Very helpful, great, many thanks!

Best, Wolff

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"Anderson M. Winkler" winkler@fmrib.ox.ac.uk 11.02.2013 15:57 >>>

Dear Wolff,

Compared to vertexwise, analysis of regions (ROIs) have different features. If the area of true signal spans the whole region, or large parts of it, then an ROI analysis tend to be more powerful as the noise is diluted when the vertices that comprise the ROI are summed or averaged. However, ROI analyses can also be less powerful if the signal does not span the whole region, or is split across multiple regions. This alone could explain your observation.

Does this help?

All the best!

Anderson

2013/2/11 Wolff Schlotz Wolff.Schlotz@psychologie.uni-regensburg.de

Dear Anderson (and all), Many thanks for your reassuring response, but I am still puzzled about the lack of association between mean area (rh_medialorbitofrontal_area) and area in qdec (see area.png). I thought that technically there must be larger areas for vertices within medialorbitofrontal? Best wishes, Wolff

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"Anderson M. Winkler" winkler@fmrib.ox.ac.uk 11.02.2013 14:22 >>>

Dear Wolff,

There is nothing wrong with your results. Your finding is one more confirmation that thickness and area are indeed different traits, which are influenced differently by different genetic and/or environmental factors, and should not be confused one with another. They represent different aspects of brain morphology and its development, and can (and should) be analyzed and interpreted each on its own right.

It is also evidence that more power can be gained by using these two measurements separately, rather than mixed up as in methods that only measure gray matter volume.

Assuming you did everything else correctly, your results look perfectly fine to me.

All the best!

Anderson

2013/2/11 Wolff Schlotz Wolff.Schlotz@psychologie.uni-regensburg.de

Dear Freesurfer experts, I tested associations between a continuous predictor and thickness and area in qdec (and command line mri_glmfit, which gives the same results) and found a cluster being negatively associated with thickness orbitofrontal, but nothing for area orbitofrontal. After exporting mean thickness and area values from aparc.stats into Stata, consistent with my expectation I found a significant negative correlation with my predictor for thickness. However, I also found a significant positive association between birth weight and area. To check what might be wrong I tested correlations between mean medialorbitofrontal thickness from rh.aparc.thickness and thickness in qdec and did the same for area. As expected, large average thickness values were positively associated with thickness in medialoribotfrontal and adjacent areas (see attached qdec screenshot thickness.png), but there were no associations using area (see attached qdec screenshot area.png). My expectatioin was that there should be positive area associations similar to those for thickness. Hence my question: Is this expectation correct? If yes, why this discrepancy between thickness and area? Thank you.

Wolff

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