[pymvpa] Searchlight radius questions

J.A. Etzel jetzel at artsci.wustl.edu
Mon Sep 10 15:39:20 UTC 2012

Your question touches on something I've been meaning to post about: how 
exactly to shape a searchlight? As far as I know, there isn't a consensus.

I posted images at 
http://mvpa.blogspot.com/2012/09/searchlight-shapes.html showing 
one-voxel-radius surrounds containing 6, 18, and 26 voxels. (If you know 
of others in use, let me know!)

I'll leave it for the pyMVPA experts to say if a one-voxel radius 
searchlight corresponds to 9 voxels; that doesn't sound right to me, 


On 9/9/2012 2:24 PM, Ping-Hui Chiu wrote:
> Dear PyMVPA experts,
> r=0, 1, 3 respectively correspond to 1, 9, 123 voxels in the following page:
> http://dev.pymvpa.org/examples/searchlight.html
> However, doesn't r=1 encompass 1 center+6 faces=7 voxels?
> Also, is there a formula for calculating the number of voxels given a
> specific radius?
> Finally, suppose a voxel=3*3*3mm, is it correct that r=3 covers up a
> neighborhood of (left 3*3mm + center 1*3mm + right 3*3mm)=21mm spherically?
> Thanks!
> Dale

Joset A. Etzel, Ph.D.
Research Analyst
Cognitive Control & Psychopathology Lab
Washington University in St. Louis

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