Why do we see optical illusions?
Albert Einstein once said “Reality is merely an illusion, albeit a very persistent one.” When we see optical illusions we perceive objects in a different way from what is physically in front of us. Our brain tries to process what we are seeing in the most logical way. However it doesn’t always get it right. We may think things are bigger than they actually are, or we might think that things are a different color than they actually are because of their surrounding color.
The optic nerve in our eye is attached to the occipital lobe where the brain processes the images that we perceive. The optic nerve receives impulses from three types of cells: 1) ganglion, 2) bipolar, and 3) cone and rod cells. The cone cells are used for interpreting color. The cone cells are part of the retina which is responsible for transmitting electrical impulses to the brain. When we perceive color a shock is passed through the cone cells and the optic nerve to the brain. But when we see an optical illusion, while the retina is processing the impulse it changes what it sees based on what is around the object. This is called context. For example image 1 shows two gray boxes stacked on top of each other. The one on the bottom appears lighter than the one on the top, but when a finger is placed between the two boxes, the viewer sees they are the exact same color. The context, in this case the shading between the boxes, causes the brain to assume that the bottom box is lighter than the top box.1
In 2009 Beau Lotto gave a Ted talk about our perception of color and light. He made the audience look at a screen with red on one side and green on the other side. After thirty seconds he showed them a picture of a landscape that he had shown the audience earlier. When the audience first saw the landscape, it had “normal” coloring. But after looking at the red and green screen for thirty seconds, the landscape image appeared to be half red and half green, but with those two colors flip-flopped from the original red and green screen. This supports the opponent-process theory of color vision. This theory states that the human visual system interprets information about color by processing signals from cones and rods. Responses to one color of an opponent channel are against those of the other color. That is, opposite opponent colors are never perceived together.
Look at image 2. Our eyes perceive that the red line is shorter than the blue line. However they are the same size. This is caused by the Müller-Lyer illusion. We use bottom-up processing when we see these two figures. When the eye sees the two figures it sends a signal to the brain. Then the brain processes what it’s seeing and comes up with a conclusion. The ends of the shapes (...