Dear Chris,
I think your approach is worth a try. I can't see any major problems with it.
Best wishes,
Klaas
At 14:32 06/11/2006, Christopher Summerfield wrote:
>hi Klaas, sorry to bother you.
>I sent this to the list last week, but didn't get a reply. I
>know you're busy...but any chance you could briefly let me know whether
>DCM on the unsmoothed data is a good idea or not?
>many thanks - Chris
>
>Christopher Summerfield
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>
>---------- Forwarded message ----------
>Date: Tue, 31 Oct 2006 08:25:25 -0500 (EST)
>From: Christopher Summerfield <[log in to unmask]>
>To: [log in to unmask]
>Subject: 2-step spatial smoothing in DCM
>
>Dear DCM experts:
>
>a while back there was a discussion on the list about
>2-step spatial smoothing (smoothing first the raw images then the con*
>images), the advantage being that for single-subject analyses one
>could use the
>first step to smooth just enough to implement random field theory, and the
>second step would bring smoothing up to a reasonable level for group
>comparisons.
>
>Given that DCM is typically carried out on the timeseries individually for
>each subject, presumably there might be an argument for using this
>2-step procedure in DCM? My question is this. For a given peak in your
>(nice,smoothed) GLM analysis, one would presumably want to use individual
>subject peaks which fell closest to this RFX peak to extract data
>for entry into
>the DCM. Therefore: you don't actually need the individual peaks to pass
>any special threshold - just to be the highest peak in the
>neighbourhood. so....is there any reason why the DCM analysis should
>not be run on completely unsmoothed data? could one not simply define
>peaks of interest with a smoothed GLM analysis (as normal), go
>back, run the model on the unsmoothed data, and extract the timeseries
>from this unsmoothed analysis?
>
>I am interested in eliminating as much smoothing as possible, because my
>VOIs are reasonably close together (~20mm), and intrinsic connectivity
>(under normal smoothing: 8 x 8 x8) is extremely high (p< 1 x 10^-12 in
>some cases). I am wondering if it is
>possible for instrinsic connectivity to hit a sort of ceiling, such that
>there is no room for improvement (giving consequently unimpressive
>bilinear terms).
>
>Many thanks,
>Chris
>
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>Christopher Summerfield
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