A circle appears much larger when surrounded by small circles than when surrounded by large ones.

The Ebbinghaus illusion is a classic optical illusion named after German psychologist Hermann Ebbinghaus, who first described it in the late 19th century. It was later popularized in the English-speaking world by Edward B. Titchener in a 1901 textbook on experimental psychology. The phenomenon occurs because the human visual system processes the size of an object relative to the objects immediately surrounding it.Research conducted by the University College London in 2011 suggests that the primary visual cortex plays a critical role in this perception. Their study found that people with a smaller primary visual cortex are actually more susceptible to the illusion. This indicates that the physical structure of the brain influences how we interpret relative size and spatial context.A related concept is the Delboeuf illusion, which specifically applies to how we perceive the size of circles within circles. This has significant real-world applications in nutrition and portion control. Studies published in the Journal of Consumer Research show that people consistently serve themselves more food on larger plates because the portion appears smaller relative to the plate's surface area.Interestingly, some studies suggest that certain groups, such as the Himba people of Namibia, are less susceptible to the Ebbinghaus illusion. Researchers believe this is because their culture focuses more on individual details rather than global context. This highlights that while the illusion is a biological process, it can be influenced by cultural and environmental factors.