Dear SPM
In my experience as a beginner of fMRI study, some deactivation
( i.e. activation in rest minus task contrast) may dissapear after
reanalyzing the same dataset with "no global normalisation".
Using ANCOVA and proportional scaling give almost identical results
both for activation and for deactivation, however analysis with "no global
normalisation" reduce deactivation particularly in the medial frontal
and retrosplenial area although activated area is similar.
Which of ANCOVA or scaling or no gobal normalisation is appropriate
for fMRI data?
Tetsuya
>Dear Ingo,
>
>> my question concerns the use of the contrast 2 results of SPM
>> calculations fo fMRI data sets. Applying contrast 2 to PET data
>> indicated areas of deactivation or inhibition. Can this option
>> (negative contrast) also be applied to fMRI data in the same way. If
>> yes, how to explain a negative BOLD effect?
>
>This question will, I am sure, excite a lot of answers (in people's
>minds at least). In principle the situation is exactly the same for
>PET and fMRI. A deactivation in fMRI is simply an activation on
>comparing the baseline condition to the index condition. This reflects
>the relative nature of fMRI measures of neurophysiology. It becomes
>more interesting in event-related fMRI where one has to posit a
>reduction in BOLD signal that is presumably mediated by a local
>reduction in mean synaptic activity (below tonic resting or control
>levels). To do this one has to suppose that the synaptic
>activity-hemodynamic response coupling is monotonic over 'resting'
>levels (if not symmetric about them). I believe there are animal
>models of this effect using optical imaging but I cannot recall where I
>saw these data.
>
>With best wishes - Karl
>
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