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Previous Article | Next Article
Volume 17, Number 18, Issue of September 15, 1997 pp. 7079-7102
Copyright ©1997 Society for NeuroscienceStructural and Functional Analyses of Human Cerebral Cortex Using a Surface-Based Atlas
D. C. Van Essen and H. A. Drury Department of Anatomy and Neurobiology, Washington University School of Medicine, St. Louis, Missouri 63110
ABSTRACT
We have analyzed the geometry, geography, and functional organization of human cerebral cortex using surface reconstructions and cortical flat maps of the left and right hemispheres generated from a digital atlas (the Visible Man). The total surface area of the reconstructed Visible Man neocortex is 1570 cm2 (both hemispheres), ~70% of which is buried in sulci. By linking the Visible Man cerebrum to the Talairach stereotaxic coordinate space, the locations of activation foci reported in neuroimaging studies can be readily visualized in relation to the cortical surface. The associated spatial uncertainty was empirically shown to have a radius in three dimensions of ~10 mm. Application of this approach to studies of visual cortex reveals the overall patterns of activation associated with different aspects of visual function and the relationship of these patterns to topographically organized visual areas. Our analysis supports a distinction between an anterior region in ventral occipito-temporal cortex that is selectively involved in form processing and a more posterior region (in or near areas VP and V4v) involved in both form and color processing. Foci associated with motion processing are mainly concentrated in a region along the occipito-temporal junction, the ventral portion of which overlaps with foci also implicated in form processing. Comparisons between flat maps of human and macaque monkey cerebral cortex indicate significant differences as well as many similarities in the relative sizes and positions of cortical regions known or suspected to be homologous in the two species.
Key words: atlas; human; macaque monkey; cerebral cortex; Visible Man; visual areas; computerized neuroanatomy
INTRODUCTION
Human cerebral cortex is a thin sheet of tissue that is extensively convoluted in order for a large surface area to fit within a restricted cranial volume. Convolutions occur to varying degrees in the cortices of many species and have long been a source of both fascination and frustration for neuroscientists. The fascination arises because of curiosity about how the convolutions develop and what they signify functionally (Welker, 1990
; Van Essen, 1997
These difficulties can be alleviated to a considerable extent by analyzing cortical organization and function in relation to explicit representations of the cortical surface. A particularly useful display format involves cortical flat maps, which allow the entire surface of the hemisphere to be visualized in a single view (Van Essen and Maunsell, 1980
A surface-based atlas is particularly useful for comparing results across the burgeoning number of neuroimaging studies that use positron emission tomography (PET) or functional magnetic resonance imaging (fMRI). Typically, the centers of activation foci are localized by reporting their stereotaxic coordinates in Talairach space (Fox et al., 1985
Human visual cortex is a suitable domain for exploring the utility of this approach. Recent neuroimaging studies of human visual cortex have revealed six topographically organized visual areas (Sereno et al., 1995
Our understanding of cortical organization in humans can be aided by comparisons with nonhuman primates, particularly the macaque monkey and owl monkey (Sereno et al., 1995
MATERIALS AND METHODS
Images, contours, and raw coordinates. Reconstructions of cerebral cortex from the Visible Man (Visible Human Project, National Library of Medicine) were derived from digital images of the cut brain surface. These images were acquired at 1 mm intervals in a plane oriented approximately transverse to the long axis of the body (Spitzer et al., 1996
Fig. 1. Image of the cut brain surface from the Visible Man cerebrum, taken 46 mm from the top of the head (26 mm from the beginning of cortex). A contour representing the estimated trajectory of cortical layer 4 is drawn for the left hemisphere. In regions where the cortex was cut obliquely, close scrutiny of several nearby sections was often needed to infer the most likely contour for layer 4. Cerebral cortex was contained in 108 images (1 mm intervals), with an in-plane resolution of 2048 × 1216 pixels (0.33 mm/pixel). The raw data coordinate system associated with this stack of images is represented by an x-y-axis with origin at the bottom left of each image. The section number relative to the topmost image indicates the value for the z-axis.
