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-----
From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On
Behalf Of Tim Behrens
Sent: 03 July 2008 14:20
To: [log in to unmask]
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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