Thank you for the information, Steve.
I'll read these papers.
-Yoshiko
On Fri, 30 Nov 2007 09:17:27 +0000, Steve Smith <[log in to unmask]> wrote:
> Hi,
>
> On 29 Nov 2007, at 20:08, Yoshiko Yamada wrote:
>
> > Dear List,
> >
> > At the last FSL course, it was recommended that we use a fixed
> > effects analysis at the mid level
> > (i.e., single-subject multiple-session) when fewer than 10 sessions
> > had been collected for each
> > subject. I have questions regarding this recommendation.
> >
> > Our data sets consist of 14 subjects with 2-4 runs each (done in a
> > single session). (These data
> > were obtained from young children, and some of the runs have been
> > excluded from the analysis
> > due to excessive head motion.) We are interested in making
> > inferences about the population from
> > which 14 samples were drawn. Thus, my understanding is that you
> > recommend using a fixed
> > effects analysis on the single-subject multiple-run level and a
> > mixed effects analysis on the
> > multiple-subject/group level. (Please correct me if Im wrong.)
>
> That's right.
>
> > When I compared the outputs of the fixed effects and mixed effects
> > analyses for each subject
> > (combining multiple runs), I see greater extent of activations, and
> > this conforms with what I have
> > reading (e.g., Friston et al., 1999), namely that a fixed effects
> > analysis is generally more sensitive
> > to activations than a random effects analysis.
> >
> > Subsequently, I performed two group-level analyses using a mixed
> > effects analysis (FLAME 1); one
> > analysis had mid-level fixed effects stats as the input (FE-ME), and
> > the other had mid-level mixed
> > effects stats as the input (ME-ME). Contrary to my expectation, some
> > significant activations
> > present in the ME-ME analysis are less robust in the FE-ME analysis.
> >
> > Why should using a mixed effects analysis at the mid level be more
> > sensitive to activations (at
> > least in some areas) than using a mid-level fixed effects analysis?
> > If we have a reason to suspect a
> > large run-to-run variability, would it be better to use a mixed
> > effects analysis at the mid level, or
> > do you think that run-to-run variance cannot be estimated well with
> > a small number of runs we
> > have in our data?
>
> It could go either way. Changing the variance model at the second-
> level means that different subject-wise variances are passed up to the
> third level. This means that the different subjects will be weighted
> (relative to each other) differently: In the 2nd=mixed approach, the
> weighting will be largely driven by the subject's cross-session
> variability. In the2nd=fixed approach, the weighting will be driven by
> the ('average' of) the first-level variabilities.
>
> Therefore it's not quite right to say that one approach leads to more
> or less 'sensitive' results at the 3rd level, just that they are
> asking different questions. For the reasons outlined in the latest
> version of the manuals (and on the course), we think that 2nd=fixed is
> the most sensible model, and so would recommend that.
>
> For more details on these issues, see the course lecture notes, and
> also the papers:
>
> M.W. Woolrich, T.E.J. Behrens, C.F. Beckmann, M. Jenkinson, and S.M.
> Smith.
> Multi-level linear modelling for FMRI group analysis using Bayesian
> inference.
> NeuroImage, 21(4):1732-1747, 2004.
>
> and
>
> C.F. Beckmann, M. Jenkinson, and S.M. Smith.
> General multi-level linear modelling for group analysis in FMRI.
> NeuroImage, 20:1052-1063, 2003.
>
> Cheers.
>
>
> > Also, Id like to have better understanding of how these two
> > different types of analyses are
> > performed in FEAT, and how the stats of these analyses are carried
> > to the higher-level analysis.
> > Are there any documents or references on these you can point to me?
> >
> > Thank you in advance for your advice!
> >
> > -Yoshiko
>
>
> ---------------------------------------------------------------------------
> Stephen M. Smith, Professor of Biomedical Engineering
> Associate Director, Oxford University FMRIB Centre
>
> FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
> +44 (0) 1865 222726 (fax 222717)
> [log in to unmask] http://www.fmrib.ox.ac.uk/~steve
> ---------------------------------------------------------------------------
>
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