H.A. Drury and D.C. Van Essen

Washington University Medical School, St. Louis, MO USA

Introduction

The cerebral cortex is a thin sheet of neural tissue that is extensively folded in order to fit within the cranium. Studies of cortical organization can be greatly facilitated by explicit surface reconstructions that preserve topological relationships within the surface. Accordingly, we have generated computerized surface reconstructions for both cerebral hemispheres of the Visible Man. Two-dimensional cortical maps were generated using our two-stage, multi-resolution flattening procedure [1]. The volume and 3-D surface reconstructions were then transformed to correspond to the Talairach coordinate space that is widely used in functional brain imaging studies. In parallel, a surface-based coordinate system was established on the 2-D maps. A one-to-one mapping exists between our surface-based coordinate system and the Talairach coordinate space.

Figure 1: Surface reconstructions of the left and right hemispheres of the Visible Man. Arrows indicate positions of artificial cuts.

Results

We obtained high-resolution cryosectioned data for the head of the Visible Male from the Visible Human Project [2]. Contours were manually traced through the estimated path of cortical layer 4, and surface reconstructions were generated for the left hemisphere (total surface area 757 cm²) and right hemisphere (total surface area 795 cm²), as shown in Figure 1. For orientation, the central sulcus (CeS) and sylvian fissure (SF) are labeled.

The sulcal and gyral convolutions of the human cortex (reflected in the shading shown in Figure 1) pose a major impediment to the investigation of many aspects of cortical structure and function. For many types of analyses it is desirable to remove these convolutions and represent the cortex as a minimally distorted flat surface.

We applied our automated flattening procedure to the 3-D cortical surface reconstructions of the Visible Man to produce 2-D maps for both the left and right hemispheres. Appropriate cuts were made to the 3-D surface reconstructions (shown with arrows in Figure 1) prior to flattening in order to reduce distortion in surface area introduced by flattening the surface.

Comparisons between the flat maps of the two hemispheres show striking similarities in their overall size and shape and unambiguous correspondence between most major sulci (represented as mean curvature as computed in the original 3-D representation). However, closer inspection of the cortical geography reveals conspicuous differences in the pattern of the many minor sulci. Although the flat maps of the human cortex are clearly more complex than those previously made of the macaque, the overall configurations of the maps share important similarities. Shape-based deformation algorithms have successfully been applied to 2-D cortical maps of the macaque in order to compensate for individual variability [3]. These same methods can be applied to make quantitative assessments of hemispheric differences as well as to make cross-species comparisons between the macaque and the human.

The development of coordinate systems for the cortex (as well as methods for transforming between these coordinate systems) is critical in order to allow investigators to analyze and compare experimental results. Standard 3-D stereotaxic coordinates (Talairach [4]) are good in some respects but lacking in others. Points that lie close to one another in stereotaxic coordinates may be far apart in terms of the minimum intra-cortical distance that separates them (i.e., points on opposite banks of a sulcus). The presence of relatively undistorted flat maps of the human cortex provides a natural substrate for surface-based coordinate systems. These were generated separately for each hemisphere by establishing a square grid that overlays the flat cortical map.

In order to link our surface-based coordinate system to the widely used stereotaxic coordinate system, we transformed the original Visible Man volume data and associated surface reconstructions to the Talairach coordinate space using a nine-parameter affine transformation [5]. Each point on the Visible Man flat map can now be assigned unique 2-D surface-based coordinates and corresponding unique 3-D Talairach coordinates. Conversely, any point (or focus location) in the 3-D Talairach space can be projected to the nearest point on the surface and then assigned 2-D surface-based coordinates. Representation of foci by an appropriate localized volume distribution allows visualization of which gyri and sulci of the Visible Man are within a specified distance of any given Talairach coordinate. To illustrate this concept, we took several activity foci identified in published studies and examined their locations using their reported Talairach coordinates and after transformation to the 2-D cortical flat maps. In this manner, the numerous activity foci reported using modalities such as PET and fMRI can be mapped to flat maps. This will aid in the generation and progressive refinement of areal partitioning schemes, such as those developed for the macaque [6].

Conclusions

We have produced relatively undistorted 2-D maps for both hemispheres of the Visible Man and have defined surface-based coordinate systems on these maps that are directly linked to the Talairach coordinate space. Flat maps provide a convenient framework for visualizing and comparing activity foci reported from both PET and fMRI studies. A surface-based coordinate system linked to the Talairach atlas will be a valuable tool for providing an objective framework to incorporate data from the many approaches available for studying cerebral organization and function.

Acknowledgments

Supported by the Human Brain Project MH52158 and NIH EY02091.

References

1. Drury HA, Van Essen DC, Anderson CH, Lee CW, Coogan TA, Lewis JW. J. Cogn. Neurosci., 1996, 8:1-28.

2. Spitzer V, Ackerman MJ, Scherzinger AL, Whitlock DJ. Am. Med. Informatics Assoc., 1996, 3.

3. Drury HA, Van Essen DC, Joshi S, Miller MI, Anderson CH, Coogan T. Soc. Neurosci. Abstr., 1995, 371.9.

4. Talairach P, Tournoux J. A stereotactic coplanar atlas of the human brain. Thieme Verlag, 1988.

5. Lancaster JL, Glass TG, Lankipalli BR, Downs H, Mayberg H, Fox PT. Human Brain Mapping, 1995, 3:209-223.

6. Felleman DJ, Van Essen DC. Cerebral Cortex, 1991, 1:1-47.