[pymvpa] Searchlight radius questions

[Jonas Kaplan]

Seems to me that radius of 1 gives 7 voxels, e.g.:

In [117]: radii = range(5)

In [118]: for radius in radii:
   .....:     sp = Sphere(radius=radius)
   .....:     size = len(sp((1,1,1)))
   .....:     print "Radius %d size: %d" % (radius,size)
   .....:     
Radius 0 size: 1
Radius 1 size: 7
Radius 2 size: 33
Radius 3 size: 123
Radius 4 size: 257

Jonas

----
Jonas Kaplan, Ph.D.
Research Assistant Professor
Brain & Creativity Institute
University of Southern California

On Sep 10, 2012, at 8:39 AM, J.A. Etzel <jetzel at artsci.wustl.edu> wrote:

> 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, either.
> 
> Jo
> 
> 
> 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
> http://mvpa.blogspot.com/
> 
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