Depth Perception and Optical Illusions

Optical Illusions

An optical illusion is an illusion caused by the visual system and characterized by visually perceived images that are different what the object really looks like. Information gathered by the eye is process by the brain to give a perception that does not go along with the actual measurement of the object/image. There are three main types of optical illusions: literal optical illusions that create images that are different from the objects that make them, physiological illusions that are the effects of excessive stimulation of a specific type, and cognitive illusions which are the results of unconscious inferences.

Hollow Face Illusion

This is a flat sheet of plastic with a facial mask pushed in on one side. It is a picture of Albert Eisenstein. The picture was actually taken from the side, with the nose being the farthest way. This where the optical illusion comes in because when you look at the picture, it looks like he is facing forward when really the it was sculpted from the side of his face. another illusion is that if you were to see this in person, when you walk away it looks like his eyes are following you, even though they cannot move. This effect happens when you change your viewing direction. The explanation for seeing these illusions is that the retina (part of your eye) sees in two dimensional, but we live in a three dimensional world. In order to perceive depth and distance, they eye and the brain must make guesses about the structure of the space. In my previous medium, I talked about depth perception cues, and this is where they come in handy. If we have reason to believe that two objects are the same size, the one that creates a larger image on the retina is perceived as closer. The brain uses different cues to make guesses on how far the away the object is and its depth. The different cues used are: relative size, occlusion, distance, disparity, and shadow.

Ponzo Illusion

The Ponzo illusion is an example of an illusion that uses monocular (one eye) cues of depth perception to trick the eye. The brain exaggerates vertical distances when compared with horizontal lines, even with two dimensional images. This creates a vertical-horizontal illusion where the two lines are exactly the same length, but it does not appear that way. The converging parallel lines tell the brain that the higher image is farther away, therefore the brain perceives the image to be larger even though the two images hitting the retina are the same size. The optical illusion seen in a diorama/false perspective also uses assumptions based on monocular cues of depth perception.

Spinning Dancer

The ambiguity of the direction of the motion of the dancer that is due to the lack of visual references for depth is shown in this illusion. The dancer seems to be rotating either clockwise or counterclockwise, depending on spontaneous activity in the brain where perception is subjective. There have been studies done using fMRI that show that there is spontaneous fluctuations in cortical activity while watching this illusion. This cortical activity is found specifically in the parietal lobe because it is involved in perceiving movement.

Rubin Vase

The Rubin Vase in is considered a figure-ground illusion. When we look at an image, we do not see everything at once. If we did, our senses would be overwhelmed and confused. Instead of seeing everything at once, our mind tends to focus on one main object. This main object is known as the “figure”. Everything else that our minds does not focus on becomes the “ground”. A figure ground illusion is an image that is unclear what the figure is and what the ground is. Because of this, our mind switches back and forth trying to make sense of what we are seeing. In this illusion, we either see two black faces or a white vase.


Depth perception plays a huge role in how we perceive different optical illusions. We use our a variety of monocular cues to perceive this images in different ways. There are many different types of optical illusions that mess with our eyes and brain.

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