Dear SPM community,
I would like to estimate beta parameters for every subject in my sample
using exactly the same explicit (whole brain) mask. I would like to have
beta estimates for exactly the same voxels in MNI space. Can SPM do this?
There is the option to specify an explicit mask when setting up a GLM.
However, SPM automatically still applies some implicit masking. From
what I understand the critical parameter for implicit masking is
mask.tresh in the spm_defaults (usually = .8, meaning that voxels with
less than 80 % of the global mean signal are discarded).
The strange thing is that when I set this to 0 in order to disable
implicit masking, the beta estimates become extremely noisy and my
effects are gone. This is very odd behavior.
When I set it to .01, the betas estimates seem to be fine again but it
is not exactly what I want. The "effective masks" that SPM comes up with
for every subject still differ a little here and there, so this is no help.
How can I achieve what I want with fMRI?
Thanks & best,