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Surface reconstructions and area estimates. Each image was thresholded to show only the traced contours, which were then converted to a discrete sequence of points using the wand tool of NIH Image. After transfer to a Silicon Graphics (Mountain View, CA) UNIX workstation, contours were subsampled to a spacing of ~1.5 mm between nodes and loaded into custom software [Computerized Anatomical Reconstruction and Editing Tools (CARET)], which is designed for the interactive viewing and editing of surface reconstructions and associated experimental data. A wire frame reconstruction was generated using the Nuages software (Geiger, 1993
A smoothing algorithm was used to remove nonbiological surface irregularities in the initial three-dimensional (3-D) reconstruction. The optimal amount of smoothing, judged by visual inspection, involved 100 iterations with a smoothing parameter of 0.05 (cf. Drury et al., 1996a
Surface geometry. A curvature estimation algorithm (Malliot et al., 1993
Two dimensionless indices were used to calculate measures of overall surface geometry, independent of the absolute scale of the hemisphere. The intrinsic curvature index (ICI) was computed by integrating across all regions of positive intrinsic curvature and dividing by 4 (the integrated intrinsic curvature for a perfect sphere of any size). The ICI is calculated as:
where k
(1) = |kmax kmin| if kmax kmin > 0, or else k
The folding index (FI) was computed by integrating the product of the maximum principal curvature and the difference between maximum and minimum curvature and dividing by 4 Any ridge or furrow having the shape of a half-cylinder increments the folding index in proportion to its length, starting at 0.5 if its length equals its diameter. Thus, the folding index increments quickly per unit length of sharply folded regions, slowly for loosely curled or folded regions, and not at all for spherical or saddle-shaped regions.
(2)
Cortical flattening. Cuts in the reconstruction were introduced to reduce distortions in surface area when flattening the cortex. The surface was taken through multiple cycles of our multiresolution flattening algorithm (Drury et al., 1996a
Coordinate spaces and transformations. We used the Spatial Normalization software (Lancaster et al., 1995
The origin and alignment of the Visible Man cardinal axes are identical to those used in the Talairach stereotaxic atlas (Talairach and Tournoux, 1988
Slices through the Visible Man surface can be visualized in different cardinal planes (parasagittal, coronal, or horizontal) using a resectioning algorithm that displays portions of the surface lying within selected ranges along the appropriate x-, y-, or z-axis. This was particularly useful for delineating isocontour lines (constant x, y, or z values) in Talairach space. We also defined a surface-based coordinate system that respects the topology of the cortical surface (Anderson et al., 1994
Projection of stereotaxic data. Any experimental data point with its location reported in Talairach stereotaxic space can be linked to the nearest point on the Visible Man surface after transformation to Talairach space. For each point of interest (generally the center of an activation focus from a neuroimaging study), the projection algorithm identifies the nearest tile on the Visible Man surface and determines the closest point within that tile (or along the perimeter of the tile if the projection to the plane lies outside the tile). Activation foci reported in the 1967 Talairach space (Talairach and Tournoux, 1967 14), 1.07z]T'67 (T. Videen, personal communication).
Activation foci were visualized on the cortical flat map by displaying the center of the focus in relation to the closest tile on the 3-D surface. To visualize nearby portions of the cortical surface that are potentially associated with each activation focus, we identified all tiles within a core region up to a defined radius from the center in 3-D space (generally set to 10 mm). An additional option allows visualization of all tiles within a surrounding shell region (generally 10-15 mm from the center; see Fig. 8). 
