Dear Boris:
It is 'legal" and may even be ideal for DCM analysis in the sense that
group differences are not explained by regional responses but rather
by the influences between regions (Sonty et al, J Neurosci, 2007).
One can consider the standard fMRI convolution model as a special case
of DCM in which the effects of coupling between regions is discounted
(Friston, Neuroimage, 2003). In a sense the standard method of SPM
analysis would be like only including direct effects on each region
and observing the responses. The result you are getting suggests that
regions are responding in similar ways between the two groups, but
that the reason for those responses may differ because of connectivity
influences, e.g., each group may use different connections or weight
the connections differentially to achieve the same activations. (I am
using the term connections in the sense of influences between regions
rather than strict anatomic connectivity.)
-----
Darren Gitelman
On Fri, Oct 17, 2008 at 4:57 AM, boris suchan
<[log in to unmask]> wrote:
> Hi all,
>
> I have a short question about the condition for a dcm analysis.
>
> I have 2 groups patients and normals which do not show any activation
> differences in a "normal analysis".
>
> Using dcm, I find some differences which makes sense and verify my
> hypothesis. Is this legal even if the results from the two sample activation
> t-test do not show any differences?
>
> Many thanks
>
>
>
> boris
>
>
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