How optical illusions trick your brain - Nathan S. Jacobs
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Visual illusions that show how we (mis)think
Together, all of the pieces of our visual system work together to rapidly create a 3D rendering of our surrounding environment. But just like detecting edges in the retina or creating 3D surfaces from gradients of light in the neocortex, we don’t see everything around us equally. We focus on what’s most important to us.We focus on faces or animals that might kill us and consume us.
For example, in the ‘what’ pathway there is a cortical region called the fusiform face area that is specifically designed to help us see faces. In analogous cortical areas in monkeys (inferotemporal cortex) there are cells that respond to facial features such as how far apart the eyes are, how big the mouth is, and how curved and angry the eyebrows look. There are cells that respond to what direction the head is faced. All of this requires many assumptions, one of which is that people aren’t upside down.
The upside down face illusion is based on 1) the assumption that faces are usually right side up, and 2) that different parts of the face are processed separately. The upside down face has the eyes and mouth cut out and turned upside down (or rights side up, I guess). Then when the image is rotated the eyes and mouth get turned upside down and the whole face looks weird.
So at every level, whether it’s the retina or the neocortex, the neurobiological basis of optical illusions demonstrates how our visual system translates confusing ambiguous light patterns into realistic 3D objects around us.It does this by making assumptions, and then sticking to those assumptions for better or worse. It gets things wrong every once in awhile (for example, if you go out of your way to make an optical illusion), but 99.9% of the time it gets it right and it does so in less than 300 ms. Optical illusions don’t show us quirks in the visual system, they provide a glimpse at the complex neurobiological systems required to turn squiggly, smudgy dots captured by the retina into a realistic, animated 3D world around us.
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