The boundary of the stria coincides with the topographic representation in V1 of the visual hemifield boundary, providing complimentary structural and functional definitions of V1 (Henschen 1890
; Bolton 1900
; Polyak 1933
; Clark 1941
). Due to its visuotopic structure, the geometric shape of V1 provides information regarding the amount of cortex devoted to processing information from different locations in the visual field. Therefore, the intersubject variability in the shape of V1 is related to the degree of population variability in the visual representation it contains.
The term intrinsic geometry refers to geometric properties of a surface that are independent of the embedding of the surface in 3 dimensions, such as Gaussian curvature and geodesic distances. The term extrinsic geometry refers to properties that are determined by the embedding, such as sulcal depth and mean curvature (Do Carmo 1976
; Griffin 1994
). Here, we demonstrate that it is critical to consider the intrinsic and extrinsic geometry of cortex separately when assessing variability of cortical structures, such as V1.
Previously, magnetic resonance imaging (MRI) of the stria has been used to identify V1 (Clark et al. 1992
; Barbier et al. 2002
; Fatterpekar et al. 2002
; Walters et al. 2003
; Bridge et al. 2005
), but the full cortical area has not been observed. Here, ex vivo samples of human visual cortex were imaged using high-field (7 T) structural MRI with custom-designed, multichannel radio frequency (RF) coil arrays, long scan times, and a pulse sequence optimized for myelin contrast. This enabled imaging with an isotropic voxel size of about 200 μm over the full extent of human V1 while maintaining a signal-to-noise ratio (SNR) sufficient for reliable identification of the stria of Gennari. Manual tracings of the stria in the MRI volumes were used to construct mesh representations of the full V1 surface.
In order to characterize the shape of macaque V1, tracings of publicly available histological serial section data were used to construct surface meshes of macaque V1. These surfaces, along with existing surface data (Van Essen et al. 2001
), were computationally flattened using the same methods used to produce the human V1 flattenings. In addition, hand-flattened V1 surfaces (Horton and Hocking 1996
) were compared with the computationally flattened macaque surfaces. The shape of macaque V1 was virtually identical for the computationally and hand-flattened data sets.
In this study, the simplest parameterization of the shape of flattened V1 was found to be an ellipse and the aspect ratio of the best-fitting ellipse was found to exhibit low variability across subjects. As an independent measure of shape similarity, a rigid-body alignment of the boundary of size-normalized V1 was performed, which demonstrated high overlap and low spread across subjects. Although the aspect ratio of the ellipses describing macaque and human V1 were found to be statistically significantly different, qualitatively the shape between these 2 species was found to be quite similar.
The nearly invariant shape of V1 observed here suggests that the developmental mechanisms determining V1 shape are guided by a process that is remarkably similar across individual humans as well as between human and macaque. Furthermore, the developmental mechanisms giving rise to the shape of V1 respect the intrinsic geometry of the cortical surface and do not appear to be influenced by its extrinsic geometry. This idea follows from the observation that the extrinsic structure of V1 is very different across macaque and human, whereas the intrinsic structure of V1 is stereotyped across individuals and between these 2 species.
Despite previous reports of substantial intersubject variability in the geometry of V1, we have shown that the shape of V1 exhibits low variability within both human and macaque subjects and is similar in shape between these 2 species. Demonstrating low variability in the intrinsic shape of V1 depends on discounting effects of the extrinsic geometric features of cortex, which are more highly variable across subjects. Because the orderly topographic map that V1 contains shares a consistent relationship with anatomical V1, demonstrating a consistent V1 shape constrains the variability in the map itself.