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
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 ?
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
> This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED
> HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is
> intended only for the use of the individual or entity named above. If the
> reader of the e-mail is not the intended recipient or the employee or agent
> responsible for delivering it to the intended recipient, you are hereby
> notified that you are in possession of confidential and privileged
> information. Any unauthorized use, disclosure, copying or the taking of any
> action in reliance on the contents of this information is strictly
> prohibited and may be unlawful. If you have received this e-mail
> unintentionally, please immediately notify the sender via telephone at (773)
> 406-2464 or email.
> On Tue, Nov 29, 2011 at 10:22 AM, Marjorie Dole <[log in to unmask]>
>> 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,