Hi Steve,
Well, I don't think it is possible with the way my analysis has been run
up to the 3rd level. If I take the example of the interaction (Al>Ar) >
(Bl-Br), I get both contrasts Al>Ar and Bl>Br only at the output of the
3rd level analysis, so I don't see how I could contrast them one against
another within the same analysis...
Should I set the differential contrasts (e.g. Al>Ar, etc...) as soon as
the 2nd level, for each subject? Then I guess I could set my interaction
contrasts at the 3rd level...
Do you see any reason why this should fail at the 4th level?
Thanks again for your help,
Stephane
Steve Smith wrote:
> Hi - it sounds like you should be finding this interaction as
> additional contrasts at the 3rd level, not via a 4th level analysis -
> does that sound possible?
> Cheers.
>
>
> On 25 Jul 2007, at 23:54, Stephane Jacobs wrote:
>
>> Hello,
>>
>> I'm trying to set up interaction contrasts in a 4th level-analysis. I
>> have 2
>> conditions (A and B) x 2 sides (l and r) design, and I'd like to
>> analyze the
>> interaction between the factors SIDE and CONDITION.
>> The second level analysis is run for each subject separately to model
>> between run variance, and outputs each of the 4 conditions vs. resting
>> baseline (bl): Al > bl, Ar > bl, Bl > bl and Br > bl.
>> The third level analysis models between-subjects variance, and is
>> basically
>> a "quadrupled" T-test, computing paired-comparisons between my 4
>> conditions:
>> Al > Bl, Ar > Br, Al>Ar, Bl > Br, and the reverse contrasts.
>>
>> Now, I want to take these copes from the 3rd level analysis to test for
>> interactions between SIDE and CONDITION. To this end, I set up the
>> following
>> contrasts in my 4th level analysis:
>>
>> (Al>Bl) > (Ar>Br) to test the difference between both sides for (A > B)
>> (Al>Ar) > (Bl>Br) to test the difference between both conditions for
>> (l > r)
>> etc... I have a total of 8 contrasts set, using 8 EVs coming from the
>> 3rd
>> level analysis.
>>
>>
>> I guess (hope!) all this makes sense so far, but FEAT fails running the
>> analysis when doing higher-level stats:
>>
>> An exception has been thrown
>> Singular design. Number of EVs > number of time points. Trace:
>> Gsmanager::ols; Gsmanager::run.
>>
>>
>> I can't figure out why this is happening... I understand it suggests
>> I don't
>> have enough data to run these contrasts, but it surprises me as I
>> have 20
>> subjects in my group, and quite a few trials per condition. This
>> being said,
>> I'm not quite sure of what the number of time points represents for
>> such a
>> high-level analysis...
>>
>> Any help would be most appreciated!
>>
>> Best regards,
>>
>> Stephane
>
>
> ---------------------------------------------------------------------------
>
> 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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