Hi,
On 30 Jul 2007, at 09:37, Kate MacIver wrote:
> Hi,
> I have a question regarding higher-level analysis with added
> covariates of
> interest. I have a group of 13 scanned before and after a clinical
> intervention, and I want to know if the difference in activation is
> related
> to the reduction in pain scores at follow up. My query relates to
> how to
> input the covariates and how to interpret the maps.
>
> I have two events in my lower level design:
> A 1 0 0 0
> B 0 1 0 0
> A-B 0 0 -1 1
> B-A 0 0 1 -1
>
> At higher level, I want to look at the within group difference
> (pre- minus
> post intervention, and post minus pre-intervention):
> 1 1 1 0 0 0...
> 1 1 0 1 0 0...
> ...
> 1 -1 0 0 1 0...
> 1 -1 0 0 0 1...
> I understand that the contrasts here would be 1 -1 and -1 1.
Is this a paired t-test model? is the first column here the group
membership or EV1? If it's EV1 it should be removed from the model
(see the example in the manual). the contrasts should just be [1 0
0 ...] and [-1 0 0.....]
Also, make sure that the diagonal parts are the same in the top and
bottom half - it's hard to tell from this snippet whether that's the
case.
> I am interested now in what activation co-varies with the pain scores.
> Should I simply demean the scores or do I need to orthogonalise
> them? Would
> the model look like this for example:
> 1 1 1 0 0 0... 2
> 1 1 0 1 0 0... 3
> ...
> 1 -1 0 0 1 0... 3
> 1 -1 0 0 0 1... -2
again - it's hard to tell what you're doing here with this
snippet...could you be more explicit about the modelling?
Cheers, Steve.
> with the following contrasts:
> Pre-Post 1 0 0 0... 0 0
> Post-Pre -1 0 0 0... 0 0
> Pre pain 0 0 0 0... 0 1
> Post pain 0 0 0 0... 0 1
> Pre-post pain 0 0 0 0... 1 -1
> Post-pre pain 0 0 0 0... -1 1
>
> Are theses contrasts going to tell me if there is any covariance
> between
> changes in activation and changes in pain scores at follow up?
>
> Thanks,
> Kate MacIver
> Pain Research Institute
> Liverpool
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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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