Thanks, Doug! 

So I tried this method using fsaverage/surf/lh.white.avg.area.mgh. The results seem to be reasonably consistent with the ones I get when eyeballing the p-maps overlayed on fsaverage in tksurfer. Would you agree that's a reasonable "sanity check"?

When I compare the results to just counting vertices, there doesn't seem to be a huge difference between proportion of #vertices and proportion of surface area. So either there isn't a lot of variance in the sizes of the faces, or the variance is evenly distributed.

Finally, if I want to convert from an individual map or the cohort mean map to mm² units, can I do it by scaling the ico area vertex values by the ratio between native and ico total area?  So:  (vertex value from mean cohort map * total area for cohort mean map) / total area for fsaverage/surf/lh.white.avg.area.mgh, where total area would be the sum of all vertex values?

Thank you!

LMR


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You don't want to get the surface area from the faces of fsaverage. 
Instead use the values in fsaverage/surf/lh.white.avg.area.mgh (this is 
the average of the group used to create fsaverage), or, probably better, 
get an average area map for your cohort
doug

On 07/30/2014 08:38 AM, Lars M. Rimol wrote:
> Hi Bruce,
>
> I would like to be able to tell what proportion of a region of 
> interest (ROI), as defined in atlas space by e.g. Desikan-Killiany, 
> that shows a significant effect (based on a p-map). For now I overlay 
> the p-map on the inflated surface of fsaverage in tksurfer and eyeball 
> the proportion.
>
> Given a p-map, if I find the FDR threshold and identify the vertices 
> within a given ROI that have a p-value greater than the threshold, 
> then I can find the proportion of the ROI that is suprathreshold. 
> E.g., I find 1986 suprathreshold vertices in "bankssts" out of 2137, 
> so 93% of vertices in bankssts show a significant effect.
>
> My question is: Does that tell me anything about what proportion of 
> the ROI's surface area is affected in atlas space? Obviously, if the 
> faces were uniform, there would be a 1 to 1 relationship between 
> #vertices and area. In the original tesselation of any dataset the 
> size of the faces is uniform, but that changes with topology fix and 
> deformation. I assume that is true also for fsaverage? (so I can't 
> assume [#sig vertices] / [# tot vertices] == the proportion of the 
> ROI's area that is significant in atlas space)
>
> I can find the surface area of the suprathreshold region for my sample 
> (or any subset thereof) by looking at a mean area map. But I'm unsure 
> how to do that for fsaverage itself? Is there information on regional 
> surface area directly available? Or would using  getFaceArea.m or 
> getFacesArea.m or similar functions be a solution?
>
>
> Thank you!

--

yours,

Lars M. Rimol, PhD
St. Olavs Hospital
Trondheim,
Norway

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