Dear List,
Karl Friston kindly provided some advice about DCM issues which we both
thought may be of interest to others - his replies to our questions are
pasted below.
With best wishes,
Narender Ramnani
-------------------------------------------
Dear Narender,
> We are investigating connectivity between two areas using DCM. Our
anatomical model simply consisted of
> forward and backward connections between them.
...[Details deleted for brevity]...
> We used random effects Bayesian model selection to distinguish
between a number of
> models (varying the modulating influence of our experimental effect
on each connection, and varying the location of the input).
...[Details deleted for brevity]...
>
> (1) Is our model comparison approach appropriate?
Yes - it is compelling and the model space is conceptually nice.
The only point I would make is that you appear to have used a random-effects
inference over subjects. This means that a priori, you expect each
subject could
have a different architecture. Usually, in straightforward systems
neuroscience
studies, one assumes that all subjects have the same basic architecture
(but different parameters). This means the fixed-effect pooling of log
evidence
is more appropriate and usually gives more significant results.
> (2) We are interested in whether or not we can say that modulations
> represent increased or decreased connectivity. Are we able to make
> inferences about this from the contents of matrix B (e.g. some values in
> some subjects are negative)?
Yes, exactly. The best way to report these is to report the % increase or
decrease in the fixed connectivity (A) implied by the condition specific
effect
(B). This means commuting (100*DCM.Ep.B{i}(j,k)/DCM.EpA(j,k)) for each
connection (j,k) and condition (i). (this is for fMRI, in M/EEG the
parameters are
already gain parameters). For group results you can use the group average,
where each subject's estimate is weighted by its precision.
I hope this helps,
Karl
--
Narender Ramnani
Reader in Cognitive Neuroscience
Cognitive Neuroscience Laboratory
Department of Psychology
Royal Holloway University of London
Egham, Surrey TW20 0EX
Tel: 01784 443519 (Direct)
Fax: 01784 434347 (Departmental)
email: [log in to unmask]
www.pc.rhul.ac.uk/staff/n.ramnani
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