Hi - yes, this makes sense. Regardless of the original contrast
setting, the contrast output in significant voxels will be positive.
Eg if the contrast is [-1], this is multiplied by the PE, and for the
significant voxels, this will be where PE is strongly negative, ie
the COPE value will be strongly positive after multiplying the PE by
the contrast.
Cheers, Steve.
On 17 Sep 2007, at 00:35, Matthew Hoptman wrote:
> Hi,
>
> I am trying to extract regression parameters from a second level
> analysis. In each first level analysis I ran (to greatly simplify) a
> positive (+1; cope1) and negative (-1; cope2) contrast on the
> variable of
> interest (in this case a seed in a resting state analysis). The
> goal is
> to examine "anticorrelated" resting state networks (a la Snyder et
> al.,
> 2005) in my data.
>
> I've extracted the regression parameters for each subject using:
>
> fslmaths -i filtered_func_data -m <cluster_mask_zstat> -o outputfile
>
> on the higher level analysis in cope1.feat. When I do the same
> thing in
> cope2.feat, I still get positive values.
>
> Thus, I'm getting positive values for the regression coefficients
> regardless of the direction of the contrast. Can I simply flip the
> sign
> for the regression parameters on the negative contrast?
>
> Thanks,
> Matt
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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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