Fig. 8. Stereotaxic projection of neuroimaging data to the cortical surface with estimation of spatial uncertainties. A, Activation foci from a study of motion analysis (Watson et al., 1993
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Reconstruction of macaque cerebral cortex. As a substrate for interspecies comparisons of cortical organization, we used a previously published reconstruction of the right cerebral hemisphere of the macaque monkey. This reconstruction (case 79-0) was based on a series of Nissl-stained sections that were aligned, reconstructed, and flattened as described previously (Carman et al., 1995
RESULTS
Geography and geometry of the Visible Man cerebral cortex
We begin with an analysis of cortical geography and geometry that provides useful information about human cerebral cortex in general, about similarities and differences between the left and right hemispheres of the Visible Man, and about the suitability of the Visible Man as an atlas on which to represent functional neuroimaging data. Cortical geography can best be appreciated by viewing the convolutions in several formats, including 3-D views of the original surface (Fig. 2, left, right columns), extensively smoothed surface representations (Fig. 2, center column), and cortical flat maps (Fig. 3). Figure 2 shows four views of each hemisphere after alignment of the Visible Man cerebrum to its cardinal axes (see Materials and Methods). The tick marks labeled VM along each axis, spaced at 1 cm intervals, represent the dimensions of the Visible Man surface in its cardinal coordinate space [x, y, z]VM-3D. Those labeled T'88 indicate the slight scale changes needed to transform the Visible Man cerebrum into the Talairach stereotaxic space.
Fig. 2. Surface reconstructions of the Visible Man. Lateral, medial, anterior, and posterior views are shown for both hemispheres. The origin was placed at the anterior commissure, the midsagittal plane was aligned to the y = 0 plane, and the anterior and posterior commissures were aligned to the x-axis. Native Visible Man (VM) and Talairach (T'88) coordinate systems are shown for each axis with tick marks at 1 cm intervals. Insets at the far left show the orientation of the original quasi-horizontal slices relative to the cardinal axes, with solid lines indicating 1 cm intervals. The maximum extent of the Visible Man surface is 68, 166, and 110 mm, respectively in the x, y, and z dimensions for the left hemisphere and 68, 171, and 106 mm, respectively, for the right hemisphere. After transforming the Visible Man brain to Talairach space, these values are 3% larger in the x dimension, 1% larger in the z dimension, and identical in the y dimension. After this transformation, the posterior pole of the Visible Man has a y value of
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Fig. 3. Flat maps of the left and right cerebral hemispheres. Top panels show flat maps with mean curvature displayed to represent cortical geography. Each map was aligned by making the mean orientation of the fundus of the central sulcus on the flat map match the visually estimated average orientation of the lips of the central sulcus in the 3-D reconstruction. For a region that is folded but not intrinsically curved, a mean curvature of ±0.5 mm1 (maximum on the scale) is equivalent to a cylinder of 1 mm radius. Middle panels show medial and lateral views of the intact hemispheres, with lobes identified according to landmarks delineated by Ono et al. (1990)
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Extensive smoothing of the surface reconstruction leads to surfaces having the shape of a lissencephalic brain similar to that of an owl monkey (Fig. 2, center column). The original pattern of folds is represented by a gray scale display of mean curvature (see Materials and Methods). Dark streaks in Figure 2 represent "inward folds," where the crease runs along the fundus of a sulcus, and light streaks represent "outward folds," where the crease runs along the crown of a gyrus. Sulci that are similar in location and overall extent in left and right hemispheres include the central sulcus (Ces) and Sylvian fissure (SF) on the lateral side and the cingulate sulcus (CiS) and calcarine sulcus (CaS) on the medial side. In many other regions the folding pattern differs markedly between hemispheres. One example relevant to functional neuroimaging results discussed below is the posterior inferior temporal sulcus (pITS), which is a single deep furrow in the left hemisphere but has a y-shaped branching pattern in the right hemisphere. Also, an additional sulcus [the anterior occipital sulcus (AOS)] is interposed between the pITS and the superior temporal sulcus (STS) in the left hemisphere but not the right. Another example is the superior frontal sulcus (SFS), which is a single long crease in the left hemisphere but is broken into several shorter creases in the right hemisphere. Cortical flat maps Figure 3 shows cortical flat maps of the entire left and right hemispheres of the Visible Man generated using our automated flattening procedure (see Materials and Methods). The top panels display cortical geography on the flat maps using the same map of mean curvature that was shown on the extensively smoothed surfaces in the preceding figure. To establish a standard orientation, the central sulcus on the map is aligned approximately parallel to its average orientation in the lateral view of the intact hemisphere. This convention makes the orientation of human flat maps similar to that commonly used for macaques and other nonhuman primates (see Fig. 13 below). To reduce distortions in surface area on the flat map, five artificial cuts were made in geographically corresponding locations in each hemisphere, as indicated by blue lines on Figure 3 and in the 3-D reconstructions. The segments representing the true margins of neocortex include its juncture with the corpus callosum (C.C.), hippocampus (HC), amygdala (Amyg.), and olfactory cortex (Olf.), as indicated along the margins of each map.
