Hi,
On 28 Oct 2006, at 21:31, Michael wrote:
>> Next, the most likely difference is that fstats don't distinguish
>> between negative and postitive activation - both show up as
>> "positive" - this is the nature of fstats. Hence your zstats only
>> show positive, the fstats show both.
>
> So my fstats setup was correct then?
Yes - if you want to ask the question "where does any linear
combination of the 3 EVs produce a significant response?"
> And why does the contrast setup:
>
>>> EV1 EV2 EV3 F1 F2 F3
>>> OC1 * 1 0 0 * 0 0
>>> OC2 * 0 1 0 0 * 0
>>> OC3 * 0 0 1 0 0 *
>
> only represent or detect positive activation ONLY as opposed to the
> F-test
> which detects both positive and negative activation? I would think
> they
> would do the same thing, no?
No - it just comes down to the definition of the fstat - it's
basically asking how much variance is explained by the space spanned
by the EVs included in the fstat - kind of like a sum-of-squares of
the individual effects - it is always positive. Whereas simple
contrasts, generating tstats, are always signed.
Note that you can always restrict the fstat reporting to areas where
one or more of the individual contrasts are positive, using the post-
contrast masking button in FEAT.
Cheers.
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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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