Sorry, I mistyped. To clarify, the original qdec analyses were run on the 10mm smoothed data from qcache, and the sims were then run with and without passing the value from fwhm.dat into the FWHM flag in mri_glmfit.
--------- Jim Porter Graduate Student Clinical Science & Psychopathology Research University of Minnesota
James Porter wrote:
I have recently run simulations with and without the FWHM flag, and the results were 100% identical, in both the mgh output and cluster summary files. From what is below, I would think that the results should have changed in at least some way. The original qdec analysis was run with 10mm smoothing, and the sims (mc-full, 10k) were run with and without 10mm smoothing.
Is there a certain point at which one would not expect smoothing in the simulations to not matter?
Jim Porter Graduate Student Clinical Science & Psychopathology Research University of Minnesota
Douglas N Greve wrote:
<div class="moz-text-flowed" style="font-family: -moz-fixed">1) The smoother the data, the more likely a cluster will be found by chance. When the data are created, they start with some smoothness level. When you smooth them you add more. So you need to match the total level of smoothing when you do the simulations, otherwise your clusters will be way too significant. mri_glmfit creates a fwhm.dat file with an estimate of the total smoothness.
- Just if they are nearest neighbors. There is no option to tinker
with this.
- Vertices in the maps created by the simulation are thresholded at
the level you pass. This is what defines the cluster.
doug
Dankner, Nathan (NIH/NIMH) [F] wrote:
Hello all,
I have a couple of questions regarding the way the cluster correction simulation in freesurfer works. I've read the wiki pages on the subject, but if I've missed something and any of this is answered elsewhere please let me know. My technical knowledge of these things is not great so I am just trying to get some background. First of all, how does smoothing the data prior to running the simulation affect the results? I've run corrections on the same data smoothed with a 10mm FWHM, and also on completely unsmoothed data, and the cluster results were different. Secondly, what determines whether vertices are neighbors or not? Is there an option to tinker with this or is it predetermined? Lastly, how do the p values of individual vertices factor into the simulation? Are they taken into account when the determination of the max cluster size under the null hypothesis is performed? Thanks in advance,
Nate
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