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Hi John,

Thanks for the comments. Yes, it's possible. Thickness gives some contribution to volume, but these two aren't the same and aren't comparable. In fact, we and others have seen that the variability of volume across subjects is better explained by cortical surface area, not by thickness.

So, one can indeed have group-level thickness effects, but if these aren't accompanied by area effects in the same direction, there may be no net changes in volume.

Btw, we have just put a paper on bioRxiv in which we use NPC to capture these effects that could remain unseen when volume is analysed alone. In the same paper we also propose a different method to measure volume in surface models that is no longer just the product of area by thickness (this new method is now the default in FreeSurfer 6.0, thanks do Doug Greve who made it available there).

All the best,

Anderson

PS: I think questions as these are more for the FS mailing list, not much for FSL...

On 2 May 2017 at 05:58, John Anderson <[log in to unmask]> wrote:
Dear Anderson,
I enjoyed studying this great manuscript! https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2891595/

Thank you very much for the effort and time spent on this study.

I highly appreciate if you can answer my questions regarding the relationship between cortical thickness and gray matter volume.

I have two groups of subjects (60 patients and 20 controls). I ran VBM on T1 images to check the difference in gray matter volume. then I ran surface based analysis to study volume on surface and cortical thickness. VBM and surface based revealed no difference in gray matter volume. Surface based approach revealed difference in cortical thickness.
Given that the: (cortical thickness = gray matter volume (GMV) X area) that means  GMV and thickness are related to each other and the change between these parameters can affect each other.

My question is how can we explain reduce cortical thickness and normal gray matter volume. Is this possible?

I highly appreciate your input;
John