Dear SPMers,
I am trying to learn Flexible Factorial design by the demo:Verbal Fluency PET data in SPM manual.In Chapter 31.3 "Multiple subjects",I follow the manual step by step but still can't get the right results.
After checking the results,I find that after I specified contrast '-1 1 -1 1 -1 1 -1 1 -1 1 ',the block of my design matrix (as attached) remain all white.However,it is said in SPM manual that "the block of the design matrix encoding the experimental conditions is now coloured differently.This is because we have allowed the variance of responses over subjects to be different between word shadowing and generation conditions.This 'nonsphericity'affects parameter estimation in a way that is……" And in the manual results(as attached) some block really change to grey after specifying contrast.
I recheck my operation several times but still can't find a way to solve it.I really have no idea of how to"allow the variance of subjects to be different".Could someone help me?
Many thanks,
Sisi
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