExperience : Rotation changes the interpretationExperience : Shading, Depth and FacesExperience : Inverted faces - Thompson's “Thatcher Illusion”



In another category of illusions that rely on the subjectivity of contours, the figure can become the background, and vice versa. The psychologists who developed Gestalt theory created many such images in which the figure/ground relationship is ambiguous. These images demonstrated the central tenet of Gestalt theory: that the whole has global properties different from those derived from the sum of its parts.

In the first, classic example shown to the right here, two profiles facing each other delimit a space that can be seen as a goblet, and your perception can alternate between the profiles and the goblet. If you focus most of your attention on the light part of the image, you will perceive that part as the figure and automatically see the dark part as simply the background. The reverse is also true. Thus, perceiving one figure prevents you from perceiving the other.

It thus seems that to interpret a complex image, your brain has to identify a main figure and relegate the rest of the image to the background. Such illusions clearly demonstrate how your visual system groups and separates the characteristics of a complex image in order to recognize objects within it. Some facetious artists take advantage of these necessary associational mechanisms of perception to leaves two possibilities open, each of which is just as plausible as the other.

Lien : Perceptual OrganizationLien : Gestalt Principles of Visual Organization Expérience : The Koffka RingExpérience : “Biological Motion”Lien : Figures ambiguës


Because the missing quarters of these 4 discs all face inward, they create the perception of a square with a subjective contour.

A circle seems to have been placed on top of these converging lines, even though there is no contour defining this circle.


Two profiles, or a goblet?

A saxophone player, or a woman's face?

What's showing here, the gentlemen's scalps, or the lady's bosom?

Your visual system takes two-dimensional images projected onto your two retinas and uses these images to reconstruct a three-dimensional perception of the world around you. To perceive the depth in a visual scene, your visual cortex relies on two kinds of information: the information that your binocular vision provides by integrating the two slightly different images from your two eyes, and the information that your monocular vision provides from the image perceived by each eye separately.

When an object is close up, you rely mainly on the disparity between the two images perceived by your two eyes—in other words, on your binocular vision. The reason is simply that the closer an object is to your eyes, the greater the difference in the angles from which your two eyes view it.

But even with monocular vision, you can receive an impression of depth, because your brain deduces it from several indicators.

- Interposition is certainly the most common depth indicator. Whenever one object hides your view of another object either partly or completely, your brain deduces that the hidden object is farther away, simply because the other object is in front of it.

- Atmospheric perspective is created by the dust particles and water vapour in the air, which cause objects to seem dimmer and blurrier the farther away they are.

Experience : The snake illusionExperience : The Haze illusion

- Texture gradients appear when you look at surfaces from a certain angle; the texture seems to become denser and less detailed as your eye moves toward the part of the surface that is farthest from you.

- Object size is another depth indicator that your brain refers to constantly. When you don't know the exact size of two objects, but you do know that they are identical, and one of them projects a smaller image on your retina, you interpret it as being farther away. In other words, you judge the two objects' relative size. Similarly, if you are very sure of the size of a familiar object, you use its size as a reference to estimate its distance, since you know that the smaller it appears, the farther away it is.

- Parallax movement occurs when when you are in motion yourself, and objects that are at different distances from you appear to be moving at different speeds. The farther away these objects are, the smaller the parallax movement.

Despite appearances, the monster being chased is the same size as the one doing the chasing. Don't believe it? Place your mouse cursor over the picture.

- Linear perspective is the well known phenomenon in which parallel lines appear to converge as they recede into the distance and appear to meet at a point on the horizon. If there are two objects, and one of them is closer to this point, your visual system assumes that this object is farther away. And generally, it is, but there are two exceptions.

The first involves drawings like the one to the left, where two identical objects are placed along converging lines that imply perspective. The two monsters in this picture are the same size, so the two images that they make on your retina are the same size too. But because your brain assumes that the monster who is closer to the virtual horizon must be farther away, your brain also assumes that this monster must be larger, in order to create the same size image on your retina.

The other exception involves heavenly bodies such as the Moon, which create the well known illusion of the Full Moon appearing larger when it first comes up over the horizon.

Experience : Hering IllusionLink : L'INTERPRETATION DES ILLUSIONS


One important source of ambiguity for the visual system is that the world is three-dimensional, but the images that it projects onto your retina are two-dimensional. Hence differing objects, depending on their distance and orientation, may occupy the same amount of surface area on your retina. Your brain therefore becomes confused, and tries to use other indicators to clarify the situation. Two such indicators are your own past experience with the object in question and the experience of the human species, which is encoded in your genes.


A given angle projected on the retina may come from various objects at various angles,
of varying lengths, and with varying orientations in space.

Experience : The Temple Illusion
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