On 2/25/2013 1:10 PM, Vadim Axel wrote:
Absolutely naive question: suppose I have single a-priori defined ROI
where I get a modest group-level beyond chance prediction of
p-value=0.01 (one-tail t-test vs. 0.5, across subjects). Now I run a
group level whole-brain search-light and I am expected to find at least
one cluster of beyond chance prediction in the environment of my ROI.
Correct?
No.
I have a paper in (hopefully the last cycle of) review that goes into
detail about these issues. But here's a brief version of some of the
relevant ideas. I'm assuming you're using a linear SVM and proper
cross-validation, and also that the searchlight is substantially smaller
than the ROI.
Two possible explanations come to mind:
1) The single searchlights are too small to hold enough voxels to
classify accurately, but the ROI can, because there is weak information
present in much of the ROI. Linear SVMs can combine weak information
from many voxels, so can sometimes classify better with more voxels.
2) There is a lot of spatial variability between subjects. Suppose only
a small part of the ROI is informative. If that part falls withing the
ROI for everyone, then the ROI might classify well at the group level.
But if each person only has a small informative area on their
searchlight map, the group map could come out non-significant (people's
maps don't overlap enough).
A few suggestions:
1) If your hypothesis is about the ROI, stick with the ROI-based
analysis, adding control ROIs (or whatever) as necessary, but not doing
the searchlight analysis.
2) If you need the searchlight analysis for a particular purpose, do
some sensitivity testing, and look closely at the single-subject maps.
For example, how much do the maps change with different searchlight
radii? Did you normalize to atlas space before or after the searchlight?
Did you smooth the data? Smooth the individual subject maps? etc.
3) Check the sensitivity of the ROI-based finding. For example, How much
does it change if the ROI boundaries are altered slightly? How much
variation is there between subjects - does the ROI classify well in most
everyone, or just a few people?
Hope this gets you started, and good luck.
Jo
--
Joset A. Etzel, Ph.D.
Research Analyst
Cognitive Control & Psychopathology Lab
Washington University in St. Louis
http://mvpa.blogspot.com/
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