Hello Michel,
1) Absolute: here, the treshold is explicitly set by the user in contrary
to "relative" where the threshold is derived from a global mean value, and
will therefore change from subject to subject.
2) Depends. If you e. g. analyse modulated maps, then the threshold 0.1
(you example) will be applied to the modulated maps. The modulated maps
(mwc*) contain not some probability information, but rather absolute
volumes (1 being "pure grey matter" in that voxel, "0" being no GM in that
voxel). For c*-images it is rather the probability of GM, indeed. Both
values, however, are correlated to a certain degree. Overall, your
interpretation is right in that this relatively low threshold will be
applied - this means, that also border zones of the GM areas are included.
In any case, best check, how large the resulting mask is.
3) First, SPM calculated a rough brain mask by a simple thresholding step
(1/8 of overall mean); then, the mean from all voxels within this area is
calculated. Should be described in the explanation window in SPM.
hth,
Philipp
Max Planck Institute of Psychiatry
NMR Research Group
Kraepelinstr. 2-10
80804 Munich
Mail: [log in to unmask]
Phone: 0049-89-30622-413
|