Dear all,
Is it possible to change the order of the columns in a design matrix in SPM?
Basically, I was reading this paper in the effects of using parametric modulation and if I understand it correctly they put the most important column of interest in as the first one, the parametric modulation.
I asked before but I think something wrong with my result or At least I cannot interrupt them.
What I have is a blocked mixed design data of a single subject last for ~16 min. The experimental contains task ( squeezing a ball ) and rest. It has 48 blocks, 24 task and 24 rest. I started with rest. Each block lasts for 20 seconds. The active task contains 30 squeezes but each block has a different target level... example: 20% 40% 60% of the grip strength.
I model it as follows:
1) I select specify first level and all the ACTIVE onset time for each squeeze so it was 720. The duration I put 0.
2) I select the parameter modulation and added the grip force values for each stimuli after I mean corrected. I used spm12 and checked orthogonal.
3) I selected up to the third order polynomial regression. ( I tried all the possibility)
4) I did with and without model interaction but it seems to be better when I add model interaction (it removes lots of false positives ) should I use it ?
In the contrast I put four names
Active : [ 1] for just the different between active and rest and the gives me activation in where I expect
Sq1 :[ 0 1] for the first order and it gives me activation in the eyes ( and occipital lope) which I do not expect.
Sq2: [ 0 0 1] for the second order and it gives me activation the opposite motor area opposite to what I think I should get.
I could not interrupt what I have as my target or my thought is that there should be a linear effects between the strength of the force and the signal.
When I read this paper, they changed the order of the columns depend on what they want to see and put it first. Should I do this or at least can someone please guide me to what I am missing?!
Thanks
AS
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