Hi Tim,
I am not sure if I understand you correctly: are you saying that I
cannot use Z-values below 2 because GRF theory is not valid anymore? Why
is that?
Thanks for your help
Nelleke
-----Original Message-----
Behalf Of Tim Behrens
Sent: 03 July 2008 14:20
Subject: Re: [FSL] thresholding question
Hi - if you are using Gaussian Random Field theory for your inference
(i.e. if you are using Flame and not randomise) then it breaks down at
low Z values (less than about 2).
Cheers
T
On 3 Jul 2008, at 12:56, Nelleke Van Wouwe wrote:
Hi all,
From some previous discussions and Matt Brett's page on cluster
thresholding I understand that you chose the z value based on the
activation you are looking for;high z for small regions and low Z for
big regions (given p<0.05, corrected).
I performed a ROI analysis (pre-masked the data with part of the
fusiform gyrus and parahippocampal gyrus)with a (corrected) cluster
significance threshold of P=0.05 and I tried a very low Z threshold of
1.1.
According to the clusterlist below I find a large cluster (232
voxels) that
is significantly active (P<0.05) with z-max 1.9.
I wonder if there are any rules of thumb to determine an appropriate Z
threshold (other than the default) and would this z = 1.1 still be a
reasonable threshold?
Thanks!
Nelleke
Cluster List
Cluster Index Voxels P -log10(P) Z-MAX Z-MAX X (mm) Z-MAX Y (mm) Z-
MAX Z
(mm) Z-COG X (mm) Z-COG Y (mm) Z-COG Z (mm) COPE-MAX COPE-MAX X
(mm) COPE-
MAX Y (mm) COPE-MAX Z (mm) COPE-MEAN
5 232 0.0356 1.45 1.83 42 -50 -22 42.7 -47.8 -24.2 193 42 -42 -30 149
4 41 0.039 1.41 2.26 -44 -62 -26 -42.7 -60.2 -25 243 -44 -62 -26 163
3 26 0.0394 1.4 1.68 34 -34 -20 31.5 -35 -19 134 34 -34 -20 107
2 24 0.0394 1.4 1.45 -32 -38 -20 -30.8 -38.3 -18.6 103 -32 -38 -20
87.6
1 1 0.0405 1.39 1.15 24 -48 -6 24 -48 -6 110 24 -48 -6 110
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