Yvonne,
to add one more voice to that, I think Tim and Joe's suggestion is
currently the simplest/clearest alternative. You may want to call it
conjunction "overlay", as opposed to "analysis" if you want to be very
clear. Even the theory on the point of conjunction is very
controversial. Try looking up "cognitive conjunction & Friston", you
will find I think 4-5 articles suggesting how to do a conjunction, and
then you can read Nichols, Brett & friends ("When a conjunction is not a
conjunction?" and "Valid conjunction inference with the minimum
statistic"), to see why that was NOT a conjunction...! All this just to
show how unclear this whole procedure is at present.
Plus, in my recollection virtually all conjunctions presented in
published papers are simple overlays (excluding the controversial SPM
conjunctions following Friston, I suppose?).
just my 5p..
best of luck
martin
Tim Behrens wrote:
> Hi
> On 13 Feb 2008, at 22:19, Joseph T. Devlin wrote:
>
>> Hi Yvonne,
>>
>> Since no one more statistically savvy than me has answered as yet,
>> I'll poke my head out and say "no" -- FSL doesn't offer an
>> implementation of the "minimum statistic" implementation of
>> conjunction analyses. To be honest, though, there are real problems
>> with interpreting those statistic values, as pointed out by Tom
>> Nichols and colleagues. In many cases it is sufficient to use
>> inclusive masking. If the contrasts are truly orthogonal, then you
>> can reasonably multiply the p-values together to get a "conjunction"
>> p-value in a given voxel. For instance, if you were interested in
>> areas in Group 1 where A>B and masked it with Group 2's A>B (at
>> p<0.001 uncorrected to pick a random threshold...), then any areas at
>> p<0.001 in the G1 A>B contrast that are also present in G2 A>B are
>> unlikely to be there by chance given that the corresponding p-value
>> would be p<10^-6.
>>
>
>
> Just to note that even this is a bit misleading, because p<10^-6
> refers to the null hypothesis that NEITHER of the activations were
> present - you certainly can't claim a conjunction with this p-value.
>
>
> I agree with Joe that just showing the overlap is fine.
>
> T
>
>
>> Personally, I wouldn't feel the need to see a computed p-value -- it
>> would be sufficient to know that the "conjunction" contrasts were
>> truly orthogonal and know the level that each one was
>>
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