Dear Stephan,

>We have three conditions, A, B, and a baseline condition C, and two
>groups. Group one has a particular response to A (for example, fear), but
>has no particular response to B. Group two has the same response (fear) to
>B but not to A.
>
>The analyses of interest are:
>  1) characterize the (fear) response in each group separately, taking the
>A - C contrast for group one and the B - C contrast for group two and
>performing a random effects analysis separately for each group.

This seems reasonable enough.  Obviously without knowing the details 
of your experiment, I can't be sure that the response which you 
observe is related to 'fear' (or whatever) rather than to some other 
difference between stimulus A and stimulus B.  (I see that you point 
this out yourself later in your message.)

>2) Compare the neural correlates of the (fear) response between the
>groups. If both groups were responsive to the same condition (say, both
>had fear of A), this would be a simple matter of entering the A - C
>contrasts for both groups into a 2-sample t-test at the second level.

Agreed.

>My guess was that since the A - C contrast is psychologically the counterpart
>to the B - C contrast in the example, it would be valid to enter A-C for
>group one, A-B for group two, and do the 2-sample t-test.

Er ... do you mean the contrasts A - C for group one and B - C for 
group two?  With the aim that you would identify 'fear' responses in 
group one with A - C and in group two with B - C?  This might be a 
bit misleading.

If you were to get reassuringly similar responses in these two 
contrasts, you might be tempted to suggest that this is evidence that 
these are 'fear' responses.  In fact they could be to do with common 
ground between the responses of your two groups to stimulus C (e.g. 
'inactivations' in response to C).  It's the same argument that one 
hears for conjunctions: each condition has to have its own separate 
baseline if they are to be valid.

If on the other hand, you see differences, you might be tempted to 
ascribe these two differences in the fear responses in the two 
groups, but as you point out...

>This would not control for activations related to specific material 
>content in A and B,
>but this could be assessed with a separate contrast.

This seems to me to be a fatal flaw with this approach.  I'm not sure 
what your 'separate contrast' would be.

>  The only alternative I can see is to qualitatively compare the two
>patterns found in each group (vs. performing a statistical test).

Perhaps it's better to forget stimulus C for the moment.  Let's 
imagine that you have identified a given brain area which shows up in 
the contrast A - B in group one and B - A in group two.  The direct 
contrast that you might use to demonstrate this area might be the 
second level comparison between the A - B contrast in group one and 
the same A - B contrast in group two (the group by condition 
interaction which you mention in your subject line).  You know from 
your psychological observations that group one shows fear to A but 
not B, whilst group two shows fear to B but not A (i.e. there is a 
similar group by condition interaction in the psychological data). 
Can you now convince a referee that this is a fear area in both 
groups?  Possibly.  You have at least ruled out the possibility that 
this is some other stimulus-related component which the two groups 
have in common (i.e. which doesn't interact with group).

>So, in general the question is: when 2 groups have the same type of
>psychological response but in different conditions, how should one go
>about analyzing this?

I am still not absolutely clear what your experimental aim is.  If 
you want to convince us that, for example, an area which in 
arachnophobes lights up in response to spiders but not toads, and 
that the SAME area lights up in bufunophobes in response to toads but 
not spiders (i.e. this single area shows a group by condition 
interaction), then the approach which I mentioned seems OK.  However, 
if you want to demonstrate that there is a DIFFERENCE in the pattern 
of the fear response of arachnophobes to spiders compared with the 
fear response of bufunophobes to toads, then you are interested in a 
group by condition by brain region interaction, which needs more 
thought (unless you are happy to leave it as a 'qualitative' 
observation).  To convince us that brain regions X and Y show 
significantly different response patterns, in that area X responds 
more in A - B and area Y responds more in B - A, you should strictly 
speaking use a statistical test in which data from one these regions 
is directly compared with data from the other, not something that is 
routinely done in SPM.

>thanks,
>Stephan

Well, I hope it was of some help, anyway,

Best wishes,

Richard.
-- 
from: Dr Richard Perry,
Clinical Lecturer, Wellcome Department of Cognitive Neurology, 
Institute of Neurology, Darwin Building, University College London, 
Gower Street, London WC1E 6BT.
Tel: 0207 679 2187;  e mail: [log in to unmask]


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