Hi Donald,
There is something I do not get with these contrasts.
Imagine a voxel "x" with parameter estimates of:
Condition A = .5
Condition B = .5
Condition C = 0.
You want to evaluate if this voxel show preferential activity for a
specific condition.
Obviously this voxel do not show a selective preference for task A.
But if you use the contrast 2 -1 -1, will it give you some "activity"
in this voxel and then wrongly suggest some selective preference for
task A, no ?
Would it not be more adequate to do 1 -1 0 conjonction(and) 1 0 -1 ?
Thanks,
Olivier.
2011/11/29 MCLAREN, Donald <[log in to unmask]>:
> For within-subject designs, you should use the flexible factorial design and
> include a subject term to properly account for subject effects.
>
> The contrasts are correct.
>
> Best Regards, Donald McLaren
> =================
> D.G. McLaren, Ph.D.
> Postdoctoral Research Fellow, GRECC, Bedford VA
> Research Fellow, Department of Neurology, Massachusetts General Hospital and
> Harvard Medical School
> Office: (773) 406-2464
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> On Tue, Nov 29, 2011 at 10:22 AM, Marjorie Dole <[log in to unmask]>
> wrote:
>>
>> Dear SPM users,
>>
>> I have a question concerning second level analysis:
>>
>> I have an experimental design with three conditions, A, B, C, and a
>> baseline. I want to evaluate fMRI activations specifically for each
>> condition versus the two others in my whole group, by testing the contrasts
>> : A-(B+C), B-(A+C) and C-(A+B).
>>
>> So I did for my first level analysis the following three contrasts:
>> A-baseline, B-baseline and C-baseline.
>> At the second level I used a full factorial design with one factor
>> (Condition; three levels: A, B, C) and then I contrasted each condition
>> versus the two others using the following contrasts : 2 -1 -1 ; -1 2 -1;
>> -1 -1 2.
>>
>> Is this approach valid?
>> Do you have any suggestion?
>>
>> Thanks for your help,
>>
>> Marjorie
>
>
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