Hi, there could be several reasons for this.
Firstly - it's probably just a typo, but did you mean for contrast 3
to be [0 0 1] ?
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.
More subtle: if for example your regressors are somewhat correlated,
then each individual contrast may be insensitive, but the f-test
still could have greater sensitivity.
Hope this helps - cheers, Steve.
On 27 Oct 2006, at 21:36, Michael wrote:
> Hello,
>
> I have been running some analyses and I have some questions about F-
> tests
>
> and contrasts.
>
> When running FEAT on individual subjects in first-level analysis,
> clickin
> g
> on the stats tab, and selecting the Full-model setup, there is a
> section
>
> for setting up F-tests and contrasts.
>
> In my experiement I have 3 tasks of interests, i.e., 3 EV's in my
> GLM mod
> el.
>
> So I set up my contrasts and F-tests as such:
>
> EV1 EV2 EV3 F1 F2 F3
> OC1 * 1 0 0 * 0 0
> OC2 * 0 1 0 0 * 0
> OC3 * 1 0 0 0 0 *
>
> (* indicating yellow clicked box)
>
> The trouble I'm finding is that when I look at the resulting
> activation
>
> maps for zstat and zfstat maps, I see two completely different
> pictures.
> I
> don't understand why. For the zstat activation map, I barely see any
> activation, but for the zfstat activation I see several clusters of
> voxel
> s
> in ROI's we typically see from our experiments. Is there something I'm
>
> doing wrong? Then when I take the individuatl subjects and make a
> group m
> ap
> of a one-sample t-test and I include and F-test with it, I get the
> same
>
> picture:
>
> hardly any activation in the zstat group map, but I do see
> activation in
>
> the several ROI's in the zfstat map.
>
> What is going on? I would think I would see the same, if not similar
> results. Is there something wrong with my contrast and F-test set-
> up at t
> he
> first-level analysis?
>
> Your help would greatly be appreciated
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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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