Dear Bo,
>now, another basic question. how to judge whether I should chose
>"none" or "scale" in fMRI->data. how to know whether the global brain signal
>was correlation with my task? I get the rd_taskcorr.m file from web which
>could computer the correlation P value, rewrote it for SPM2 and test my data.
>I found in same one study, some subjects could be scaled, and others could
>not. what's your opinion about this question?
Personally, I would usually use scaling. The reason is simple; If you
are interested in regionally specific effects, then, by definition global
effects are confounding (irrespective of whether you can detect them
or not). I appreciate some people find it difficult to interpret models
with global normalization in absolute physiological terms but these
difficulties have to be seen in the context of partitioning responses into
regional and global components It is perfectly OK to do a T-test (i.e.
a correlation) on the global activity and an SPM{T} of regional effects.
In designs with low-level baselines and widespread activation one can
sometimes
get a significant global effect and no regional effects. This may seem
paradoxical from a physiological point of view but is quite proper in relation
to the differences between global and regional effects.
There are many qualifications here. For example, two regions may express
a task-related response that is highly correlated with global activity.
However,
one of the regions expresses it with twice the amplitude. Is this a regional
effect or a global effect?
With very best wishes,
Karl
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