My -little- understanding of GRF is that:

The rationale behind GRF is that the -expected- euler characteristic (EC) approximates the FWE probability.
EC basically counts the number of peaks minus the number of holes (after thresholding). If your threshold is too low, you will have many holes, and the EC will not represent the number of peaks, which you want in order to approximate the FWE..

Saad.


On 3 Jul 2008, at 13:52, Wouwe, Nelleke van wrote:

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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Saad Jbabdi, 
Postdoctoral Research Assistant,  
Oxford University FMRIB Centre

FMRIB, JR Hospital, Headington, Oxford  OX3 9DU, UK
+44 (0) 1865 222545  (fax 222717)
[log in to unmask]    http://www.fmrib.ox.ac.uk/~saad
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