We think when we see something, say a small red rosebud, our friend will see the same rosebud, same shape, same size. But this isn’t exactly the case. There’s a surprising amount of variability in how each of us perceives things.
Now some studies have shown that the differences are likely cultural: the Müller-Lyer visual illusion, which shows two lines of equal length where one is often perceived, at least by American undergrads, as longer than the other, is actually not an illusion at all for the San foragers of the Kalahari.
A recent study in Nature Neuroscience provides evidence that individual perceptual differences may not be so influenced by culture as by the size of our primary visual cortex. (This part of the brain, by the way, can differ in individual size by up to three times.)
Scientists presented various visual illusions to subjects, like one where two equal circles are often perceived as being different sizes. Individuals with a smaller primary visual cortex perceived a much more pronounced visual illusion than those individuals with a larger visual cortex.
Notes one of the authors: “How much your brain tricks you depends on how much 'real estate' your brain has put aside for visual processing.”