Are the orange circles in this picture (right) different sizes? Your answer can reveal the size of your brain.
Most people perceive the central circle to be smaller, an effect known as the Ebbinghaus illusion. and colleagues at University College London created a series of images in which the relative sizes of the two circles varied, and asked 30 volunteers to estimate which of the two was larger.
The team then scanned each volunteer’s brain using fMRI while they were shown a black dot in various points of their visual field. From the scans, they were able to assess the size of the visual cortex.
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They found that people with a smaller visual cortex experienced the more strongly.
Schwarzkopf suggests that this is because the circuits in the visual cortex responsible for the illusion are the same size in everyone, but cover a greater proportion of a smaller visual cortex, causing a stronger effect.
The team also found that people with a smaller visual cortex tended to have bigger brains overall, though it is not clear why.
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