Dear SPM'ers,
In a post-hoc random analysis of PET data, I am interested to perform a "conjunction" between a contrast [A-B] performed in Group 1 and a contrast [C-D] performed in Group 2. So, I have two different contrasts in two different populations of normal subjects, assuming that [A-B] and [C-D] share in common a cognitive component of interest. All individuals contrasts were computed, and the con*.img were forwarded to the 2nd Ievel, then a one-sample t-test computed (usual procedure). I would be grateful for your comments on the following points :
1) Since [A-B] and [C-D] are computed in different subjects, a single contrast per subject is forwarded to the second level analysis, and the sphericity assumption is not violated. Is this correct ?
2) Is this true that a one-sample t-test on all con*.img (i.e., [A-B] and [C-D]) is equivalent to a conjunction analysis, since the aim of this 2nd level analysis is to highlight the areas where a common activation was detected in individual contrasts?
3) Is there a caveat with the (non) orthogonalization between [A-B] and [C-D] ?
thanks very much
Philippe
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PEIGNEUX Philippe, PhD, Researcher
E-mail: [log in to unmask]
Cyclotron Research Centre
Liege University
Bâtiment B30, Sart Tilman
Allée du 6 août, 8
B-4000 Liege
BELGIUM
Also at Neuropsychology Department, B33
http://www.ulg.ac.be/neuropsy/
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