PERCEPTUAL ILLUSIONSA perceptual illusion is defined as ‘the perception of something objectively in such a way as to cause misinterpretation of its actual nature’. However, an illusion is not always a misinterpretation, but rather it convinces us that the real life version of an object is untrue or false.Perceptual illusions are divided into two parts, i.e. sensory illusion and optical illusion.

Sensory Illusions Sensory illusions occur when the brain misinterprets signals received from the sensory receptors present in our body such as ears, tongue, nose, etc.Sensory illusions are further divided into auditory, tactile, olfactory and taste illusions.

Auditory IllusionAuditory illusions are illusions that distort what we hear. Examples of auditory illusions are the Shepard scale and the McGurk effect.

Tactile IllusionsTactile illusions are caused when there is misinterpretation of signals by the brain which are received by the receptors present in our skin. For example, if you pull your top lip to left and the bottom lip to the right, then prod the middle of the lips with a pencil, it feels like there are two. Another example is of the Phantom Limb, where an amputee’s brain still believes that the limb is still there, causing her/him to feel the limb or even feel pain.

Olfactory IllusionsOlfactory illusions occur when the brain misinterprets signals received from the receptors present in the nose.

Taste IllusionsTaste illusions take place when the brain misinterprets signals received from the receptors present on the tongue. For example, mixing barley tea and milk produces a coffee milk taste, while cucumber and honey tastes like melons.

Optical IllusionsOptical illusions occur when the brain misinterprets signals received from the sensory receptors present in the eyes.There are three types of optical illusions, i.e. literal, physiological and cognitive.

Literal IllusionsA literal illusion is an image created by smaller images that are in no way related to the larger, overall image created. Here are some examples of literal illusions:

(a) (b)

In picture (a) a dog, a man and a woman carrying a baby are part of a larger image, which is a portrait of an old man. In picture (b) two horses, a bird and mountains are part of a overall image of a woman.

Physiological IllusionsA physiological illusion can occur after prolonged visual stimulation. An afterimage, for example is a physiological illusion. According to theorists, a viewer’s perception may be changed as a result of a physiological imbalance. This imbalance is usually caused by over-stimulated nerve paths caused by competition between the light and dark receptors in the retina. This competition is essentially what leads to the physiological imbalance. Here are some examples of physiological illusions:

(a)

(b)

Look at both the images (one at a time) for more then 2 minutes and then look away to a blank wall or screen and you will be able to see the images on the wall or screen because they are retained in your brain for some time. After looking at image (a) we can see the dots on the screen and in image (b) we can see the image of Queen Elizabeth on the screen after looking away from the image.

(a) (b)Keep staring at the above images for some time and you will see them move. In the first image the rings (circles) look as if they are moving and in the second image you will see the dancer slowly moving either to the left, right or both. This occurs because our brain tries to construct space around the rings in the first image and around the spinning figure in the second image. The same effect also applies to (c) where it looks as though the center of the picture is going into the page.

(c)

Cognitive Illusions A cognitive illusion can be defined as the viewer’s knowledge and assumptions about the world, or unconscious inferences. Cognitive illusions are categorized into three parts namely ambiguous illusions, distorting illusions and paradox or fiction illusions.

Ambiguous IllusionsAmbiguous illusions are images or objects that allow for the viewer to have two valid interpretations of what the object represents or what it actually is. The viewer is usually able to mentally visualize one interpretation right away and eventually the second, after some time. However both interpretations cannot be seen at the same time because that would interfere with the full perception of either one and the brain simply does not allow that. Examples of ambiguous illusions are:

(a)The Necker Cube (b)In image (a) it is hard to tell whether the corner marked red is coming out or going into the page. In image (b) from one perspective we can see the image of a duck while from the opposite side we can see the image of a rabbit.

Distorting illusionsDistorting illusions are images or objects that are distorted in their geometrical make (i.e. size, length, position or curvature).Examples of distorting illusions are:

(a) Muller-Lyer

illusion (b) In image (a) both the lines are of the same size but the line which is on top looks bigger because the arrows at the ends of the line are facing inwards as opposed to the line below which has the arrows at the ends facing outwards which makes it look smaller. In image (b) the circles at the center are of the same size but one looks bigger than the other because of the size of the surrounding circles.

Paradox or Fiction IllusionA paradox or fiction illusion is an image or object that is simply impossible in 3-D or in real life, however they look very real and very possible in 2-D representations. An example of paradox illusions are the Penrose stairs:

As we look at the Penrose stairs it seems like a normal set of stairs, however it becomes evident that we are either ascending or descending, but getting nowhere.