Michael Hanke listed the number of voxels for different searchlight radii in pyMVPA. He also wrote that pyMVPA considers neighboring voxels to be those that share two edges (i.e., a face).
Here's a diagram of following the faces-touching rule. The center voxel is black, the one-voxel radius surrounds red, the two-voxel radius surrounds purple, and the three-voxel radius surrounds blue.
This gives 1 + 9 + 21 + 24 + 21 + 9 + 1 = 86 surrounding voxels for the three-voxel radius.
But Michael wrote that a three-voxel radius searchlight in pyMVPA has 123 voxels (counting the center). If so, at left is how I could come by that count: 13 + 28 + 40 + 28 + 13 = 122.
This shape is not quite symmetrical: there are four voxels to the left, right, front, and back of the center voxel but only two above and below.
Anyone know if this is what pyMVPA does?
UPDATE: Neither of these sketches is correct, see pyMVPA searchlight: solved.