We used two approaches to assess whether the cortical convolutions of the Visible Man have any gross abnormalities that would make this brain unsuitable as an atlas. First, we compared the pattern of convolutions with those described and illustrated for 25 normal brains by Ono et al. (1990) 
Fig. 13. Interespecies comparisons between macaque and human cerebral cortex. A, 3-D surface reconstructions and a flat map of the macaque monkey (case 79-0; Drury et al., 1996a
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The bottom panels in Figure 3 display sulci and gyri in an alternative format, in which buried cortex (not visible from the exterior of the hemisphere) is shown in darker shades. Black lines indicate sharply creased folds (fundi) within each sulcus. In addition, the five lobes of the hemisphere are colored on the map and on the accompanying lateral and medial views of the hemisphere. This helps in visualizing the locations of the various cuts, which include deep cuts into the occipital and frontal lobes, plus smaller cuts at the parieto-frontal junction, the fronto-temporal junction, and near the occipito-temporal junction. Surface geometry The overall extent of the dark and bright streaks in Figure 3 indicates that crease-like folds occupy a significant fraction of total cortical surface area. However, in many places the surface is not just folded along a single axis but instead has significant intrinsic (Gaussian) curvature. The intrinsic curvature of the surface in 3-D is displayed on a flat map of the right hemisphere in Figure 4A, with dark regions denoting positive intrinsic curvature (rounded bulges or indentations) and light regions denoting negative intrinsic curvature (saddle-shaped regions). The map is peppered with hundreds of foci of elevated intrinsic curvature. Individual foci are typically 1-2 mm across and are mainly concentrated along crests of gyri and fundi of sulci, as can be seen in relation to the pattern of sulcal margins (Fig. 4A, fine white lines). High intrinsic curvature tends to occur where creases terminate or bifurcate, as can be determined by comparison with the map of mean curvature in Figure 3.
To determine surface areas for each lobe and for the entire hemisphere, we summed the area of all individual tiles over the relevant portion of each 3-D reconstruction (Table 1). The total cortical surface area of 1570 cm2 for both hemispheres is similar to the estimate of Jouandet et al. (1989)
Table 1. Surface area measurements of cortical lobes
Region Left hemisphere [cm2 (%)] Right hemisphere [cm2 (%)] Total neocortex 766 (100) 803 (100) Frontal 278 (36) 297 (37) Temporal 161 (21) 161 (20) Parietal 139 (18) 161 (20) Occipital 144 (19) 145 (18) Limbic 46 (6) 40 (5) Total sulcal 536 (70) 554 (69) Total gyral 230 (30) 249 (31) Areal measurements are based on summing the areas of tiles in the 3-D reconstructions, not on the flat maps. 
