Dear SPMers,
I'm very puzzled by results of an SPM99 covariate analysis that are
making me question my basic understanding of what a covariate analysis
does. I hope you can set me straight. We have 18 subjects who were PET
scanned (6 scans), then got 3 weeks of a drug, then were scanned
again. Since the drug seemed to work, by the criterion of reduced symptom
severity (in ALL patients), we wanted to see what this did to regional
brain activity. A straightforward contrast of session (all pre drug scans
vs all post drug scans) showed a number of activations and deactivations
that made sense in terms of the relevant literature. The problem is that
when we did a covariate only analysis using the symptom severity rating,
the activations and deactivations seemed to reverse with each other. Since
covariate score is the same for all scans in a given session, it seems to
me that both analyses are primarily modeling change between sessions. I
was happy to see the same areas, only larger, in the covariate analysis,
because it suggested that the important difference between sessions was
related to symptom change, but why should the activations become
deactivations and vice-versa?
This is my thinking. Since all subjects went from a higher covariate
(more symptoms) in session 1 (pre-drug) to a lower rating, and the
+covariate contrast shows voxels whose activity is positively correlated
with the covariate, these will primarily be voxels that were higher on day
1 than on day 2. The minus covariate contrast will show voxels whose
activity is negatively correlated with the covariate, and so these will
primarily be voxels that were higher on day 2 than on day 1. Is this
wrong? Why do the areas that were more activated in the session when
covariate scores were HIGH come out in the -covariate, while voxels more
activated when covariate scores were LOW show up in the +covariate?? The
covariate was subject specific & centered, and I entered scan order as a
nuisance covariate.
One other feature of this data puzzles me, so perhaps it is
related. Of the 18 subjects, 17 have between-session changes between .5
and 8.0 for the covariate. The 18th has a change of 8.0 to 8.01 (.01
change). When I plot the fitted and adjusted response at any voxel, for
either + or - covariate, the otherwise flat baseline of the plot display
becomes very jagged vertical lines only for this subjects scans. What does
this mean, and should I be concerned?
Thanks very much for your help,
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