Hi-

Thanks for sending the mask. I show it overlaid on the one I had been testing---it is indeed a bit narrower, esp around the ventricles. I don't know that that is an inherent problem, but those narrow regions are kind of non-ideal for the 3D cluster calcs and smoothing. It might just be part of life when using a GM mask. When I ran with just the default 10,000 iterations, the cluster sizes were 45, 47 and 49 (see below for full output).

I am not sure why you are switching to a different voxel size in that other mask? Note that regridding does introduce blur/smoothing into data, and that may change things slighty in your final analysis.

It looks like the cluster size has a reasonable convergence around 47+/a couple. Note that in all these things, it is good to keep an eye on the main goal: presenting reasonable results. If results appear/disappear because of having 47 vs 48 voxel thresholding, that is a sign of deeper instability in analyses. Reducing the sensitivity to these kinds of thresholds is discussed in this draft, with transparent thresholding:
https://www.biorxiv.org/content/10.1101/2022.10.26.513929v2
and here with ETAC:
https://pubmed.ncbi.nlm.nih.gov/31115252/

--pt

# 3dClustSim -mask MNI152_T1_2mm_brain_GM_02182017.nii.gz -acf 0.5128 3.8451 11.3839 -nodec -prefix TMP_CSIM_TEST2a
# bi-sided thresholding
# Grid: 91x109x91 2.00x2.00x2.00 mm^3 (160742 voxels in mask)
#
# CLUSTER SIZE THRESHOLD(pthr,alpha) in Voxels
# -NN 2  | alpha = Prob(Cluster >= given size)
#  pthr  | .10000 .05000 .02000 .01000
# ------ | ------ ------ ------ ------
 0.050000     694    828   1024   1203
 0.020000     268    332    415    477
 0.010000     155    190    242    277
 0.005000      93    116    147    173
 0.002000      54     66     84    100
 0.001000      36     45     59     70
 0.000500      25     32     42     52
 0.000200      15     20     27     34
 0.000100      11     14     19     24


# 3dClustSim -mask MNI152_T1_2mm_brain_GM_02182017.nii.gz -acf 0.5128 3.8451 11.3839 -nodec -prefix TMP_CSIM_TEST2b
# bi-sided thresholding
# Grid: 91x109x91 2.00x2.00x2.00 mm^3 (160742 voxels in mask)
#
# CLUSTER SIZE THRESHOLD(pthr,alpha) in Voxels
# -NN 2  | alpha = Prob(Cluster >= given size)
#  pthr  | .10000 .05000 .02000 .01000
# ------ | ------ ------ ------ ------
 0.050000     728    881   1088   1262
 0.020000     287    351    441    505
 0.010000     164    202    251    287
 0.005000     101    124    157    183
 0.002000      58     72     91    108
 0.001000      39     49     62     73
 0.000500      27     34     44     52
 0.000200      17     22     28     34
 0.000100      12     16     21     26


# 3dClustSim -mask MNI152_T1_2mm_brain_GM_02182017.nii.gz -acf 0.5128 3.8451 11.3839 -nodec -prefix TMP_CSIM_TEST2c
# bi-sided thresholding
# Grid: 91x109x91 2.00x2.00x2.00 mm^3 (160742 voxels in mask)
#
# CLUSTER SIZE THRESHOLD(pthr,alpha) in Voxels
# -NN 2  | alpha = Prob(Cluster >= given size)
#  pthr  | .10000 .05000 .02000 .01000
# ------ | ------ ------ ------ ------
 0.050000     698    851   1058   1223
 0.020000     277    335    434    505
 0.010000     156    194    247    292
 0.005000      96    120    153    177
 0.002000      55     68     88    105
 0.001000      37     47     61     72
 0.000500      26     33     43     51
 0.000200      16     21     28     34
 0.000100      11     15     20     25