a. Ebbinghaus illusion13: the two central circles are physically identical but appear different in size due to the presence of the surrounding circles. b. A variant of the Ponzo illusion14: the two checkerboard circles are physically identical, but appear different in size due to the three dimensional context. c. The smaller the V1, the stronger the illusion. Representative maps showing cortical regions V1-V3 on a reconstructed 3D mesh of the left hemisphere gray-white matter surface of three participants. The surface area of the left V1 and Ebbinghaus illusion strength are given for each participant. Red: V1. Green: V2. Blue: V3.

Scatter plots showing the inter-individual variability of the size of the visual regions V1-V3 plotted as a function of the psychophysically measured strength of the Ebbinghaus (a) and the Ponzo (b) illusions (see Supplementary Material for full details). Each data point represents a measurement from one participant. The solid black lines show the linear regression for each panel. Correlation coefficients and statistical significance are denoted above each panel. The numbers in brackets denote the bootstrapped 95% confidence intervals for the correlation coefficient.