Right, thank you for your very prompt reply. Browsing through several handfuls of recent papers, I find authors mentioning that their measures are combined across hemispheres, but they never explicitly state how they did the merger. Do you think most people just take the simple average? And you're absolute right in that the difference ought to be minor.

Thank you, Kelvin

P.S. As for the denominator, ah... the unfortunate conveniences and consequences of copy-n-paste :/ ________________________________________ From: Douglas N Greve [greve@nmr.mgh.harvard.edu] Sent: Tuesday, June 29, 2010 5:26 PM To: Liang, Kelvin Cc: freesurfer@nmr.mgh.harvard.edu Subject: Re: [Freesurfer] Surface-Area Weighted Average of Cortical Thickness

I doubt it's going to make that much difference, but if I had to choose, I'd choose the formula that you have below (note the the areas should be added together in the denominator).

doug

Liang, Kelvin wrote:

Dear FreeSurfer Experts,

I'd like to merge my cortical thickness measures across left and right hemispheres. Simple averaging of the ?h.thickness values would probably be the most straightforward method, but would it be more accurate to use a surface-area weighted average?

i.e. entorhinal_combined = (lh_entorhinal_thickness * lh_entorhinal_area + rh_entorhinal_thickness * rh_entorhinal_area) / (lh_entorhinal_area * rh_entorhinal_area)

Alternatively, the average could be weighted by the number of vertices (discussed in an old FreeSurfer mailing thread, but never quite concluded). Please let me know what you think.

Thank you, Kelvin Liang

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