A quick reality check:
In SPM'96 it is possible to plot the covariate against "response and estimate"
in a conditions (2) and covariates (1) design. Looking at the plot for
contrast 0 0 -1 1, I think I can see that the first scan is centred over
covariate1 = 0, and 2nd scans with lower covariate values are plotted to the
left, 2nd scans with higher covariates to the right. There are two types of
lines: blue unbroken and red dashed lines. Am I right in assuming that the
end-points of the blue lines give the measured covariate-perfusion points for
both scans, whereas the dashed red lines (each of which has the same covariate
values as one of the blue lines and dissect this paired blue line at its centre)
give the slope of the perfusion-covariate regression as predicted from the first
scan only?
These two coloured lines also appear for the main effects (0011 and 00-1-1) - in
those contrasts the blue lines are presumably the only relevant plots, and the
reds are to be ignored?
Klaus
Professor K P Ebmeier
Department of Psychiatry
University of Edinburgh
Royal Edinburgh Hospital
Edinburgh EH10 5HF
UK
Tel/Fax: *44-131-5376505
http://www.pst.ed.ac.uk/staff_pages/keb/
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