Thank you, as always!
As it turns out I have run a similar model before I sent you that
email, but only for one group of subjects (so basically half of the
model below) with the same contrasts 1 and 2. I have noticed however,
that the results from that model differ (not very much but they do
differ) from results of the model below, (the model with both groups,
corresponding contrasts). Why is that? I would expect the model below
to be the same as doing two separate analysis (for each group) followed
by another (third-level) analysis to find differences between groups?.
Thanks!
On Thursday, March 20, 2003, at 12:52 PM, Stephen Smith wrote:
> Hi Zrinka,
>
> Your design matrix is not quite right. The most obvious problem, apart
> from it not achieving the modelling that you want, is that you have as
> many EVs as data points, so all your variances will be zero!
>
> I suspect that you are effectively wanting to do paired t-test
> comparisons. have a look at:
> http://www.fmrib.ox.ac.uk/fsl/feat5/
> detail.html#PairedTwoGroupDifference
>
> so you probably want a slightly more complex version:
>
> group EV1 EV2 EV3 EV4 EV5 ......... EV23
> S1pre 1 1 0 1 0 0 ......... 0
> S1post 1 -1 0 1 0 0 ......... 0
> S2pre 1 1 0 0 1 0 ......... 0
> S2post 1 -1 0 0 1 0 ......... 0
> ...
> S21pre 2 0 1 0 0 0 ......... 1
> S21post 2 0 -1 0 0 0 ......... 1
>
>
> con1 1 0 0 0 0 ......... 0 = group1 pre-post
> con2 -1 0 0 0 0 ......... 0 = group1 post-pre
> con3 0 1 0 0 0 ......... 0 = group2 pre-post
> con4 0 -1 0 0 0 ......... 0 = group2 post-pre
> con5 1 -1 0 0 0 ......... 0
> con6 -1 1 0 0 0 ......... 0
>
> con5 = group1(pre-post)-group2(pre-post) OR can alternatively be seen
> as
> group2(post-pre)-group1(post-pre); you can disambiguate these choices
> by
> using contrast masking.
>
> Good luck!
> Steve.
>
>
>
> On Wed, 19 Mar 2003, Zrinka Bilusic wrote:
>
>> Hello to everyone... I have tried running a second-level
>> across-session
>> analysis in FEAT yesterday, and have run into some problems... The
>> model
>> is as follows: I have 21 subjects in two groups (10 in one and 11 in
>> the
>> other group) that were scanned twice with the same stimuli but under
>> different conditions, pre and post. I want to see whether there are
>> differences between the sessions for each subject and then eventually
>> across subjects (third-level analysis). So, in the end, I have 42 EVs
>> and 21 contrasts:
>>
>> group EV1 EV2 EV3 EV4
>> EV41 EV42
>> 1. subject1 pre 1 1 0 0 0.....
>> 0 0
>> 2. subject1 post 1 0 1 0 0.....
>> 0 0
>> 3. subject2 pre 1 0 0 1 0....
>> 0 0
>> 4. subject2 post 1 0 0 0 1.....
>> 0 0
>> .....
>> 41.subjec21 pre 2 0 0 0 0.....
>> 1 0
>> 42.subject21 post 2 0 0 0 0.....
>> 0 1
>>
>> contrasts:
>> EV1 EV2 EV3 EV4...
>> EV41 EV42
>> 1. subject1 1 -1 0 0....
>> 0 0
>> 2. subject2 0 0 1 -1...
>> 0 0
>> ...
>> 21. subject 0 0 0 0...
>> 1 -1
>>
>>
>> I have run first-level anlysis on all these before, and there were 2
>> EV for each subject/session.
>
>
>
>
> Stephen M. Smith MA DPhil CEng MIEE
> Associate Director, FMRIB and Analysis Research Coordinator
>
> Oxford University Centre for Functional MRI of the Brain
> John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK
> +44 (0) 1865 222726 (fax 222717)
>
> [log in to unmask] http://www.fmrib.ox.ac.uk/~steve
>
>
Zrinka Bilusic-Vezmar
UCLA Brain Mapping
660 Charles Young Drive South
Los Angeles, CA 90095
310-794-5060
[log in to unmask]
www.brainmapping.org
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