Dear Joe
To create an SPM which represents what is common to a group of individuals,
we find conjunction analyses with masking very useful. To use conjunction
analyses all subjects are entered into the same statistical model but the
parameters for each subject are estimated independently in a subject
seperable design matrix. You then enter your contrasts seperately for each
subject.
To identify areas where there are consistent effects for each subject, you
do a conjunction of the 5 contrasts (one for each subject). This will sum
over the effects and eliminate any significant interactions. To ensure that
the individual effects are significant for each subject, you can also mask
the conjunction with each of the 5 contrasts. The resulting SPM will
display only those voxels that are significant for each subject.
If you had more subjects an alternative approach would be to use a random
effects analysis. This allows you to make generalisations to the
population but does not determine the consistency of your activations.
Best wishes
Cathy Price
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