Hi Dr. McLaren and SPM list members,

I have two follow up questions re: these alternative approaches to statistical corrections and VBM (VBM8) analyses in SPM.

1) 3dClustSim: Re #1 below, I'm also interested in applying AFNI's 3dClustSim to define the cluster extent (I.e., number of voxels) needed to reach significance for my results from VBM8 analysis. I assume I would just use my custom DARTEL GM template as the mask in 3dClustSim (we used a custom template bc this was a pediatric study).

We used a 8mm smoothing kernel, but I am not sure how to obtain the average blur estimate to use as input for "-fwhm" flag in 3dClustSim, given that these data are from VBM8 analysis and not fMRI. What are the steps for identifying the XYZ blur estimates for structural T1 data processed with the VBM8 toolbox?

2) Dr. McLaren,  for the latter approach you suggested in #2 below (I.e., " apply the mask to the results and then click SVC you will get the FWE corrected value for the GM mask region"), would it be appropriate to use our custom DARTEL GM template as the mask in the SVC procedure in SPM? I think this fits what you suggested, but it would be helpful to clarify.

Thanks,
Andrea

From: "MCLAREN, Donald" <[log in to unmask]<mailto:[log in to unmask]>>
Date: Friday, January 23, 2015 1:43 PM
Subject: Re: exploratory VBM and thresholds

Colin,

Two thoughts:
(1) Have you tried using cluster FDR or 3dClustSim to define the cluster extent needed to reach significance?

(2) Have you applied a GM mask to your data to reduce the search region - has to be applied at the level of model estimation to make a difference. Alternatively, you can apply the mask to the results and then click SVC you will get the FWE corrected value for the GM mask region. This isn't really a SVC, but simply a way to restrict the correction to only considering the voxels in the mask.

Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Postdoctoral Research Fellow, GRECC, Bedford VA
Website: http://www.martinos.org/~mclaren
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On Fri, Jan 23, 2015 at 1:27 PM, Colin Sauder <[log in to unmask]<mailto:[log in to unmask]>> wrote:
Good day all,

We're completing an exploratory comparison of two clinical groups re: GM volume. Unfortunately, with our small sample size a whole-brain FWE correction does not reveal any significant clusters.

However, utilizing a .001 uncorrected threshold w/ a extent threshold of 1000 voxels (in this case equal to 1 cubic cm) reveals several large clusters of GM differences, which span regions previously identified as important in these disorders.

I was hoping for feedback regarding this approach and the likelihood of spurious results. Of course, we could always complete an ROI analysis w/ SVC but I'd prefer a whole-brain analysis if possible.

Thanks,
Colin


--
Colin L. Sauder, M.S.
Doctoral Candidate
Interim Assistant Director
Krasner Psychological Center
Department of Psychology
Stony Brook University
Stony Brook, NY 11794-2520
Phone:  (631) 306-4096<tel:%28631%29%20306-4096>
Fax:  (631) 632-4632<tel:%28631%29%20632-4632>
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