FIG. 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.

viding a familiar view, has several inherent drawbacks. First, sulcal regions are largely occluded from view. Second, dimensions along the image are distorted by foreshortening wherever the surface is oblique to the viewing angle. Third, the stereotaxic coordinate system in which the cortex is embedded does not respect the topology of the cortical surface; points that are close together in 3-D space may be widely separated along the cortical surface.

Surfaces can be modified in several ways to address these problems. The major options include smoothing the surface, cutting it at suitable locations, and mapping it to a geometrically well defined shape. Various combinations of these steps lead to four alternative display formats shown in Fig. 1: slices cut through the hemisphere (shown at two coronal levels in Fig. 1B); an extensively smoothed surface, which makes buried regions visible (Fig. 1C); a geometrically well defined ellipsoidal representation (Fig. 1D); and a cortical flat map (Fig. 1E). Darker shading represents cortex that is buried within sulci in the native 3-D configuration, thereby preserving an explicit representation of cortical geography. For each display format, Table 1 summarizes the tradeoffs between the improvements related to one set of characteristics (visibility, compactness, foreshortening, and parameterization) vs. drawbacks that are introduced, including distortions of surface geometry, topological changes (cuts) in the surface, and shape changes that obscure relationships to the native 3-D configuration.

Slicing a surface into sections, such as the coronal slabs shown in Fig. 1B, reveals buried regions without changing the shape of the surface contours. This format has the advantage of familiarity, insofar as the contours have shapes similar to that shown in standard stereotaxic atlases. Moreover, with computerized reconstructions, it is feasible to slice a given surface in different sectioning planes (e.g., coronal or horizontal). On the other hand, slicing the surface makes it difficult to discern important topological relationships within and between sections. This problem is exacerbated when sections