On Mon, 13 Jan 2003 15:57:12 +0000, Stephen Smith <[log in to unmask]>
wrote:
Dear Dr. Smith,
thanks for your fast response. So the problem with different TRs has
nothing to do with unbalanced designs. Am i right in assuming that scaling
all scans from all subjects to a commom mean is the critical point, so if
scaling is used any mixed (random) effects model would be valid?
So i couldnīt analyse the data from all subjects in one first level model
(fixed effects model, not that i want to do that)but in any second level
(mixed effects)?
Just a second short question, i have talked to the people who did this
study and it contains two groups, controls and patients (parkison disease)
and the TR seems to be dependent on brain volume in this particular
sequence. Could i run into any problem if the TRs of the both groups are
significantly different (which i donīt know, but will check), because if
scaling is the solution, differences in TR beetween groups shouldnīt be any
problem, i assume?
Thank you very much,
Martin
Martin Kronbichler
Institute of Psychology
University of Salzuburg
Hellbrunnerstr. 34
5020 Salzburg
Austria
tel.: 0662/8044-5162
fax.: 0662/8044-5126
e-mail: [log in to unmask]
>Hi - yes, FEAT5 does indeed allow you to analyze unbalanced designs, but
>in fact, that's not really the point here:
>
>The first thing to note is that changing TR from 0.2 to 0.6s probably
>won't have a huge impact anyway on the baseline and activation BOLD
>signal. However, in any case, any effect would largely be a scaling of
>both baseline and activation signal change, so the
>percentage-change-activation would be the same, ie the grand-mean (4D)
>intensity normalisation that FEAT always carries out in the preprocessing
>would rescale the baseline AND the activation signal so that it is
>comparable across subjects.
>
>So - you should hopefully be safe in your analysis.
>
>Thanks, Steve.
>
>
>
>
>
>
> Stephen M. Smith
> Head of Image Analysis, FMRIB
>
> Oxford University Centre for Functional MRI of the Brain
> John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK
> +44 (0) 1865 222726 (fax 222717)
>
> [log in to unmask] http://www.fmrib.ox.ac.uk/~steve
|