dear listmembers,
I have a number of [naive] questions about dynamic causal modelling. any
input would be very much appreciated.
1) are there obvious advantages/disadvantages to different types of design
(event, blocked) and trial parameters (ISI, ITI) in DCM analyses, other
than having a rapid TR?
2) It seems from Ollie Hulme & Barrie Roulston's powerpoint presentation
(very useful, thanks: www.fil.ion.ucl.ac.uk/spm/doc/mfd/dcm_practical.ppt) that
it's often best to use a different design matrix for DCM analysis than you
might use for a normal SPM analysis. in the example they give, a 2x2
factorial motion/no motion x attention/no attention is reduced to 3
regressors: no motion, motion and attention. why is this?
3) this has been asked before, but there seem to be conflicting accounts
about how to deal with it. What is the best way to use DCM for random
effects analysis? Would it be reasonable to exactly match VOIs across
subjects/sessions by using an absurd P threshold (for example, p<1) such
that exactly the same VOI could be extracted, by using the activation
cluster from
the group as an inclusive mask? assuming the scans are normalised, would
this be a reasonable approach?
4) I am struggling to conceptualise what is meant when the connectivity
values in the output matrices A,B or C are negative. Does this mean that
an area is exhibiting 'less than zero' modulatory influence on another
region?
5) any thoughts on why would I get a warning: 'Returning NaN for out of
range arguments'
(these seem to occur in the probability matrices when the connectivity is
zero).
6) are there any constraints on how you define your VOIs? for example, if
I have 2 trial types, and my first VOI is centered on the peak voxel for
A>B, and the other is the same for B>A, and another is the peak voxel
responding to A *and* B, am I not introducing artificial correlation into
my timeseries...should I be accounting for this in my model?
7) when I display 1st order kernels, what are 'neuronal responses'? (left
graphs).
thank you very much.
chris
Christopher Summerfield
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