Dear All:
I have a simple question but it is really bugging me :-)
I have the following study design consisting of two conditions and a
covariate in the presence of each condition:
Condition: drug on or off
Covariate: Stimulation during drug on and off
I have chosen "single subject: conditions and covariates" for my
first level analysis and entered the drug scans as conditions giving
me the first two columns in my design matrix and then two covariates
of interest giving me the next two columns.
I believe the following to be true:
[1 -1 0 0] ---> rCBF decreases due to drug (condition 1 is pre-drug)
[-1 1 0 0] ---> rCBF increases due to drug (condition 2 is on drug)
[0 0 1 1] ----> Main effect of stimulus rating (covariate)
[0 0 1 0] ----> Effect of stimulus rating during condition 1 (no drug)
[0 0 0 1] ----> Effect of stimulus rating during condition 2 (drug)
What I am wondering is where I get my covariate by condition
interactions from. Intuitively I think it is some combination of
condition and covariate contrasts [1 -1 1 0] but logically I think it
is simply the contrast of my two covariates [0 0 -1 1].
I realise there must be a simple answer to this but I searched in
vain through the archive. Can someone supply me with correct logic?
Many thanks,
Stuart.
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