Thursday, October 25, 2012

searchlight shapes: Stelzer

This is the first of what will likely be a series of posts on a paper in press at NeuroImage:

Stelzer, J., et al., Statistical inference and multiple testing correction in classification-based multi-voxel pattern analysis (MVPA). NeuroImage (2012),

There is a lot in this paper, touching some of my favorite topics (permutation testing, using the binomial, searchlight analysis, Malin's 'random' searchlights).
But in this post I'll just highlight the searchlight shapes used in the paper. They're given in this sentence: "The searchlight volumes to these diameters were 19 (D=3), 57 (D=5), 171 (D=7), 365 (D=9), and 691 (D=11) voxels, respectively." The authors don't list the software they used; I suspect it was custom matlab code.

Here I'll translate the first few sizes to match the convention I used in the other searchlight shape posts:
diameter radius number of surrounding voxels notes
3 1 18 This looks like my 'edges or faces touch' searchlight. 
5 2 56 This has more voxels than the 'default' searchlight, but less than my two-voxel radius searchlight. Squinting at Figure 1 in the text, I came up with the shape below.

Here's the searchlight from Figure 1, and my blown-up version for a two-voxel radius searchlight.
It looks like they added the plus signs to the outer edges of a three-by-three cube. This doesn't follow any of my iterative rules, but perhaps would result from fitting a particular sphere-type rule.

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