For conjunctions and the 2nd level, would inclusive masking using Imcalc
also be a valid option?
For example, we have conducted this analysis with
two contrasts [aversive - neutral], one using word stimuli [Word(A) -
Word(N)] and the other using picture stimuli [Pict(A) - Pict(N)] with 14
subjects. The aim was to characterize regions jointly activated by
aversive emotion regardless of stimulus modality.
After conducting separate RFX analyses for word and picture data, we
"write filtered" the two SPMS at .07 and then used Imcalc to
inclusive-mask these images (i1.*(i2>0)), the idea being to end with a
joint p of approx. .005, (.07)^2. This seems to follow the spirit of
SPM99-style conjunctions.
Perhaps a new section of the RFX page could be added, in which FAQs
regarding conjunctions could be referred to? Several people have told
me they are trying to work out how to do similar things and it would be
much appreciated.
thanks,
Stephan Hamann
[log in to unmask]
On Wed, 1 Nov 2000, Karl Friston wrote:
> Dear Philippe,
>
> > 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 ?
>
> That is 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?
>
> Not quite. First you are assuming homogeniety of variance over the
> contrasts from the two groups, but I think this is very tenable. Your
> one-sample T test simply tells you where the activation (averaged over
> both groups in significant. To get the conjunction you would have to
> enter the two groups contrasts as different effects at the 2nd level
> and do a conjunction of contrasts testing for each alone. This is best
> done (I think) by using 'simple regression' in 'basic designs' and
> entering [1 ... 1, -1 ... -1] as the regressor. The conjunction of
> contrasts [1 1] and [-1 1] should give you what you want (this gets around
> the fact the SPM always includes a constant term in the design matrix).
>
> > 3) Is there a caveat with the (non) orthogonalization between [A-B] and
> > [C-D] ?
>
> No.
>
> I hope this helps - Karl
>
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