How the Eye Sees Optical Illusions

The eye sees optical illusions in many different situations. They can be seen in everyday situations such as perceived motion in a stationary object. All such illusions are due to the brain's visual area misinterpreting visual information.

Rod and Cone Arrangement

• The photoreceptor cells in the eye are the rods and cones. They are located in the retina. There are approximately 126 million rod and cone cells that collect light from the environment. The input from these cells is funneled into approximately one million ganglion cell axons. These ganglion cell axons constitute the optic nerve. The optic nerve transports the visual signal to the brain. This funneling of visual information creates a situation where visual data may be sent back to the visual area of the brain and misinterpreted in the form of an optical illusion.

Occipital Lobe

• The occipital lobe in the back of the brain constitutes the visual area of the brain. The left occipital lobe visualizes the right half of the visual field, and the right occipital lobe visualizes the left half of the visual field. The combination of the visual images is what allows us to have depth perception. Certain types of spatial arrangements and certain types of motion will cause the brain to misinterpret the information, creating an optical illusion.

Figure/Ground Illusions

• This type of illusion deals with how the eye and visual system interprets a stationary form in the environment. The best example of this is the face/vase illusion. This optical illusion contains two faces in profile that are facing each other. The two profiles create the outline of a vase. Depending on which of the images the person pays attention to, he will see either a vase or the profile of two faces. Te visual area of the brain needs to quickly work to make sense of the image that it is viewing. In the case of a figure/ground illusion, it chooses an object to focus on, the figure, and the rest is the background. In certain situations, that can be changed.

Motion Illusions

• There are special types of visual cells in the occipital lobes that process the vision of objects in motion. These cells can undergo sensory adaptation. An example of this is the waterfall illusion. If a person views a moving picture of a waterfall for several minutes and then immediately views a still picture of the same waterfall, the picture of the waterfall will appear to be moving upward. This is because the visual cells in the occipital lobe have adapted to the downward motion. In order to keep us from getting motion sickness, the other visual cells must counteract that motion. When the downward motion has stopped, the cells that were working to keep our equilibrium will be fooled. Their neural adaptation will cause them to view the stationary object as moving upward.

Size Illusions

• The visual area of the brain uses distance cues to tell how large an object is. This is why we know a car is still a large object when it is one quarter mile away. The car would only measure approximately one inch on a ruler at that distance if it were measured from our arm's length. However, the visual area of the brain knows that a car is large, and so it magnifies our perception of the image. This can best be seen at sunset. The sun is the same 92 million miles from the center of the earth as it moves across our sky. When the sun is up in the sky in the afternoon, it looks much smaller than when it is on the horizon at sunrise or sunset. When it is on the horizon, your brain can interpret it as being a long distance away, inducing perceived magnification. When it is in the sky, there are no depth cues and thus no perceived magnification.