Fig. 4. A, Intrinsic curvature of the cortical surface, displayed on a map of the right hemisphere. There are numerous regions of positive (spherical) curvature (dark) and of negative (saddle-shaped) curvature (light). A histogram of intrinsic curvature values is shown to the right. The mean value (0.004 mm
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An interesting question relating to surface geometry is whether human cerebral cortex is dominated by intrinsic curvature (like the pock-marked surface of a golf ball), as suggested by Griffin (1994) A geographic atlas Gyri and sulci that can be recognized by their characteristic shape and location represent useful geographical landmarks that facilitate comparisons of results across hemispheres. We identified 47 sulci and 34 gyri in one or both hemispheres of the Visible Man using the atlas of Ono et al. (1990)
Significant distortions of surface area are unavoidable when a surface containing an irregular pattern of intrinsic curvature is transformed to a flat map (or even to a smooth 3-D surface such as an ellipsoid). To reduce global distortions associated with the overall rounded shape of the hemisphere, we found it necessary to make five cuts along the margins of the hemisphere. The residual areal distortions on the flat map were quantified by taking the ratio between the area of each tile in the 3-D reconstruction and its area on the cortical map. These distortion ratios are displayed as a gray scale representation for the right hemisphere (Fig. 4B). The map shows numerous regions of local compression (darker regions) or expansion (lighter regions) relative to surface area in the intact hemisphere but only modest variations in the average distortion for the different lobes. Comparison of the maps in Figure 4, A and B, reveals significant correlations between the patterns for distortion and for intrinsic curvature. Most notably, local compression occurs at many hot spots of positive (spherical) intrinsic curvature, as should be expected when flattening a bumpy surface. The histogram to the right of the map shows the number of tiles having different distortion ratios. Only 4.5% of the surface tiles (triangles) are expanded by more than 50% (distortion ratio, >1.5), and only 2.4% are compressed to an equivalent degree (distortion ratio, <0.67). The total area of the flat map divided by the total 3-D surface area is 1.10, signifying that the flat map is 10% expanded overall in areal extent, equivalent to ~5% in linear dimensions. 
Fig. 5. A geographical atlas showing sulci and gyri in the Visible Man. Sulcal and gyral abbreviations are listed in the Appendix, alphabetically for each lobe. Designations are based mainly on the atlases of Ono et al. (1990)
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Coordinate systems on the cortical surface The irregular shapes and uncertain boundaries of most gyri and sulci limits their utility in describing precise spatial locations across the cortical surface. One option for assigning spatial coordinates to the cortical surface is to display isocontour lines, where the surface is intersected by planes of constant x, y, or z value in stereotaxic space (cf. DeYoe et al., 1996
Only a few of the larger sulci, such as the CeS and CiS, contain a single uninterrupted fold and also are completely isolated from their neighbors. Some sulci are broken into multiple creases separated by intervening gyral protrusions, as occurs for the SFS in the right hemisphere. Other sulci merge with one or more neighboring sulci to form a larger expanse of completely buried cortex, often with ambiguity as to the exact border between one sulcus and the next. One prominent example includes the region of the STS, angular sulcus, and postcentral sulcus, near the center of each map. These sulci are confluent with one another in both hemispheres and in addition merge with different sets of neighboring sulci in the left and right hemispheres. 
Fig. 6. Stereotaxic (Talairach) isocontours displayed on the Visible Man surface. Contours at 10 mm intervals in 3-D are displayed on flat maps for constant x (A), constant y (B), and constant z (C) values. For any point on the map, its Talairach coordinates can be determined by interpolation between contours on each panel. In the reverse direction, given a set of Talairach coordinates, the nearest point on the cortical map can be estimated by looking for intersection points on the appropriate isocontours.
