Hi Ged,
On 24 Oct 2007, at 16:00, DRC SPM wrote:
> Hi,
>
> I think the set notation and Venn diagrams are unhelpful here... when
> people talk about A>B they mean a contrast with null hypothesis A-B =
> 0, and a right-tailed (single-tailed) alternative hypothesis. This
> might sound like pedantry, but with more complex contrasts, I think it
> clarifies things. [1 -1 -1 1] has null hypothesis Al-Ar+Br-Bl = 0,
> with right-tailed alternative. Which could be expressed as Al-Ar >
> Bl-Br or Al+Br > Ar+Bl, or various alternatives. Now, you can see that
> several things could happen to reject the null hypothesis, for
> example, Al could be *less* than Ar and Bl less than Br (by a larger
> amount). In other words, you are not testing ((Al - Ar) > (Bl - Br)) &
> (Bl > Br), where the & is the crucial bit. The contrast vector is used
> as a whole, and the result compared with zero, so there is no question
> of "ordering" of any "component" contrasts.
>
> You can mask the results of the [1 -1 -1 1] contrast by the sign of a
> simpler contrast, for example [0 0 1 -1], to ensure that Bl > Br and
> (Al-Ar) > (Bl-Br). SPM has some kind of contrast-masking buttons you
> can push; I don't know if there is anything similar in FEAT, but you
> could use fslmaths with T or Z images to reproduce the effect, I
> think.
Well, you should just look at the FEAT GUI then - yes these options
are in there ;-)
>
> I hope that helps,
> Ged.
>
>
> On 24/10/2007, Jose Paulo Santos <[log in to unmask]> wrote:
>> Hello,
>>
>> I'm a bit confused with the problem below.
>>
>> A graphical visualization could be as in Fig1. But if the question is
>> (Al>Ar) > (Bl>Br), the contrast should be:
>>
>> [ 1 -1 0 0 ] - [ 0 0 1 -1 ] = [ 1 -1 -1 1 ]
>>
>> which is the same!? How can it be? Graphically (Fig2), the answers
>> are
>> similar, but not equal. It seems that the order of the operations
>> matters.
>> In this case, at the third level, it should be processed the
>> contrasts
>> that are in the parentheses and then, at the fourth level, the
>> overall
>> question.
>>
>> How does FSL computes the contrast [ 1 -1 -1 1 ]? Linearly or by
>> any order
>> (Fig3)?
>>
>> Kind regards,
>> Jose Paulo Santos
>>
>> On Mon, 30 Jul 2007 09:26:07 +0100, Steve Smith
>> <[log in to unmask]>
>> wrote:
>>
>>> Hi - I'm pretty sure you can do this at the third level. If your 3rd
>>> level EVs are:
>>>
>>>>>> Al > bl, Ar > bl, Bl > bl and Br > bl
>>>
>>> Then do ask
>>>
>>>>>> (Al>Bl) > (Ar>Br)
>>>
>>> At the third level you just want the contrast:
>>>
>>> [ 1 0 -1 0 ] - [ 0 1 0 -1 ] = [ 1 -1 -1 1 ]
>>>
>>> which is a typical interaction contrast. Note that you may want to
>>> use contrast masking to ensure that (e.g.) Al>Bl and Ar>Br (etc) to
>>> help the interpretation of the interaction.
>>>
>>> Cheers, Steve.
>>>
>>>
>>>
>>> On 26 Jul 2007, at 18:16, Stephane Jacobs wrote:
>>>
>>>> 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
>>>>> ------------------------------------------------------------------
>>>>> ---
>>>>> ------
>>>>>
>>>>>
>>>>>
>>>
>>>
>>> --------------------------------------------------------------------
>>> ----
>>> ---
>>> 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
>>> --------------------------------------------------------------------
>>> ----
>>> ---
>>> ====================================================================
>>> =====
>>
>>
>>
------------------------------------------------------------------------
---
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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