I'm using SPM5, fMRI data.
I might be misreading it... but in the HBF book chapter 8 on contrasts, page 9;
http://www.fil.ion.ucl.ac.uk/spm/doc/books/hbf2/pdfs/Ch8.pdf
"When testing for the second regressor, we are effectively removing that
part of the signal that can be accounted for by the first regressor."
Maybe I'm misreading this... but as you're only ignoring signal related to
the first regressor in the model, it does sound as if they are treated
sequentially.
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What version of SPM are you using? What operation are you using to do the
estimation? Is this PET or FMRI data?
I work with fMRI and have been under the impression that everything is done
simultaneously. Do you happen to have the page numbers and version of the book?
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