How we perceive the world is determined by the visual cortex, but not everybody's cortex is created equal. One person's visual cortex can be up to three times bigger than someone else's, and size matters when it comes to perception.
The primary visual cortex has a far more active role in shaping how we see the world than anyone had suspected. Researchers at University College London discovered this when they were testing a pair of optical illusions on some test subjects. Here's the first one, known as the Ebbinghaus illusion:
Which of the two circles at the center of the other circles is smaller? Most people will say the circle on the left is smaller than the circle on the right, but they're actually the exact same size. Here's another, known as the Ponzo illusion:
Again, which line is longer? To most people, the line further back in the tunnel looks longer than the one that appears closer to the front. As the researchers tested these illusions on their subjects, they discovered a lot of variance in how much people actually perceived these differences. Some people saw a very large illusory difference, while others were barely fooled and saw the circles as more or less the same size.
The researchers then took MRIs of the subjects' brains. What they discovered astonished them - there was an almost perfect link between the size of somebody's visual cortex was and how much the optical illusion affected them. The smaller the visual cortex, the more a person was taken in by the optical illusion. Those with the largest visual cortices were also those most able to see the circles' true sizes.
Chief researcher Dr. D. Samuel Schwarzkopf explains the result:
"Our work is the first to show that the size of part of a person's brain can predict how they perceive their visual environment. Optical illusions mystify and inspire our imagination, but in truth they show us that how we see the world is not necessarily physically accurate, but rather depends a lot on our brains. Illusions such as the ones we used influence how big something looks; that is, they can trick us into believing that two identical objects have different sizes. We have shown that precisely how big something appears to you depends on the size of a brain area that is necessary for vision. How much your brain tricks you depends on how much 'real estate' your brain has put aside for visual processing."