Dear Dr Crivello,
>Our design is
>
>Contrast 1 = activation 1 - control 1
>Contrast 2 = activation 2 - control 2
>
>we first tested the following design
>contrast 1 - contrast 2 and we observed an activation in the occipital cortex.
>This means that there is a significant difference between these two contrasts
>Then we tested the conjunction of contrast 1 and contrast 2, and we find
>also an activation of the same voxels.
>how can we interpret these results ?
The conjunction of contrasts 1 and 2 gives you voxels within which there is
a significant difference between the combined activations and the combined
controls, excluding any voxels in which there is an interaction at a
statistical level which you enter separately.
Thus if you set a high statistical threshold for excluding 'interaction'
voxels (such as p < 0.05 corrected), then voxels may survive which
nonetheless show up in an interaction (which I presume is what you mean by
'contrast 1 - contrast 2') for which a lower threshold (such as p < 0.001
uncorrected) has been set.
Your area of occipital cortex, for example, may give a large positive
response to 'activation 1' compared with its control, and give a smaller
positive response to 'activation 2' compared with its control. The
interaction (activation 1) - (control 1) - (activation 2) + (control 2),
may exceed your threshold for your 'interaction spm', but not exceed your
higher 'voxel-exclusion' threshold for your 'conjunction spm'.
However, my understanding is that if you have set the same statistical
threshold for the interaction per se, and for excluding voxels from a
conjunction because they show an interaction, then there should be no
voxels which show up in both of these spms! If there are such voxels, then
you will need someone with more expertise than I have to explain what has
gone wrong,
Best wishes,
Richard Perry.
from: Dr Richard Perry BM BCh MA PhD MRCP(UK),
Clinical Research Fellow, Wellcome Department of Cognitive Neurology,
Darwin Building, University College London, Gower Street, London WC1E 6BT.
Tel: 0171 504 2187; e mail: [log in to unmask]
Pager: 04325 253 566.
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