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Abstract:
The large size of the human brain imposes computational constraints that are reflected in its structural organization. For example, specialized visual processing of spatial and categorical information is partially segregated in dorsal and ventral regions in the occipital and temporal lobes; these regions are widely separated in cortex. For effective vision and action, the processing performed in these regions must be coordinated. Classical as well as recent studies identify the human Vertical Occipital Fasciculus (VOF), as a likely white matter bundle that includes axons that communicate between the dorsal and ventral regions. In this study, we compare the human vertical occipital pathways with similar tracts in the much smaller macaque brain in order to better understand similarities and differences across species. Methods. We obtained diffusion MRI (dMRI) at several spatial resolutions, and we used fiber tractography to estimate the trajectories of several different occipital pathways. DMRI data were acquired from 4 macaque monkeys and many humans (Takemura et al., 2015). We analyzed the data using constrained spherical deconvolution (CSD) and an ensemble of probabilistic tractography methods. We optimized the tractography results and tested statistical hypotheses using Linear Fascicle Evaluation methods (Pestilli et al., 2014). Results. A substantial bundle of vertical occipital fibers could be found in all the macaque and human datasets. The location of the macaque VOF (mVOF) is consistent with a schematic description in a post-morterm monkey brain described by Wernicke (1881). The mVOF terminates near cortical areas V3d, V3A, V4d and MT dorsally and V4v/TEO ventrally. The pattern of mVOF terminations is similar to those of human VOF, which are principally V3A/B dorsally and hV4 ventrally. In both species, the VOF is lateral to the optic radiation (sagittal stratum). In human, the VOF is also lateral to the Inferior Longitudinal Fasciculus (ILF) and clearly distinguishable from the ILF; but the mVOF intermingles with the vertical component of macaque ILF and does not form a very distinct bundle. The estimates of the position, size and cortical terminations of the mVOF depend on instrumental parameters, such as dMRI resolution; but in all cases the core of the tract can be identified and the estimates are consistent. These findings suggest that the human and macaque vertical occipital fiber systems diverged from a common ancestor. The human system significantly enlarged and became separated from the ILF. The change in white matter may be part of the general evolution of the size and position of extrastriate maps with the increase in brain size.