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A complementary strategy is to establish a coordinate system that respects neighborhood relationships on the cortical surface (Anderson et al., 1994 
Fig. 7. A surface-based coordinate system for the left hemisphere [u, v]L-SB and right hemisphere [u, v]R-SB of the Visible Man, displayed on a map of cortical geography. The origin corresponds to the ventral tip of the central sulcus, and grid lines are spaced at 20 map-mm on each map. The horizontal (u) axis extends from
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The overall dimensions of the left and right hemisphere maps are similar, as reflected by their total horizontal extent (424 vs 426 map-mm, respectively) and total vertical extent (333 vs 306 map-mm, respectively). This consistency makes surface-based coordinates useful for quantitative comparisons between hemispheres. From visual inspection, it is evident that corresponding sulci in the two hemispheres generally have similar surface-based coordinates, just as they have similar 3-D stereotaxic coordinates except for the mirror reflection about the horizontal axis. We confirmed this quantitatively by determining the geometric center of gravity on the flat map for each of nine sulci that are well delineated in both hemispheres (Fig. 7, white dots). In Table 2, the two-dimensional (2-D) map coordinates are listed on the left, along with the misalignment between corresponding points on each map (R-L2D). Overall, the difference between the left and right centers of gravity on the cortical flat maps was small (9 map-mm median value, 15 map-mm mean value) and in no case was >7% of the total length of the flat map.
Table 2. Surface-based and 3-D coordinates of geographic landmarks (sulcal centers of gravity)
Left map (map-mm) Right map (map-mm) R-L2-D 3-D (left) (mm) 3-D (right) (mm) R-L3-D u v u v x y z x y z- CeS 21 62 20 60 2 48 37 49 8 PoCeS 60 62 64 70 9 52 34 56 10 STS 83 107 27 18 48 14 14 CaSd 196 104 192 118 15 11 1 0 15 CaSv 237 241 8 7 3 1 9 CoS 194 188 6 1 24 6 OlfS 150 120 30 34 7 35 1 SRS 140 108 32 0 42 0 10 42 3 10 FOS 8 37 1 41 32 11 12 Median 9 10 Mean 15 9 For full names of sulci in column 1, see Appendix. Column 6, R-L2-D indicates the misalignment between left and right hemisphere surface-based (map) coordinates. Column 13, R-L3-D indicates the misalignment between left and right hemisphere 3-D (stereotaxic) coordinates.
The corresponding 3-D coordinates are listed in Table 2 on the right, along with the misalignment between points in 3-D (R-L3D). The misalignment between the corresponding centers of gravity in 3-D was comparable in the absolute extent (10 mm median, 9 mm mean value), making it a higher percentage of the total length of the hemisphere. This degree of misalignment is about what should be expected, given that the scatter in position of major sulci is typically ~2 cm after transformation to stereotaxic space (Steinmetz et al., 1990
By these measures, surface-based coordinates are at least as consistent as 3-D stereotaxic coordinates in describing positional relationships in the two hemispheres of the Visible Man. The lack of perfect symmetry reflects a combination of individual variability in the pattern of convolutions and various technical factors such as alignment of the hemispheres and, for the flat maps, the exact placement of the cuts. It is likely that there will be somewhat greater variability between flat maps made from hemispheres of different individuals, but this does not pose a problem for how we have used surface-based coordinates in the examples illustrated below. Mapping functional organization
Projection via stereotaxic coordinates A common analysis strategy in neuroimaging studies involves transforming data from individual brains into Talairach space and reporting the stereotaxic (Talairach) coordinates for the center (or peak) of each statistically significant activation focus (Fox et al., 1985
To estimate the aggregate uncertainty from all factors combined, we analyzed neuroimaging data in which the coordinates of activation foci were reported for individual subjects as well as for the group means within that study. Figure 8 illustrates the distribution of foci in a study that compared responses to moving versus stationary stimuli using the PET technique (Watson et al., 1993
The full extent of this pattern is shown for the right hemisphere on 3-D lateral views and on flat maps of the occipito-temporal cortex for the left (Fig. 8C,D) and right (Fig. 8E,F) hemispheres. On both flat maps, cortex within 10 mm 3-D distance from the mean (red) occurs as two separate blobs of comparable size, whereas cortex within 15 mm is almost entirely contained in a single larger region. The irregular shapes of these shaded regions (e.g., elongated vertically for the right hemisphere and horizontally for the left) reflect the particular way that the local convolutions of Visible Man surface intersect the spheres of 10 and 15 mm radius in each hemisphere.
