Dear Dhananjay,
I don't think you can necessarily infer that with
certainty. Although it is true that you need SPM activations for all
regions of the DCM in the first place, they do not necessarily have
to be super-strong. Imagine the following simple model:
STIM --> Area 1 ---a21--> Area 2
A moderately or even weak response of Area 1 to a direct input STIM
could be associated with a strong coupling estimate of its afferent
intrinsic connection (a21) provided that the responses of the target
region Area 2 are tightly coupled to the first area. In other words:
two weak regional responses that are, however, tightly coupled. But
this scenario can probably only be detected under conditions of good SNR.
Best wishes,
Klaas
At 01:59 09/02/2006, you wrote:
>Hi,
>This is a decription of my experiment setup. If I could get some
>pointers about my DCM work it will be great.
>
>ROIs were selected based on group activation maps during the
>performance of a task. Using modification indices a SEM for this
>group was built. This SEM had good model fit (chi^2=0.003, p =
>0.955). The connections derived for this model were used to test
>effective connectivity with DCM on each individual. The task was
>additionally included as an external influence affecting two ROIs.
>The DCM results showed very little intrinsic connectivity between
>these ROIs (<0.02) with low probabilities (<0.05). No bilinear
>interactions were included in the model. Close examination of the
>activation maps for each individual showed that high task activation
>was not present for each individual at the ROIs chosen.
>
>Is it correct to infer that low connectivity was seen with DCM as
>strong task activation is necessary for connectivity? As I
>understand it intrinsic connectivity is a results from the responses
>at the ROIs and low activation will not result in high connectivity.
>
>Regards,
>
>Dhananjay
_____________________________________
Dr Klaas Enno Stephan
Senior Research Fellow
Wellcome Dept. of Imaging Neuroscience
12 Queen Square, WC1N 3BG, London, UK
phone: +44-207-8337481
fax: +44-207-8131420
web: http://www.fil.ion.ucl.ac.uk/~kstephan/
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