Hi Laura,
could you please clarify what your regressors in your model are (you say you
have 3 conditions and model either hrf or hrf+ time derivatives [incl.
dispersion?], but it appears you always have 6 colums in your design). I
cannot work it out, even should you have put one colum per block (B C A A C
B), which gives six colums, including the derivatives should give you
more...
In principle: onbe [/another] way to look at 'deactivations' is this: change
your perspective and see it as: the set of brain areas that 'deactivates'
during your respective condition is in fact more active at the "off-times"
of your condition.
Maybe this is a start?
Helmut
----- Original Message -----
From: "Laura Mancini" <[log in to unmask]>
Sent: Thursday, June 30, 2005 6:52 PM
Subject: deactivation
> Dear SPMers,
>
> I wonder if you can help soving a puzzle.
>
> I have 6 fmri acquisition, two for conditions A versus rest, two for B
> versus rest, two for C versus rest.
> The order of the acquisitions is: B C A A C B
> A = only hand movement
> B = only speech production
> C = hand mov + speech prod
> I did only t-tests, not F-tests.
> If as basis set for the SPM analysis I consider only the hrf I have the
> following.
> When I consider the contrast [0 0 1 1 0 0] I see a huge activation in the
> motor cortex, while with [0 0 -1 -1 0 0] there is little or no
> (de)activation
> When I consider [1 0 0 0 0 1] or [0 1 0 0 1 0], I see little activation in
> some parts of the brain, while with [-1 0 0 0 0 -1] or [0 -1 0 0 -1 0] I
> see
> large (de)activation almost everywhere in the brain.
>
> If as basis set for the SPM analysis I consider only the hrf+time
> derivatives I have the following.
> When I consider the contrast [0 0 1 1 0 0] I see much less activation in
> the
> motor cortex, while with [0 0 -1 -1 0 0] there is maybe more
> (de)activation
> When I consider [1 0 0 0 0 1] or [0 1 0 0 1 0], I see much more activation
> in some parts of the brain, and with [-1 0 0 0 0 -1] or [0 -1 0 0 -1 0] I
> see less (de)activation.
>
> I am quite puzzled. Has anybody find something similar?
> Is it more likely that there was a problem during the acquisition or the
> analysis of the data, or that it is a real behaviour of the brain?
>
> Many thanks,
> Laura
>
>
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