All but two of the individual activation foci lie within 15 mm of the group mean, and most of them lie within 10 mm. The 3-D distance between each individual activation focus and the associated group mean is shown quantitatively in the histogram of Figure 8G (black bars). The gray bars show analogous results for seven subjects (14 hemispheres) in a study that used a similar visual stimulation paradigm but was based on fMRI instead of PET (McCarthy et al., 1995 Estimating the extent of area V1 We used data from conventional architectonic analyses as well as modern neuroimaging studies to estimate the extent of primary visual cortex (area V1) on the Visible Man reconstruction. Our starting point was the postmortem histological analysis of architectonically identified V1 in 20 hemispheres by Rademacher et al. (1993)
The uncertainty zones visualized using this method reflect the geographical range over which the center of an activation focus might have occurred if the neuroimaging paradigm had been performed in the Visible Man. It implies nothing about the extent of cortex actually involved, which requires information about the number of voxels activated in each individual hemisphere by a given paradigm. Despite the obvious importance of this issue, we have not incorporated it into the analyses presented below, because the requisite quantitative data are not available. 
Fig. 9. Estimated location and variability in extent of visual area V1 in the Visible Man. A-C, Medial, posterior, and lateral views of V1 determined from the postmortem architectonic study of Rademacher et al. (1993)
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As an independent basis for localizing area V1, we used results from an fMRI study of visual topography by DeYoe et al. (1996) Extrastriate visual areas To estimate the boundaries of topographically organized visual areas V2, V3, VP, V3A, and V4v on the Visible Man atlas, we used both surface-based measurements and the stereotaxic projection method. The surface-based measurements involved determining the average width of each area on the summary flat maps published in the fMRI studies of Sereno et al. (1995)
Results for 10 topographically identified points along the V1 boundary are plotted on a flat map of occipital cortex in Figure 9F. The map is violet where the surface is within 10 mm (in 3-D) of a superior vertical meridian site, green where it is within 10 mm of an inferior vertical meridian site, and blue-green where it is within 10 mm of both meridia. For half of these sites, the closest point on the cortical surface (blue or green dots) lies within the estimated boundary zone for V1 (thin orange lines). The remaining sites are all within 10 mm 3-D distance from this zone. As expected, central visual fields (3° and 5° eccentricity) are represented posteriorly in the cortex, toward the occipital pole, whereas peripheral fields are represented more anteriorly (toward the top and bottom of the map). The steps in this progression are somewhat irregular because of errors inherent in the stereotaxic projection method. Also, there are two systematic asymmetries evident on the map. First, for the 20° and 24° superior vertical meridian loci, the closest point on the Visible Man surface lies dorsal rather than ventral to the calcarine sulcus. Second, any given eccentricity is represented closer to the occipital pole for the inferior vertical meridian than for the superior vertical meridian. These asymmetries are attributable to individual differences in the orientation of the calcarine sulcus and in the shape of the occipital lobe, which are not compensated by the methods used to transform different brains into Talairach space. Altogether, these neuroimaging-based stereotaxic estimates for the V1 border are consistent with those based on architectonic data, but they show greater variability and provide looser constraints for border localization. 
