Look at the image above. Which of the central orange circles looks bigger? Most people would say the one on the right – the one surrounded by the smaller ‘petals’. In truth, the central circles are exactly the same size. This is the Ebbinghaus illusion, named after the German psychologist Hermann Ebbinghaus. It has been around for over a century, but it still continues to expand our understanding of the brain.
Samuel Schwarzkopf from University College London has just discovered that the size of one particular part of the brain, known as primary visual cortex or V1, predicts how likely we are to fall for the illusion. V1 sits at the very back of our brains and processes the visual information that we get from our eyes. It’s extremely variable; one person’s V1 might have three times the surface area of another person’s. While many scientific studies try to average out those differences, Schwarzkopf wanted to explore them.