Lily Cohen Optical Illusions What is an Optical Illusion? • An optical illusion is something that plays tricks on your vision. Optical illusions teach humans how their eyes and brain work together to see. • Since we live in a three-dimensional world, the brain gets clues about depth, shading, lighting, and position to help humans interpret what they see. • When looking at a two-dimensional image, the brain can be fooled because it doesn’t get the same clues as something that has three dimensions. Why Humans Perceive Illusions The Way That They Do? • Your senses gather information and send it to the brain. The brain does not receive this information, therefore it creates your perception of the world. This means that your brain fills in the gaps where there is incomplete information or creates an image that isn’t there by making assumptions and using prior knowledge. • This essentially happens because of evolution. Survival is dependent on fast reactions, which enables the brain to work quickly to piece together whatever bits and fragments that it can get. Basic Illusions Face or vase? • Your brain has to make a decision about which part of the picture is the object and which part is the background. • If you see the faces-- that means your brain is interpreting the rest of the picture not as a vase but as the background. • If you see the vase -- your brain is interpreting the rest of the picture not as faces but as the background. Basic Illusions • The illusion is triggered by eye movements. Even when you stare at a still object, your eyes dart around. • Most of the time, your brain can tell the difference between your eyes moving versus an object moving. • Since there are strong color contrasts and shapes in the illusion, your brain gets confused, which causes your motion sensors switch on. This process allows the image to turn. • The brain is evolved to focus attention on movement because it can be a sign of danger. Danger will automatically trigger a sense of survival. Lateral Inhibition • A process which distinguishes the lines and boundaries of shapes. • The goal of lateral inhibition is to facilitate edge detection. Edge detection is the process of distinguishing where one object ends and the next begins. • When comparing one color with the color farthest to the left, it looks darker, which causes the brain to interpret it that way. • When we take the same color and compare it to the color on the right, it looks lighter, which causes our brain to interpret it this way. Hermann-Hering Grid • Scientists refer to the illusion as a “stimulus lightness contrast illusion”, but philosophers contrast that this is actually a hallucination. • If this were a hallucination, one would be hallucinating the appearance of non-existent grey smudges or dots. • This is often caused because we are viewing a mind-independent object. Ballerina Illusion Explained • The spinning dancer, also known as the silhouette illusion, is a kinetic, bistable (something can be seen in two different perspectives) optical illusion. • Also an example of an “ambiguous illusion” because the images are able to shift from one direction to another based on a person’s perception. • With the spinning dancer, because the image has no depth or directional cues from the surrounding environment, it’s up to the viewer to figure out which direction the silhouette is turning. • This causes the brain to fill in the missing information and assume the direction that the dancer is turning in. Blue & Black or Gold & White? • The brain’s perception can be deceived by the colors of nearby objects and the reflected light falling on the object in focus (the dress). • The brain is actually interpreting the background lighting. • If your brain thinks the dress is more illuminated, you will see blue and black. • If your brain thinks that it is more reflective (referring to its shadows), then the dress appears white and gold. Lilac Chaser • In the lilac chaser illusion, the viewer sees a series of lilac-colored blurry dots arranged in a circle around a focal point. As the viewer stares at the focal point, a few different things are observed. • At first, there will appear to be a space running around the circle of lilac discs. After about 10 to 20 seconds, the viewer will then see a green disc moving around the circle instead of the space. • This is an example of what is known as apparent movement or beta movement. • When we see something in one spot and then again in a slightly different spot, we tend to perceive movement. • Example of a negative afterimage effect. • When a color is presented in the visual field for an extended period of time, an afterimage results. An afterimage involves continuing to see colors briefly even after a stimulus is no longer present. Lilac Chaser Video Recap • Your senses gather information and send it to the brain. The brain doesn’t always receive this information, therefore it creates a perception of the world. • Perception indicates how the brain fills in gaps where there is missing information or creates an image that isn’t even there. • This happens because of evolution. The brain is trained to make quick decisions with whatever information it can get. Hering Illusion Bibliography • https://www.cnbc.com/2017/04/04/the-brain-bending-science-behind-optical-illusions.html • This source explained lateral inhibition and the science behind why the brain perceives things the way it does • https://psych2go.net/optical-illusion-explained-way-dancer-spinning/ • Spinning Dancer explanation and introduces the idea of the brain making assumptions • http://www.indiana.edu/~p1013447/dictionary/lat_i.htm • Another lateral inhibition source. Provides information about how parts of the brain get involved with famous illusions. • https://www.illusionsindex.org/i/hermann-grid • Hermann Grid description. This source was used to develop a understanding on simultaneous lightness contrast illusion. • http://www.rosslab.neurobio.pitt.edu/making-sense-of-the-hermann-grid-illusion/ • Another Hermann Grid resource. Science behind why the dots are observed between the grid lines . • https://www.amnh.org/explore/ology/brain/optical-illusions-and-how-they-work • Background information on optical illusions and provides examples on basic illusions, while introducing factors like color, filling, priming, etc. .