Dear SPM users and PPI enthusiasts,
I've noticed that running the PPI module with SPM8 and SPM12 gives completely different results.
Few words on my design:
I have an n-back paradigm with blocks of ['X' 'One-back' 'Two-back' 'Three-back'], and these are modeled in the fist-level SPM.mat of each subject. I am interested in the negative and positive modulation of two-back on the left and right DLPFC, as VOIs. Thus, when specifying my PPI for each subject I use the contrast weights vector [3 1 1] to tell spm I'm interested in the two-back only.
Then, from the variables [PPI.P PPI.Y PPI.ppi] I'm interested in the negative and positive PPI interactions with my VOI, so when viewing the PPI results I define the contrasts [0 0 -1] and [0 0 1], for negative and positive interactions respectively. Following the subject-level PPI estimation, I create a two flexible factorial models, one for the left DLPFC and one for the right, with factors 1) Group (controls - patients) and 2) PPI interaction (negative - positive). When I do these steps with PPIs created with SPM8 and SPM12, I noticed completely different results from the 2nd-level analysis.
The issue:
I've inspected the subject-level PPIs from SPM8 and SPM12 and spotted that the variable PPI.P have 0s assigned for every scan except where the two-back is supposed to be, as expected, since spm was instructed to ignore the other blocks. However, the same PPI specification and estimation with SPM12 does not produce PPI.P with 0s in the blocks of no interest, instead it assigns some random values at the beginning at the first X block and then a stable value (different from 0) up to where the two-back begins. The PPI.Y variables are the same from both SPM versions, but since PPI.P are different, PPI.ppi are also different. In the end, the within and between group results have no relationship with each other.
Has anyone else encountered this issue? Is it normal that with SPM12 I don't get 0s for the PPI.P corresponding to the blocks of no interest? If not, how could I correct this and calculate the correct PPI.ppi?
Many thanks!
Vasilis
|