Hi,
On 21 May 2009, at 04:10, Liam Nestor wrote:
> Dear FSl users
>
> I have compared two independent groups of subjects (drug users versus
> healthy controls) in FSL with respect to BOLD activity differences.
> I have
> found a number of clusters where the drug user group shows higher
> activity.
>
> The drug user group also scores higher on a psychiatric measure. I
> wish test
> whether this psychiatric measure might explain these BOLD activity
> group
> differences
>
> I read a previous message on a two-grouped analysis using an
> additional
> co-variate, but this query didn't appear to be fully resolved.
>
> My questions:
>
> First, is the best solution to follow the link below, and simply
> enter in my
> drug user group fMRI data in EV1 and their demeaned psychiatric
> score data
> in EV2, to simply see where there might be a correlation between
> these two
> EVs in regions where I have observed group differences?
>
> http://www.fmrib.ox.ac.uk/fsl/feat5/detail.html#SingleGroupAveragewithAdditionalCovariate
Yes - that's right.
> Second, how do I interpret the result? Look at the EV2 map and hope
> not to
> see a cluster in areas where I observed group differences?
This is more tricky, with no clear general answer. IF you could
argue that the psychiatric measure varied linearly with BOLD, and IF
you had a meaningful 'zero' on the scale that corresponded with
normals, you could use a different model that included the normals as
well, and the additional covariate, with a different approach to
demeaning etc... but it's probably not worth going into the details of
that given that these IFs are very big!
Cheers.
>
> Many Thanks,
>
> Liam.
>
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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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