Fig. 2.

Comparison of voxel selection methods in information mapping. A: schematic representation of a brain slice, with white matter, gray matter, and matter outside the brain indicated. The curved lines represent the white matter–gray matter boundary, the gray matter–pial surface boundary, and the skull. With the traditional volume-based voxel selection method for multivoxel pattern analysis, a voxel (blue) is taken as the center of a sphere (red; represented by a circle) and all voxels within the sphere are selected for further pattern analysis. B: an improvement over A, in that only gray matter voxels are selected. The gray matter can be defined either using a probability map or using cortical surface reconstruction. A limitation, however, is that voxels close in Euclidian distance but far in geodesic distance (i.e., measured along the cortical surface) are included in the selection, as illustrated by the 3 voxels on the left. C: using surface reconstruction, the white matter–gray matter and gray matter–pial surfaces are averaged, resulting in an intermediate surface that is used to measure geodesic distances. A node on the intermediate surface (blue) is taken as the center of a circle (red; represented by a solid line), the corresponding circles on the white–gray matter and gray matter–pial surfaces are constructed (red dashed lines) and only voxels in between these 2 circles are selected.