Hello,
Since nobody competent has responded. Here's a reply from an enthusiastic
amateur.
I am assuming that you are exploring activations treating inter-subject
variability as a random effect. Your question about common areas across the
two groups can be addressed by either masking or a conjunction analysis.
One easy way to do it using masking would be to select the contrast of
interest for group 1 (doesn't matter which group) and then convert it into
a binary image using ImCalc (input file is SPMTxxx.img; output file = mask,
function is something like i1>2 (or wahatever t value you deem
appropriate)). When running the analysis for group 2, select the explicit
masking option and use the binary image that you have created. This will
ensure that your analysis is confined to areas that show an activation in
group 1 so that effects that are ultimately shown will be common to both
groups. Since the data sets are independent, the uncorrected p value for
this conjoined activation is given by the product of the p value used in
creating the mask and the p value in the 'maskee'.
A conjunction analysis will allow a more refined statistical inference
since you can obtain the corrected p value for the conjunction. This will
require, as far I know, that you use newish code in SPM_Devel to allow you
to do this (see previous SPM emails).
Hope this is of some help to you?
Paul Fletcher
At 11:42 29/06/01 +0200, Andreas Jansen wrote:
>Dear SPM,
>
>so far nobody answered my questions, so I'll try it a second time.
>
>Using fMRI I have scanned 16 subjects from 3 different groups: 9
>subjects, 3 subjects and 3 subjects, respectively.
>The first group consists of "normal" subjects , the other two are
>composed of "atypical" subjects.
>It is rather difficult to find such "atypical" subjects. That explains
>the different group size.
>
>I have the following hypothesis:
>Although the activation pattern of the subjects belonging the "atypical"
>groups is different from the activation pattern of the "normal" group,
>they all have some cortical areas that are used by subjects from all
>groups.
>
>What sort of analysis is appropriate?
>
>I can think of the following:
>1. A conjunction analysis for each group to get the activation pattern
>that is typical for each group.
>2. Comparing the activation pattern of each group...
>
>Any comments are appreciated,
>
>Andreas
>
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Paul Fletcher,
Box 189,
Research Department of Psychiatry,
University of Cambridge,
Addenbrooke's Hospital,
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