Hi Darren,
Not only does your suggestion make good sense, it indeed turns out to
be the case. The ResMS.img shows nice focal "black holes" of very low
variance/SD in these areas (SD ~1.3).
Thanks for the lead.
-David
> -----Original Message-----
> From: [log in to unmask]
> [mailto:[log in to unmask]]
> Sent: Saturday, June 18, 2005 3:42 PM
> To: Kareken, David A.
> Cc: [log in to unmask]
> Subject: Re: [SPM] Random vs Fixed Effects discrepancy in fMRI
>
>
> Hi David:
>
> This sounds like a case in which, for unclear reasons, the
> intersubject variance is less than intrasubject variance.
> Thus despite possibly small parameter effects they are
> consistent across subjects. I suggest looking at the variance
> images in the random effects data (ResMS.img files). Using
> imcalc with a function of sqrt(i1) will give you standard
> deviation. Also look at these images for some fixed effects
> data. Plot the "time series" (actually subject
> series) for some voxels using the RanFX analysis in the area
> of activation. Presumably you'll see consistent effect sizes
> across subjects.
>
> Darren
>
> Quoting "Kareken, David A." <[log in to unmask]>:
>
> > Hi all,
> >
> > I wonder if someone has insights into the following discrepancy
> > between a random (RFX) and fixed effect (FFX) result in fMRI. The
> > essential problem is that there is a much stronger RANDOM effects
> > result in selected areas in which not one single subject shows
> > activation in their single-subject, fixed effect analysis.
> There is
> > huge "activation" spanning the lateral ventricles, and an uncinate
> > focus (all < 0.001), but NONE of the single subject FFX
> analyses show
> > anything like this at < 0.01.
> >
> > I would certainly expect much stronger FFX signals than RFX
> signals,
> > but have never seen the opposite-- and wouldn't expect any.
> >
> > Some quick details are:
> > n= 7 subects (yes, I know, quite small, but not a final sample).
> > Blocked design. FFX analyses with corrections for serial
> correlation.
> > The FFX results are a somewhat complicated interaction consisting of
> > conditions
> >
> > 1/1 1/2 1/1 1/2 2/1 2/2 2/1 2/2
> >
> > where the number to left of the ''/" is a drug condition (drug v
> > placebo) and the number to the right of '/' is a task (1 or 2).
> >
> > The contrast images analyzed consist of the drug x task
> interaction: 1
> > -1 -1 1 -1 1 1 -1.
> >
> > Thanks for any help,
> > David K
> >
> > David A. Kareken, Ph.D., ABPP/ABCN
> > Board Certified Neuropsychologist
> > Associate Professor & Director of Neuropsychology
> > Department of Neurology (RI-1773)
> > Indiana University School of Medicine
> > Indianapolis, IN 46202
> > Tel: 317 274-7327
> > Fax: 317 274-1337
> >
> >
> >
>
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