Dear SPM users,
We have a PET study where we have 2 conditions (Awake and NREM) and 1
covariate PSQI. There is only one scan per condition per subject.
We want to test for areas where:
(1.) Increased activation from Awake to NREM is associated with higher
PSQI scores;
(2.) Increased activation from Awake to NREM is associated with lower PSQI
scores;
(3.) Decreased activation from Awake to NREM is associated with higher
PSQI scores;
(4.) Decreased activation from Awake to NREM is associated with lower PSQI
scores.
We looked at the SPM mailbase and found a suggestion given by Andrew
Holmes (about August 4, 1998). That suggestion given was to (i.) center
the covariates so that the "new scores" all add up to zero, and then (ii.)
enter the covariates as [-a/2, a/2, -b/2, b/2, .....] where a is the "new
score" for the first person, b is the new score for the second person,
etc.
Suppose that our scans are ordered (per subject) so that the first is the
Awake and the second is NREM. Does the manner in which the covariates are
entered above mean that the contrast [0 0 1] is testing for areas where
**increased** activation from Awake to NREM is associated with higher PSQI
scores and that the contrast [0 0 -1] is testing for areas where
**increased activation** from Awake to NREM is associated with lower PSQI
scores?
Suppose that we want to test for areas that have decreased activation from
Awake to NREM being associated with higher PSQI scores. Do we simply
(i.) change the signs of the covariates (i.e., use [a/2, -a/2, ...]) and
(ii.) form the contrast [0 0 1]?
Is this equivalent to
(i.) keeping the covariates the way they were: [-a/2, a/2,...]
(ii.) but change the order of the scans -- this time NREM first and then
Awake second and
(iii.) form the contrast [0 0 1]?
If my understanding is incorrect, please advise on how I should form the
contrasts?
Thanks for your help.
Hernando Ombao
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