> 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?
I don't think you need to .. depends what you want to show and what the
reviewer asks :-P
> (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"?
well the model goes like data = mean + effects and the effects are
reflected by the beta ; so the normalization is only across subjects
otherwise the ratios effect1 vs effect2 should be the same before after
dividing each beta by its own mean
> (3) Is there a situation that there is positive signal change but no
> activation shown in the activation map?
i guess this is always the case .. the fMRI signal changes over time
> (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?
deactivation? if relative to the mean I would say decrease of neural
activity (tempting) but these are so many interpretation of what's going
on with negative bold ..
> (5) Why does negaitve signal change not give deactivation in the
> activation map?
did you look at the con or t image without thresholding? this might just
be stat problem ...
> (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?
I skip those ones ...