How variable is the functionally-defined structure of early visual areas in human cortex and how much variability is shared between twins? Here we quantify individual differences in the best understood functionally-defined regions of cortex: V1, V2, V3. The Human Connectome Project includes retinotopic measurements from 181 subjects, most of whom are twins. We trained four "anatomists" to manually define V1-V3 using retinotopic features. These definitions were more accurate than automated anatomical templates and showed that surface areas for these maps varied more than three-fold across individuals. The cortical magnification function also differed substantially among individuals: the relative amount of cortex devoted to central vision varied by more than a factor of 2. Whereas our twin sample sizes were too small to make precise heritability estimates (50 monozygotic pairs, 34 dizygotic), they nonetheless reveal high correlations, consistent with strong effects of the combination of shared genes and environment on visual area size. In V1, intraclass correlations of surface area between twin pairs were 84% and 68% for monozygotic and dizygotic pairs, respectively. The correlations were also high for V2 (81%, 73%) and V3 (75%, 43%). A trend for higher monozygotic than dizygotic size correlations, as well as greater similarity in map properties amongst monozygotic twins, suggest that visual area size and topography are partly genetically determined. Collectively, these results comprise the most accurate account of individual variability in visual area structure to date, and provide a robust population benchmark against which new individuals and developmental and clinical populations can be compared.
bioRxiv Subject Collection: Neuroscience