Hello,
I conducted an fMRI experiment and performed a contrast using FEAT cluster correction for multiple comparisons, setting the z-threshold to 2.3 and the p-value to 0.05. This analysis yielded a nice result, with large clusters (around 400-600 voxels). However, when I did the exact same contrast, correcting for multiple comparisons with FDR and setting q to 0.01, the clusters are much smaller (around 20 voxels), with the peak voxels in the same location. Would you happen to know why this is? I may be making a simple calculation error, but I believe the z-threshold of 2.3 in cluster correction should the same as a q of 0.01 in the FDR analysis, right? And the FDR should be less conservative than the cluster correction in FEAT, right?
A couple notes:
1. I used outlier de-weighting in my FEAT analysis
2. I performed the following 4 command line steps for the FDR analysis:
ttologp -logpout logp1 varcope1 cope1 `cat dof`
fslmaths logp1 -exp p1
fdr -i p1 -m ../mask -q 0.01
fslmaths p1 -mul -1 -add 1 -thr .999972559 -mas ../mask thresh_1_minus_p1
You can see that the number I get from the "fdr" step is quite small. Any chance you know what's going on?
Thanks so much and happy holidays!
Dar
|