Steve,
I just now realized that when saying PE vs COPE I in fact meant (CO)PE
vs ZSTAT.
Just to make sure I understand, zstat is COPE-mean(COPE)/sqrt(VARCOPE)?
my point is that I compare patterns of activations across different EVs,
and some are collected in separate runs (with a run being machine ON to
machine OFF), hence comparing measures that do not account for
differences in signal mean and variability across runs I may be getting
non-interpretable results.
cheers
martin
Steve Smith wrote:
> 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
>> --------------------------------------------
>
>
> ---------------------------------------------------------------------------
>
> 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
> ---------------------------------------------------------------------------
>
>
--
--------------------------------------------
"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
www.webmartin.net
--------------------------------------------
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