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

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. Sur- face-based visualization involves reconstructing cortical sur- faces 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 f lat 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 indi- vidual hemispheres to a surface-based atlas while respecting surface topology, improving registration of identifiable land- marks, and minimizing unwanted distortions. Surface-based warping also can aid in comparisons between species, which we illustrate by warping a macaque f lat map to match the shape of a human f lat map. Collectively, these approaches will allow more refined analyses of commonalities as well as individual differences in the functional organization of pri- mate cerebral cortex. The mammalian cerebral cortex contains numerous anatom- ically and functionally distinct areas arrayed in a complex mosaic across the cortical sheet. Efforts by cortical cartogra- phers to determine the number, arrangement, and internal organization of these areas represent a major thrust in systems neuroscience over the entire 20th century. Nonetheless, a precise and definitive charting of the entire neocortex has yet to be attained for any species. In the human neocortex, classical neuroanatomists identified '50 architectonic subdi- visions (1). However, the actual number may be 100 or more areas, given the plethora of cortical areas recently discovered in nonhuman primates (2-6) coupled with the disproportion- ate expansion in cortical surface area in humans compared with nonhuman primates. The prospects for accurately mapping the arrangement of functionally specialized areas in the human cerebral cortex have been enhanced greatly by recent advances in noninvasive neuroimaging techniques. Positron emission tomography al- lows localization of activation foci with an accuracy on the order of 1 cm, and functional MRI has even better spatial resolution, comparable to the 3-mm thickness of the cortical sheet. By combining functional MRI with structural MRI, it is possible to map activation patterns precisely in relation to the specific pattern of cortical folds in the same individual. The ability to chart function at near-millimeter resolution on a cortical sheet whose surface area is '80,000 mm 2 y hemisphere poses major technical problems in the arenas of data analysis and visualization. The basic challenge is how best to visualize large amounts of complex experimental data with minimal loss of important information about spatial and topological relationships. The problem is compounded by individual variability in cortical structure and function. This variability includes pronounced differences in the pattern of folding, in the size and shape of cortical areas, and in their location relative to these folds. Together, these factors intro- duce a degree of spatial uncertainty well in excess of the resolution available with current neuroimaging techniques. To help meet this challenge, new visualization and analysis methods have been developed that emphasize surface repre- sentations and surface-based coordinates. Here, we discuss three sets of recently developed tools that collectively offer major advantages over conventional methods that emphasize volume representations and stereotaxic coordinates. The first is surface-based visualization, which includes methods for reconstructing surfaces and changing the shape of the surface to facilitate the analysis of important geometrical and topo- logical relationships. The second involves surface-based at- lases, in which reconstructions of particular hemispheres are used as standard substrates for analyzing data obtained from many individuals. The third involves surface-based warping, which uses surface representations to constrain the deforma- tion from a source to a target hemisphere, thereby preserving critical topological relationships when mapping data from individual subjects onto an atlas. Surface-Based Visualization of Cerebral Cortex