On Tue, Jul 10, 2012 at 8:20 AM, Krishna Miyapuram
<[log in to unmask]> wrote:
> Dear Atesh,
>
> The conjunction principle as in Nichols et al. paper is that the minimum t value of all the contrasts will become the t value of the conjunction. Hence the correct method of doing conjunction is to do a random effects analysis and use the contrasts [1 0 0], [0 1 0] and [0 0 1] and do a conjunction between them.
This approach only works if the groups/conditions are independent. If
they are dependent, then these contrasts would be invalid because you
have told the program that the values are dependent and the error term
would be incorrect for the above contrasts.
If A is from group1, B is from group2, C is from group3 and so on,
where the subjects are different, then you can setup a one-way ANOVA
with 6 groups. This would be a random effects analysis. If the same
subjects are in two groups, then you run into problems as the
statistical assumptions are violated.
The alternative option is to do three one-sample t-tests, threshold
them and do an imcalc to get a map where all the three overlap. if you
want to know how much is the overlap, example in contrast A, the
t-value may be 5 where as in contrast B, the t value may be 3. This
you can do by using MRIcron or slover by projecting blobs of different
colors e.g. red, blue, green and selecting the transparency of the
colors, you can get the overlap areas using the usual RGB
combinations. In general the Random effects analysis will be useful if
you want to further probe into the results of your experiment such as
finding the difference between contrasts A and B etc.
>
> The other methods like implicit masking should not be used for conjunctions, because the t-value you get are from the main contrast selected. Masking only filters out voxels not active in the second contrast.
>
> HTH
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