Hello everyone,
In the process of trying to understand cluster thresholding, I played around
with some different and sometimes non-sense values both for the Z score and
the P value thresholds.
My understanding of the cluster thresholding is that it comprises 2 steps:
1) a Z score threshold is apply to every voxel, keeping only the ones that
pass the threshold.
2) the map obtained for the first step is thresholded again using the chosen
P value, which determines a limit size for the clusters to be considered
significant. This size is obtained from the gaussian field theory, but it's
unclear to me how exactly the P value and the size correspond, if anybody
wants to take a shot at explaining this... (this is actually my first
question here I guess!)
So, in one of my thresholding experimentations, I have thresholded my data
to Z = 0 and P = 1.0, which means to me NO THRESHOLDING at all - all the
voxels pass the Z score threshold (0), and the P value allows all the
clusters as small as a single voxel to be considered significant. This is a
stupid thresholding, but I just wanted to check that I had understood
everything properly. Thus, I expected to get a really noisy map, with the
whole brain lighting up (nice for Christmas...). To my wonder, I get totally
reasonable patterns of activations!
Of importance is that what I describe here happens when I do a
paired-comparison analysis. When I just contrast each of my experimental
conditions vs. rest, I do get a nice Christmas tree, as expected. Could
anybody tell me the reason for that?
Thanks a lot for any help,
Stephane
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