Abstract
The primate visual cortex comprises a series of retinotopically organized areas. Visual cortex is expanded in humans compared to macaques (Van Essen et al. 2001). Here, we investigated whether this expansion is due to individual retinotopic areas within visual cortex becoming larger or the presence of additional areas. We used fMRI to map the retinotopic organization of the visual system in 53 humans (Wang et al. 2015) and 7 macaques (Arcaro & Livingstone, 2017). Cortical surface models for each subject were created from high-resolution anatomical MRIs to measure the size of individual areas in each subject. The human cortex has ~10-fold larger surface area. To compare visual areas across species while controlling for this difference in overall brain size, the size of each extrastriate area was normalized to the size of V1. The number of retinotopic areas and their extent across cortex relative to other sensory systems was similar between species. However, there were substantial species differences in the sizes of individual areas. All extrastriate areas were larger in humans compared to macaques. V2 was ~25% larger and V3 was ~65% larger, demonstrating that even early visual areas are expanded in humans. Moving from posterior to anterior visual cortex, areal size differences between species increased indicating greater cortical expansion for high-order visual areas. Dorsal areas were more expanded than ventral areas. Though the degree of homology between higher-order visual areas is controversial (e.g., Orban et al. 2004), size differences between humans and macaques were apparent when comparing areas based on broad anatomical regions. For example, each area in human ventral visual cortex was larger than each area in macaque ventral visual cortex. These findings suggest that the increased size of areas, rather than the addition of new areas, may be the key factor underlying differences in visual functioning between primates.