Hi,
On Tue, 26 Oct 2004, Dost Ongur wrote:
> Hi Joe,
> About negative PE values as a result of Featquery:
> - Looking at the covariance matrix, my design does not appear rank
> deficient, so I would like to use the PEs reported by Featquery to further
> understand patterns in my ROI. As a reminder, I have 5 conditions and
> Featquery gives me negative numbers for PEs and tstats for all 5 in my ROI
> (though I have positive COPEs of interest).
> - Doesn't the PE represent beta from the GLM? Is it possible that PEs
> would be negative because the blood flow changes are the opposite of what
> I modeled?
That's all correct, yes.
> - In my dataset, one group has consistently MORE negative PEs than the
> other (both have all negative PEs). Is this related to lower overall
> levels of blood flow in my ROI in one group, or to worse fit of BF changes
> to my model?
It depends really on what, qualitatively, you have put into your model.
For example, if you had created an EV that was 1s during baseline and 0s
during activation then the interpretation would be simple.... negative PE
means standard positive activation..... So it doesn't necessarily follow
that your results mean lower overall levels of blood flow (it's going to
be relative anyway when considering a given EV) or a worse fit (worse
would normally imply closer to zero rather than more negative)....
Cheers, Steve.
--
Stephen M. Smith DPhil
Associate Director, FMRIB and Analysis Research Coordinator
Oxford University Centre for Functional MRI of the Brain
John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222726 (fax 222717)
[log in to unmask] http://www.fmrib.ox.ac.uk/~steve
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