Cognitive illusions are based on previous knowledge and experiences of the world. The eye may perceive more than one image in a picture but the brain chooses one based on previous experiences and knowledge about the world. There are three types of cognitive illusions, ambiguous, paradox and distorting illusions.

Ambiguous illusions deal with pictures or objects that change in appearance. The perception will switch between the possible views of the picture or object. These illusions are fairly common and occur in areas such as math with the “Necker Cube.”* The Necker Cube shows that the mind perceives both images that are present but chooses to pass on to the brain only one of the two alternatives. The mind can also switch freely between the images, but not comprehend both at the same time.

Paradox illusions show objects or pictures of objects that are paradoxical or impossible. Often, paradox illusions are created for the purpose of being a paradox, and are rarely found in nature. The most common paradox illusion is the “Penrose Triangle”* also known as the impossible triangle. In the case of distortion illusions, the mind sees a disordered or distorted 2D image and tries to create an image that makes sense to the logical part of the brain. The retina sees an image but the picture the mind sees is an imperfect representation of reality (in the same way the mind cannot accurately represent 2D images and 3D space). The human visual system is capable of processing complex information to create a representation of reality, but sometimes this representation is inaccurate. An interesting aspect of distortion illusions is that even though the mind knows how reality should be perceived, the visual system still creates the false representation. This is especially prominent in the “Café Wall Illusion”*, where even though it is possible to use a ruler to check that all lines are straight, the lines still appear crooked when observed. Cognitive illusions are rarely found in nature, and therefore do not play as large a role in an individuals understanding of the world. It is interesting however, that the mind can be fooled into perceiving something that is not reality, unless reality is what we perceive. Cognitive illusions have been used entertainment purposes since they were discovered and do not affect the perception of reality as much as physiological illusions. They do, however, prove that what we see is not always what is really there.