Hi Ada, hi SPM community,
if you want to present your results in a way that is closer to the
original %-signal change, you may be interested in my toolbox MASCOI:
http://homepages.uni-tuebingen.de/matthias.reimold/mascoi/
At the link, you will also find a reference and an explanation of why
contrast images are spatially more precise than t-maps.
Since I am more of a PET than fMRI person, I am not an expert of how
to calculate the original %-signal change from SPM's contrast.
Probably I should go through the related comments and add a button to
the toolbox that converts SPM's contrast to a quantitatively correct
%-signal change. If you have any suggestions of how to actually do
this, please let me know.
-Matthias
Am 21.02.2008 um 16:13 schrieb Ada Leung:
> Dear SPMers,
> I like to arise some questions about the concepts of percent signal
> change and welcome answers from anyone who are interested in it.
> (1) Why do we need to report percent signal change in addition to
> activation maps?
> (2) Percent signal change is the difference between betas divided
> by mean activity and multiply by 100. Then it seems to mean that
> the value represents a normalized brain activity for an individual.
> If this holds, then comparison of percent signal change is a better
> indicator than general activation map. If this is correct, that why
> do we need to present the activation map since the result from the
> activation map is just a test of significance difference between
> two betas which is not "normalized"?
> (3) Is there a situation that there is positive signal change but
> no activation shown in the activation map?
> (4) What is negative percentage signal change? I have experience
> using Marsbar to compute the percent signal change of say, 3
> conditions. Then there are situations that the 3 conditions come up
> with some showing negative percent signal change. Because in that
> stage, there is no substraction of betas but just a computation of
> percent signal change for individual condition, then what is that
> negative percent signal change mean?
> (5) Why does negaitve signal change not give deactivation in the
> activation map?
> (6) What research group or who first proposed the use of percent
> signal change? Is there any reference?
> (7) Is there any reference or suggested readings about the concepts
> of percent signal change, its methodological issues and calculation?
> Thanks,
> Ada
|