On Thu, 9 Sep 1999, Klaus Ebmeier wrote:
> Steven Grant wrote:
>
> * I am using SPM96 to perform a conditions and covariates analysis in
> a PET FDG study. I have two sessions per subject, one scan per session,
> and a single task performance measure for each scan. The first session
> is the control condition, the second session is the active condition.
>
> * I would like to test for correlations between the change covariate
> scores across the two scans/sessions with the change in the PET images.
> I have followed the recommendation of Andrew Holmes of creating a
> mean-centered covariate difference score and entered the covariate as
> -1*Difference/2 and +1*Difference/2.
I concur with Klaus Ebmeier's exposition in reply to this, and the bottom
line on the confusion surrounding the whole issue of interaction analyses.
Some insight from the authors would be welcome, particularly as SPM99 now
explicitly allows interaction modelling.
In the example given above we have adopted 2 different approaches
depending on the version of SPM we are using:
1. Our approach using SPM94/96 has been to take the difference between the
two "scores" then enter them as a single covariate multiplied by a
sign-swap matrix [-1, +1...]. Interactions are sought using [0 0 -1] and
[0 0 +1] contrasts.
2. In SPM99 we have entered the scores from the two conditions as a single
matrix in their native format i.e.
[score1_scan1, score2_scan1...score1_scan_n, score2_scan_n]
Selecting the interaction x condition option appears to split this single
covariate vector into two separate vectors, with a 0 entered against scan
1 for condition 2 score and a zero against scan 2 for condition 1 score.
We then sought interactions using [0 0 -1 -1] and [0 0 1 1]
The results for these interactions appear to be identical to those derived
using approach 1 in SPM96 i.e. [0 0 -1] is equivalent to [0 0 -1 -1].
However, I have found the interpretation of these results difficult when
examining these correlations at the single voxel level using RESULTS.
Perhaps the authors could help us clear this one up?
Also, it would be very useful if the covariate and confound values entered
were displayed on the SPM99 print out, as in SPM96, just to help ensure
data are entered correctly.
Regards - Mike
--
_________________________________________________________________
Mike Glabus, PhD
Visiting Fellow in Functional Neuroimaging Tel: + 301 402 3682
Unit on Integrative Neuroimaging
Clinical Brain Disorders Branch FAX: + 301 496 7437
National Institute of Mental Health, NIH
9000 Rockville Pike, Bethesda, MD 20892-1365, USA
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