Comparative analysis of visual pathways using neuroimaging and high-resolution neuroanatomy datasets across species

Primates, including humans, possess unique capabilities in visual functions, such as visually-guided grasping and object recognition. An accurate understanding of the evolution of the visual system is essential for revealing how brain evolution leads to primates’ specialization in these visual functions. Over the past several decades, visual neuroscientists have highlighted the roles of two distinct visual streams: the dorsal stream for spatial information processing and the ventral stream for categorical information processing (Ungerleider & Mishkin, 1982; Goodale & Milner, 1992). Recent diffusion-weighted MRI (dMRI) studies have renewed interest in the vertical occipital fasciculus (VOF), a white matter pathway connecting the human brain's dorsal and ventral visual cortices (Yeatman et al., 2014; Takemura et al., 2016). We examined the evolutionary trajectory of this pathway by analyzing dMRI data acquired from the brains of 12 mammalian species, including primates and non-primates (Takemura et al., 2024). Our findings suggest that the VOF is a prominent feature of the visual system preserved across all investigated primate species. In contrast, we did not find clear evidence for the VOF in all non-primate dMRI datasets. To complement the finding in non-primates dMRI data, we analyzed the rat brain's 3D-polarized light imaging dataset (Zilles et al., 2016; Schubert et al., 2016), providing micrometer-resolution visualization of fiber orientation. We discuss how the evolution of the VOF may be interpreted in the context of inter-species differences in the spatial organization of white matter pathways and primates' specialization of visual functions.