Dear Feng,
these terms are somewhat arbitrary. Personally I try to prevent the term "density" or "concentration" because this rather implies something which is on a microscopic (cell) level, which is VBM definitely not. However, some of the VBM method papers started to use these terms and for a distinction between modulated and non-modulated images this might be sometimes helpful. Please take a look at the paper from Mechelli (page 4) for a more detailed explanation:
http://www.fil.ion.ucl.ac.uk/spm/doc/papers/am_vbmreview.pdf
Best,
Christian
>Dear Christian,
>Thanks a lot for your help! I will read these references carefully.
>Another question is that I noticed "relative density" on the page http://www.neuro.uni-jena.de/vbm/segmentation/modulation/
>No modulation:
>Correction Interpretation
>nothing relative density
>globals “localised” relative density after correcting for total GM or TIV (multiplicative effects)
>AnCova “localised” relative density that can not be explained by total GM or TIV (additive effects)
>Modulation:
>Correction Interpretation
>nothing absolute volume
>globals relative volume after correcting for total GM or TIV (multiplicative effects)
>AnCova relative volume that can not be explained by total GM or TIV (additive effects)
>The absolute and relative volume is easy to follow. If I perform modulation with the "affine+non-linear", I could obtain the absolute volume. If I perform modulation with the "non-linear" or modulation with the "affine+non-linear" and regress the ICV, I could obtain the relative volume.
>But I am wondering how to understand the "relative density"? Is there any absolute density? In addition, how to understand “localised” relative density?
>Best,
>Feng
|