Hi,
This is a good question; you might well expect PE1 and COPE1 (if it
just uses PE1) to give the same answer. In certain simple designs,
they will.
However, when you have partial correlation between EVs, the effective
regressor height is changed, and this has a direct impact on the %
signal change calculations (see Smith, NeuroImage, 2007 for
mathematical details). Hence the COPE result is different from the PE.
You can see the estimated effective contrast heights in the design.con
file.
Note that you only see this difference when you turn on the % change
option - the raw PE and COPE values are still identical.
Cheers, Steve.
On 15 Nov 2007, at 11:21, [log in to unmask] wrote:
> Dear all,
>
> I have a question about the results that I got with the featquery
> option of
> FSL. In my study I have 6 PE's with their temporal derivatives and I
> applied motion correction. The COPE's of my study are partly the
> same as my
> PE's or they are the substraction of certain PE's. But PE1 is the
> same as
> COPE1 and PE3 is the same as COPE2.
> When I run the featquery over the first-level feats, I get different
> results for PE1 and COPE1. However, the max data that featquery uses
> to
> calculate the min, max, mean etc. is the same for the PE1 and COPE1.
> Can
> anyone explain this difference in results? I have used the convert
> PE/COPE
> in % option for the featquery. And if I want to calculate the
> correlation
> of mean signal change, do I use the mean PE or mean COPE?
>
> Thanks!
>
> E.E.Meerman
> email: [log in to unmask]
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Stephen M. Smith, Professor of Biomedical Engineering
Associate Director, Oxford University FMRIB Centre
FMRIB, JR 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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