Functional and structural mapping of human cerebral cortex: solutions are in the surfaces

Abstract

The human cerebral cortex is notorious for the depth and irregularity of its convolutions and for its variability from one individual to the next. These complexities of cortical geography have been a chronic impediment to studies of functional specialization in the cortex. In this report, we discuss ways to compensate for the convolutions by using a combination of strategies whose common denominator involves explicit reconstructions of the cortical surface. Surface-based visualization involves reconstructing cortical surfaces and displaying them, along with associated experimental data, in various complementary formats (including three-dimensional native configurations, two-dimensional slices, extensively smoothed surfaces, ellipsoidal representations, and cortical flat maps). Generating these representations for the cortex of the Visible Man leads to a surface-based atlas that has important advantages over conventional stereotaxic atlases as a substrate for displaying and analyzing large amounts of experimental data. We illustrate this by showing the relationship between functionally specialized regions and topographically organized areas in human visual cortex. Surface-based warping allows data to be mapped from individual hemispheres to a surface-based atlas while respecting surface topology, improving registration of identifiable landmarks, and minimizing unwanted distortions. Surface-based warping also can aid in comparisons between species, which we illustrate by warping a macaque flat map to match the shape of a human flat map. Collectively, these approaches will allow more refined analyses of commonalities as well as individual differences in the functional organization of primate cerebral cortex.

Figure 1

Surface-based representations of the Visible Man cerebral cortex. (A) Native 3-D view of the right hemisphere, with lobes differently shaded. (B) Two coronal slices (6 mm thick), whose location in the other panels is shown in blue. (C) An extensively smoothed surface. (D) The same surface mapped onto an ellipsoid. (E) A cortical flat map, with selected cuts to reduce distortions. The gridwork surrounding the map defines a surface-based coordinate system, with a grid spacing of 1 map-cm, equivalent to 1 cm along the cortical surface in regions that are not distorted.

Figure 2

Functional specialization in human visual cortex. (A) Topographically organized visual areas shown on a flat map of occipito–temporal cortex in the Visible Man. (B) The same areas on a medial view of the occipital lobe. (C) The same regions on a lateral view of the hemisphere. (D) Cortical regions implicated in processing of color. Green dots denote centers of activation foci, and lighter green denotes cortex with the 10-mm uncertainty limit. (E) Cortical regions implicated in the processing of motion (M); form (F); form and color (FC); form and motion (FM); and motion, color, and spatial relations (MCS). Question marks denote additional regions potentially involved in processing of color (green), motion (red), and spatial relations (yellow). (F) The same functional specializations shown on lateral and ventral 3-D views. (G) The same pattern on extensively smoothed surfaces. [Adapted, with permission, from ref. (Copyright 1997, J. Neurosci.)].

Figure 3

Surface-based warping from the left to the right hemispheres of the Visible Man. (A) Mirror-flipped flat map of the left hemisphere, with selected sulci labeled, and geographic landmarks used to constrain the deformation shown in black. (B) Equivalently labeled flat map of the right hemisphere. (C) Deformation field associated with a warping of the left hemisphere map to match the right hemisphere map. (D) Overlay of right and deformed left hemisphere maps, color-coded as shown in Inset to indicate buried (Bur.) cortex or exposed (Exp.) cortex in each hemisphere.

Figure 4

Surface-based warping from macaque to human cortex. (A) Flat map of cortical areas in the macaque, with visual areas indicated in various colors. (B) Flat map of the Visible Man, with areas and functional specializations for vision indicated. (C) Deformation grid that warps the macaque map to the shape of the human, constrained by the perimeter and selected functional regions. (D) Deformed macaque cortical areas, for comparison with the functional specializations of human visual cortex in B.