Hi Laurence:
> -----Original Message-----
> From: SPM (Statistical Parametric Mapping)
> [mailto:[log in to unmask]] On Behalf Of <Laurence> <Wang>
> Sent: Thursday, December 27, 2007 10:56 AM
> To: [log in to unmask]
> Subject: Re: [SPM] DCM with multiple sessions per subject
>
> Hi Dr. Stephan and SPMer,
>
> My further question on this issue is that how to characterize
> the individual's coupling parameters, if I have a cohort of
> subjects and each subject has multiple seesions and now I try
> to do the correlation analysis between the coupling
> parameters derived from the DCM and behavioral data.
>
> Actually, I have analysed the data each session separately,
> however I am not sure what is the appropriate way to produce
> the model parameters representing the individual subject's
> property. Could I just average the model (identical model
> structure) across the sessions in each subject and consider
> the parameters from the averaged model could reflect the
> individual's characteristics? or just calculate the
> arithmetic mean of the parameters across session-specific
> models in each subject?
I am not sure what you are averaging at the model level. If it is the
parameters then you've answered your second question. You can average the
parameters across sessions for identical models.
>
> I read through the posting and it seems like that you experts
> thought it is better to concatenate the data from different
> sessions so that it can generate a subject-specific model
> under such a condition. Am I correct?
I think the reasons that concatenation has been recommended are
- you get one set of DCM parameters per model rather than separate sets for
each session
- it is more likely that you will have adequate numbers of trials/blocks
across an entire run vs. in individual sessions. For example, you might have
20 trials of some stimulus across all your sessions, but individual sessions
might only have 2-5 trials of that type. Having only a few trials in a run
might not produce good estimates of the parameters, but I suppose this would
also be true for the standard statistics.
In any case, assuming an adequate number of trials per session there should
be no particular barrier to running the DCM's on individual sessions and
then averaging the parameters.
Hope this helps,
Darren
>
> I would really appreciate your help!
>
> Laurence
>
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