When you specify the sign, it changes the way the thresholding is done. If you use absolute (the default), then it ignores the sign. So if your threshold is 3, then anything above +3 and below -3 will survive, and so you end up with more and bigger clusters.
doug
Dan Dillon wrote:
Dear FreeSurfers,
I'm trying to replicate some functional analyses that were originally done in SPM2, and am using mri_volcluster. Here's my command: mri_volcluster --in Old8/bold/taumax30_2_21/tal-ffx/RSF_v_NIF/sig --in_type bfloat --reg Old8/bold/taumax30_2_21/tal-ffx/register.dat --thmin 3 --minsizevox 8.
I've noticed that the size (in voxels) and number of clusters changes if I play with the -sign flag. I am confused by this.
Here's the output not using the -sign flag:
Cluster Size(n) Size(mm^3) TalX TalY TalZ Max
1 28 1792.0 -19.80 -96.70 8.53 5.28907
2 8 512.0 39.60 26.79 -8.07 4.15380
3 9 576.0 99.00 16.69 -54.67 4.09661
4 10 640.0 87.12 63.11 18.95 -3.82155
5 13 832.0 67.32 -42.44 5.81 3.79067
6 11 704.0 -39.60 -16.43 59.78 -3.62503
7 11 704.0 -87.12 -75.98 -43.30 -3.59305
8 73 4672.0 43.56 -22.33 19.54 -3.43893
9 18 1152.0 43.56 -44.81 -41.49 -3.16065
10 8 512.0 -51.48 -32.30 53.20 -3.09981
11 15 960.0 -63.36 -20.49 56.30 -3.00613
12 48 3072.0 -71.28 -7.20 11.41 -3.00547
And here's the output with sign set to positive:
Cluster Size(n) Size(mm^3) TalX TalY TalZ Max
1 35 2240.0 -19.80 -96.70 8.53 5.28907
2 21 1344.0 39.60 26.79 -8.07 4.15380
3 13 832.0 99.00 16.69 -54.67 4.09661
4 10 640.0 -71.28 33.20 -35.31 3.91335
5 21 1344.0 67.32 -42.44 5.81 3.79067
6 9 576.0 35.64 -88.58 15.49 3.63267
Why does the size of the first cluster change? Also, where is cluster 4 from the second output in the first output? Is there some re-calculation going on here?
Dan Dillon, Ph.D.
Post-doctoral Fellow
Affective Neuroscience Lab
Dept. of Psychology, Harvard University
Phone: (617) 496-5222
E-mail: dillon@wjh.harvard.edu mailto:dillon@wjh.harvard.edu
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