Hi all,
I have a few questions regarding the thresholding in FEAT:
1) I understand that with cluster thresholding, the voxels are first
thresholded by the Z threshold and then the resulting clusters are tested to
meet the Cluster P threshold.
What I want to know is, how do I find the p-value corresponding to the Z
threshold? Do I simply use the ztop (and ptoz for the reverse)? Just to
verify, this function calculates the p-value for that Z-score and NOT the
Cluster P threshold. Is that correct?
Also, if I'm conducting a whole brain analysis, I would want to use the #of
resels given at the bottom of the smoothness file under each COPE's stats
folder and not the resel count (# of resel/volume ?) for the ptoz function.
Am I misunderstanding this?
2) On a similar note but a different analysis, I have a set of localizer
data to find the FFA and I was hoping to get some advice on the threshold
settings. I am defining the FFA as a sphere (of 8mm radius) centered on the
most significantly activated voxel (within a certain range in the x,y,z
directions) from the contrast ( Faces - Items ). At the default settings for
the cluster thresholding ( Z threshold = 2.3 and Cluster P threshold =
0.05), I am only able to find FFAs in 31 out of the 47 participants. I was
hoping that I would be able to localize more FFAs with less stringent
thresholds.
I tried the no thresholding option under the Post-stats tab of FEAT but the
Post-stats report was blank except the the Post-stats title. Is this
supposed to happen?
I was considering Voxel thresholding but I need coordinates for the most
significant voxel; not sure how I would find it if it's not reported in the
results.
Since the FFA is a rather small area, I'm thinking that I should lower the Z
threshold and increase the Cluster P threshold but from previous posts, it
sounds like lowering the Z threshold below 2.0 isn't a good idea. I'm new to
GRF theory so I don't quiet understand why that is yet but from the ztop
function, a Z threshold of 1.65 corresponds to a p-value of 0.049471.
Doesn't that mean a Z threshold of 1.65 would still give me significant results?
Lastly, I am planning to re-run the post-stats with Z threshold=2.3 and
Cluster P threshold=1. Does that sound like a good idea? Does anyone have
any suggestions?
Thanks in advance everyone!
Have a great weekend,
Lin
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