George,
Unfortunately I do not understand your flirt question (please elaborate),
but as for your other questions:
> 1)
> In the Oxford Functional MRI book p 258 there is a formula for the
F-statist$
> makes sense; however, I'm unclear what the distribution is. That is,
what ar$
> numerator and denominator degrees of freedom (it depends on both the
number of
> contrast vectors, and length of beta vector, true?)
if:
length of beta vector is P
number of contrast vectors is K
length of time points is T
then the f-stat is distributed with degrees of freedom K (numerator) and
(T-P) (denominator). Using (T-P) assumes known autocorrelation with
prewhitening, or that there is no autocorrelation.
> 2)
> In the FEAT GUI under the Post-stats tab there is a field "Z threshold"
and a$
> "Cluster P threshold"
>
> is "Z threshold" for individual voxels? (it doesn't make sense to be for
clus$
> is a separate, unlinked, field for clusters, that is, "Cluster P
threshold")
Z threshold is used voxelwise to produce the clusters.
Cheers, Mark.
Mark Woolrich.
Oxford University Centre for Functional MRI of the Brain (FMRIB),
John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK.
Work: +44-(0)-1865-222713, Mobile: +44-(0)-7808-727745
Email: [log in to unmask]
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