Great, thanks for the info!
Best,
Sophie
Quoting "Stephen J. Fromm" <[log in to unmask]>:
> On Wed, 27 Jan 2010 10:36:45 +0000, Sophie Green <Sophie.Green-
> [log in to unmask]> wrote:
>
>> Dear SPMers,
>>
>> I have another question regarding PPI analyses..
>>
>> Is it possible to see significant PPI effects in voxels that are not
>> significantly present in a standard analysis?
>>
>> I would have thought that this could be the case since you are testing
>> for a change in regression slope between two conditions, rather than
>> activation of an area above a certain threshold per se. Presumably,
>> you could still get coupling of areas, which may not have 'great
>> enough' activation in a standard analysis, but if the contribution of
>> one area explains the activation in another area under a given
>> condition and this changes with condition, this could be significant?
>
> As a theoretical matter, yes, it's certainly possible. More
> abstractly, you can
> have significant interaction terms even if main effects aren't significant,
> assuming the former isn't highly confounded with the latter (which
> is the case
> in a truly multifactorial experiment).
>
> One could ask the empirical question---how often does this happen. I'm not
> sure about that.
>
>>
>> Very best wishes,
>> Sophie
>>
>>
>>
>>
>>
>> --
>> Sophie Green
>> PhD Student
>> Neuroscience and Aphasia Research Unit (NARU)
>> Zochonis Building, 3rd Floor
>> Brunswick Street
>> University of Manchester
>> Manchester M13 9PL
>>
>> Tel: +44 (0)161 306 0451
>
>
>
--
Sophie Green
PhD Student
Neuroscience and Aphasia Research Unit (NARU)
Zochonis Building, 3rd Floor
Brunswick Street
University of Manchester
Manchester M13 9PL
Tel: +44 (0)161 306 0451
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