Hi Martin,
On 31 Oct 2006, at 20:35, Martin M. Monti wrote:
> Dear Lords of FSL,
>
> I have a simple question re the contents of the PE and COPE images.
>
> At a first level (single sub - single run) PE1 is a matrix
> containing the GLM parameter estimate (b) for each voxel for EV1.
> If I created a contrast with (say) a 1 in EV1 and zeros in all
> other EVs I would then get COPE1 to be in fact the same thing as
> PE1 except it's a beta as opposed to a non standardized estimate
> (i.e. b). [the betas are what I need..]
>
> So far so good (right? ..I think we previously discussed this
> already..)
Not quite - in this case (contrast [1 0 0...]) then COPE1 is
_exactly_ PE1 - neither is standardised in any way at this point. PE1
is the beta, and for this contrast COPE1=PE1.
I'm not sure what distinction you're making between b and beta. In
the GLM:
Y=Xb+e
Then b (ie beta) is the parameter estimates.
> Now, say that I have multiple runs and multiple subjects, and all I
> do is average (i.e. no subtractions) EV1 across multiple sessions
> (for each given subject) and across multiple subjects.
> This group analysis will yield again a PE1 and a COPE1 (obtained
> again just by creating a contrast with 1 in all EV1s and 0 for all
> other EVs).
>
> Are the EP1 and COPE1 matrices just (group) average bs and (group)
> average betas, respectively?
Apart from the fact that the "bs" _are_ the betas, then yes, the rest
is correct.
Cheers, Steve.
>
> cheers
> Martin
>
>
> --
>
>
>
>
> --------------------------------------------
> "Upward, not Northward!" [Edwin Abbott - FlatLand]
> I became insane, with long intervals of horrible sanity. [E.A. Poe]
> Would you tell me, please, which way I ought to go from here?
> ''That depends a good deal on where you want to get to.'' Said the
> Cat. I don't much care where -- Said Alice. Then it doesn't matter
> which way you go, said the Cat. [Lewis Carroll]
> Che si trattasse di una gara? ..Un colpo il mondo e un colpo tu, a
> chi stupiva un po' di piu'.. [Per Un Vecchio Bambino - Roberto
> Vecchioni]
> --------------------------------------------
> Martin M. Monti
> Princeton University
> Department of Psychology
> Green Hall 3-S-8
> (609) 258-5679
> --------------------------------------------
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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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