Your approach was quite reasonable, except that you have made the
assumption that the contrasts 1 0 and 0 -1 are orthogonal, and I
guess that this is probably not the case. (After all, when you have
stimulus A, presumably you can't also have stimulus B at the same
time?) If they are not orthogonal, then the conjunction which you
have tried is not really interpretable.
To take an over simplistic example, imagine a situation in which
there were only two conditions, A and B, and these were both modelled
with box cars, and in fact the covariate for B was equal to that for
A multiplied by -1 and with +1 added (i.e. when A had ones, B had
zeros and when A had zeros, B had ones). In this overspecified
model, the same variance in voxels which were actually 'activated' by
condition A but not B could either be modelled using covariate A
(with a positive parameter estimate) or with covariate B (with a
negative parameter estimate). Many of these voxels would end up
being modelled by some combination of the two, and these would show
up in the both contrasts, 1 0 and 0 -1, and would therefore also show
up in the conjunction of these two, in spite of the fact that in this
example there is no response at all to B.
Really to answer your question you need to have more conditions.
Ideally you should have condition A and its own baseline condition,
and condition B with its own baseline condition (you can't use the
same baseline for two conditions which you want to use for a
conjunction analysis). With these four covariates you could do the
conjunction of 1 -1 0 0 and 0 0 -1 1, and get a meaningful answer. I
guess that what you have ended up with, in your conjunction of 1 0
and 0 -1, are many more voxels than you expected. These are not
necessarily voxels in which 'areas of activation from one stimulus
overlap with inactivations from the other stimulus'; it may just be
telling you that your covariates are significantly co-linear.
Best of luck,
>I want to run an analysis that I have done by you just to make sure that
>my logic is sound.
>I have an fmri study with multiple stimulus conditions in each series. I
>am interested in two questions. The first is to locate the areas of
>activation that overlap across stimulus conditions. I have performed
>conjunction analyses at the individual and group level (fixed effects).
>My conjunction included the positive contrasts for both stimulus
>My second question is: Do areas of activation from one stimulus overlap
>with inactivations from the other stimulus. To do this I performed
>conjunctions analyses (again at the individual and group level) and
>selected the positive contrast from one condition and the negative
>contrast from the other condition. In other words the did a conjunction
>of the contrasts 1 0 and 0 -1.
>Are these analyses valid? They seem to follow what has been addressed on
>the mailbase but I am quite surprised by my results and want to verify
>that I have not made some stupid misinterpretation.
>Thanks in advance.
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from: Dr Richard Perry,
Clinical Research Fellow, Wellcome Department of Cognitive Neurology,
Darwin Building, University College London, Gower Street, London WC1E
Tel: 0171 504 2187; e mail: [log in to unmask]