We tried to contrast mask (Z<0) on second level, but this didn't affect the
end result. ((Pc1-Pc2) - (Cc1-Cc2)) and ((Cc2-Cc1) - (Pc2-Pc1)) are still
the same. I really thought that this would do it and that I had figured it
out... Should we use contrast masking on first level instead? This sound
counter intuitive to me, thats why I wanted to ask first.
Thanks in advance
Hallvard
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Hi,
This sounds exactly right - so you have in one case
(Pc1-Pc2) - (Cc1-Cc2)
and in the other
(Cc2-Cc1) - (Pc2-Pc1)
These are indeed the same....!
On 16 Feb 2010, at 12:19, Hallvard Røe Evensmoen wrote:
> Hi,
>
> Yes, your assumption is correct with regards to setting within subject
> 2nd to fixed effects.
>
> To be precise, we are unsure what the problem is, other than not the
> output not making sense. With our setup we get two contrasts out of
> 3rd level that are identical to each other when we examine the
> opposite contrasts sent up from second level.
>
> To explain in more detail: As shown in the attachments in the first
> mail, the contrast c1 from 2nd level is condition 1 > condition 2, and
> contrast c2 is condition 2 > condition 1. On third level we want to
> examine the contrast C1 patient > control and C2 control > patient.
> Since C1 c2 is identical (down to individual voxels) to C2 c1,
> something is very wrong here - but we are unable to see what is wrong
> here.
>
> We have also conducted the same analysis, only difference was that we used a
> unpaired, instead on paired, t-test between the two groups (normal vs
> patients). The result was the same...
>
> Do you think contrast masking could be an idea?
>
> Thanks in advance
>
> Cheers.
>
> Hallvard
>
>
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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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