Actually, I see that I made a mistake in my previous code listing: I forgot to *use* the mask MMM.nii.gz that was created in the 2nd command in the third command... whoops! This is what should have been written initially (and I will edit my previous answer to be correct and match this, so that future generations are not thrown off by my error):
# concatenate data sets of interest
3dTcat -prefix ALLCAT.nii.gz DSET1 DSET2 DSET3 ...
# make a mask of anywhere that is nonzero across the original data sets
3dTstat -nzcount -prefix MMM.nii.gz ALLCAT.nii.gz
# find the volume index+1 of the subbrick with the maximum value within the mask, per voxel
3dTstat -argmax1 -mask MMM.nii.gz -prefix MAP ALLCAT.nii.gz
--pt
