searchlight shapes

How exactly the searchlight was shaped is one of those details that is usually not mentioned in searchlight analysis papers but is not obvious, mostly since voxels are cubes.

Nikolaus Kriegeskorte (the father of searchlight analysis) uses the illustration on the left.

Expanded, I think this would look like the picture at left. There are 32 voxels surrounding the center; the center voxel in black and surrounding in blue.

But there are other ways of thinking of a searchlight. Here are three ways of defining a one-voxel radius searchlight.

Personally, I implemented the middle version (edges and faces, not corners, 18 voxels) as a one-voxel radius searchlight in my code. Larger radii are calculated iteratively (so for a two-voxel radius, combine the one-voxel radius searchlights defined by treating all 18 surrounding voxels present in the one-voxel radius searchlight as centers). I don't think one version of defining surrounds is necessarily better than others, but we do need to specify the voxels we are including.


update 11 September: I changed the caption of the searchlight shape images for clarity.