Dear Serge,
>Dear SPM Users,
>
>In our analysis of fMRI data with simple visual stimulation (light flashes)
>we observed the following:
>using spm96 with global normalization we unexpectedly see large regions
>outside V1 with a signal decrease during stimulation. Most of these signal
>decreases disappear when we do not apply global normalization. It seems that
>the signal intensity of the images obtained during visual stimulation is
>scaled down due to the large signal increases in V1, resulting in artificial
>signal decrease in areas outside V1.
This is a well known problem when there are large activations present (you
may check Aguirre et al. NeuroImage 1998;8:302-306. or Andersson NeuroImage
1997;6:237-244.) and we are currently looking into it.
>However, when using spm97, these areas
>with a signal decrease persist, even when global normalization is not
>applied.
I am not clear about what you mean here. To the best of my knowledge SPM97
was never publically released. Could you please specify precisely what
version you are using, since I will probably need to look into the code to
be able to answer this one.
>We see the same effects of unexpected signal decrease with other
>paradigms too.
>
What is it you see? The same type of bias in the global resulting from
large activations, or the same type of differences between SPM96 and SPM97?
>Can anyone explain what causes this difference or how to solve it using
>spm97?
I will certainly give it a try once I know precisely what version of SPM
you are using.
Jesper
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