Dear FreeSurfer experts and developers I am confronted with some technical question of a reviewer. I am not sure about "volume = thickness-by-surface area" and the review's opinion (attached below) about volume, surface area, and thickness was correct. Any comments from FreeSurfer's experts would help me.
Sincerely yours Woo-Suk Tae Seoul, Korea I added the reviewer's comments. --------------------- 1. Volume/thickness: The authors cite many papers that show volume and thickness differences in MDD. The unresolved part here, however, is the RELATION between cortical thickness and cortical volume: There is no doubt, that both measures are found affected in MDD. This is, because cortical thickess multiplied by the surface area of a gyrus results in its volume, so volumse = thickness-by-surface area. Surface area values themselves are technically more noisy and usually thus less sensitive (due to the problem of false attributions to an area). So, volume is influenced by thickness and surface and is the less specific measure. If a volume effect is detected in a study, it is unclear, if it is driven by thickness, or surface area, or both. In this respect, the study of Schmaal et al. is telling, as it analyzed BOTH measures, finding only thickness effects of MDD, and (practically) no surface area changes except for adolescent MDD. In the adolescent MDD samples gross surface area differences were detected. This means, that the question of "superiority" is rather a question of "specificity": (Cortical) volume findings in adult MDD are mostly driven by thickness differences and are in on way independent from thickness differences. In this respect, the authors should follow the basic geometric principles of morphometry and point out the relatedness of the two. They can simply check this in their FreeSurfer variables (e. g. for total grey matter volume). Not reporting surface area is leaving an interpretational gap as surface area differences could in addition drive volume differences. The authors may want to decide not to present surface area results, but then they should discuss this as limitation to disentangle the origin of volumetric (cortical) effects. This seems important as methylation effects and FKBP5 interact with early life time stress, so effects on surface area as seen in adolescent MDD highlight that surface area effects could play in. ---------------------------------------------------------------------------------------- Woo-Suk, Tae Ph.D. Research Professor Brain Convergence Research Center, Medical Research Center Anam Hospital, Korea University Medical Center, Seoul, Korea mobile: 82-10-9120-4629 office: 82-2-920-6831 email: woosuk....@gmail.com, woos...@gmail.com <woos...@naver.com> ----------------------------------------------------------------------------------------
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