Dear Tom & list,
we have performed a VBM analysis giving as a cluster
of about 1400 voxels at p uncorr 0.001, with a cluster
corrected p-value of < 0.001.
(The design was a two-group comparison (quite imbalanced: 40 : 140),
with 3 covariates and a constant; preprocessing incl 12 mm smoothing
and estimation in SPM5).
We also used SnPM (version 3, in SPM2) to gain permutation based p-values,
but now are stuck - statistically and technically.
Questions:
1. Reading that cluster corrected p-values are not valid for VBM data due to
non-stationary smoothness, what is the alternative? Are we going into the right direction by gaining NP p-values?
Is the non-stationary smoothness problem somewhat 'compensated' by the large N?
2. SnPM (after 5000 permutations) showed a more or less identical cluster after thresholding the uncorr. p-values at 0.001.
We tried to collect spruathreshold clusters, but when we try to view the results an error appears 'SnPM_ST.mat corrupt',
i. e. this file cannot be read in (3.5 GB).
Is there anything we forgot to consider? (Voxel results can be viewed).
3. Do you have any alternative suggestions how we can gain a p-value for the whole cluster?
4. Looking at the ResMS image of SPM, we see that the cluster lies in an area of relatively high residuals
(compared with other areas). May be this roots in more residual gyral variation in this area after spatial normalisation.
Is this issue 'solved' by using a permutation method?
Thank you very much for any hints!!
Philipp Saemann
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