Dear SPM/fMRI experts,
On my SPM8 T-maps I am performing cluster-size thresholding using
3dClustSim of the AFNI package (formerly known as AlphaSim), which
requires an estimation of the correlation of the voxels in a 3D dataset.
I use a tool of the same package (3dFWHMx) to estimate the degree of
voxel spatial correlation, based on an algorithm by Forman and
However, a colleague uses a different approach by extracting the FWHM
values for the 3 axes from the SPM.mat file - SPM.xVol.FWHM.
I tried both methods and noticed that they produce different results -
here is an example for one contrast where the values don't seem to
differ that much, but sometimes the differences are greater than 1.0:
FWHMx = 9.82782
FWHMy = 10.0744
FWHMz = 9.31861
FWHMx = 10.2832
FWHMy = 10.3741
FWHMz = 9.4135
My questions are:
(1) Do the values in the SPM.mat file even reflect the voxel spatial
correlation of the contrast/the T-map?
(2) If so, what algorithm is their estimation based on (or: are they
based on a different calculation than Forman et al.)? and
(3) What would be the recommended method?
Last but not least:
(4) Is there an (SPM-based) alternative method for cluster-size
Thanks in advance!
All the best,
Franziska M. Korb, Ph.D.
Postdoctoral Associate (Egner Lab)
Center for Cognitive Neuroscience
LSRC Building, Room C03D, Research Dr
Durham, NC 27708