Fig. 10. Boundaries of topographically organized visual areas on the Visible Man cortex estimated using surface-based and stereotaxic projection methods. A, Boundaries between V2/V3 (top of map) and V2/VP (bottom of map) shown in green, relative to the most likely V1/V2 boundary transferred from Figure 9 and shown in black. Green contours represent boundaries estimated on the basis of distances along the cortical surface, taken from the summary cortical maps of DeYoe et al. (1996)
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Figure 10, B and C, extends this analysis to additional borders progressing away from V1. In particular, Figure 10B shows the estimated vertical meridian representation along the V3/V3A border dorsally and VP/V4v ventrally (blue lines, dots, and shading), based on average widths of 12 map-mm measured for V3 and VP. Figure 10C shows the estimated horizontal meridian representation in V3A and V4v (purple), based on average widths of 15 map-mm measured for V4v and 13 map-mm for the lower-field representation in V3A.
As with the stereotaxic projection method, the limitations of which have already been discussed, several sources of error can affect surface-based estimates of areal boundaries. For example, errors would occur wherever the representation of linear distances is compressed or expanded to a different degree on the Visible Man map and on the maps used for measuring the widths of areas. Unfortunately, insufficient data on distortion values are available to quantify these errors for the maps in Figure 10. Nonetheless, the fact that the stereotaxically based points and halos are distributed fairly evenly around each surface-based border estimate signifies reasonable agreement between the two methods and suggests that systematic errors are modest, at least for this particular data set. Analyses of functional specialization We used the stereotaxic projection method to analyze functional specializations for visual processing that have been reported for 118 foci in a dozen PET and fMRI studies (Figs. 11, 12, Table 3). In Figure 11, the data are grouped according to the type of activation involved (processing of color, motion, objects, faces, or spatial relationships) and are displayed on 3-D views as well as cortical flat maps. The shaded regions represent cortex within 10 mm (3-D distance) of at least one focus of that class. The boundaries of topographically organized areas, transferred from Figure 10, are shown in black.
The overall arrangement of topographically organized areas is summarized on a cortical map in Figure 10D and on medial and lateral views of the right hemisphere in Figure 10, E and F. Area V2 is yellow, V3 and VP are blue, and lower-field V3A (designated V3A 
Fig. 11. Distribution of activation sites associated with specific aspects of visual function. Each panel shows individual activation foci, identified according to the labels in Table 2, and cortex within the surrounding 10 mm uncertainty zone, for activations associated with processing of color (A, green), motion (B, red), form (inanimate objects or textures; C, light blue), faces (D, dark blue), and spatial relationships (D, yellow).
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Fig. 12. Combined analysis of activation foci for all foci listed in Table 3. Top panels, All 118 foci projected to flat maps of the left and right hemispheres. Note that activation foci originally reported for both the left hemisphere (x < 0 in Table 3, column 5) and the right hemisphere (x > 0) are plotted on each map. When patterns were examined separately for data obtained from the left and right hemispheres, no pronounced hemispheric asymmetries were evident. Although not labeled on either map, individual foci can be identified by determining their surface-based coordinates and finding the corresponding focus in Table 3. The distribution of foci is generally similar on the two maps, but with a number of notable exceptions (see text). Note that on the left hemisphere map a few foci lie outside the perimeter of the map (blue dots near the parahippocampal gyrus on the bottom right). This is because of their distance and orientation relative to the nearest tile on the surface. Bottom panels, Estimated extent of cortex likely to be specialized for particular visual functions, including motion (M) in red, form (F) in blue, spatial analysis (S) in yellow, and of cortex involving overlapping or closely interdigitation of multiple functions, including form and color (FC) combined (blue-green); form and motion (FM) combined (purple); and motion, color, and spatial, (MCS) combined (orange). Results are displayed in three formats for each hemisphere, including lateral and ventral 3-D views, lateral and ventral smoothed views, and flat maps of the posterior half of each hemisphere. The total extent of visually responsive cortex estimated from the pattern of activation foci is shown in dark gray.
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Table 3. Vision-related activation foci
Focus Task Baseline Z stat Talairach Left map (map-mm) Right map (map-mm) R-L2-D x y z u v u v Color Lueck et al., 1989 View color Gray 1a (T1) - 22 13 1b (T1) -
ABSTRACT
