A simple LR registration on fsaverage is not ok because you would be resampling one hemisphere and not another. The resampling inevitably incurs some interpolation so one hemi would be smoothed more than the other which would cause bias. If you just want something quick, then you can sample your fsaverage surfaces to fsaverage_sym (the registration has already been done in v6), then just follow the steps in the wiki using fsaverage as your subject name. I have not tested whether this is the same as doing the full xhemi process, so I would not use the results in publication. But probably they won't change much.

On 3/20/17 2:36 PM, Dorian P. wrote:

Hi all,

I have a bunch of thickness files in fsaverage space (already smoothed at fwhm). 

I was wondering whether is possible to compute quickly an asymmetry overlay for each one, but it's turning more difficult than I thought. Vertex indices are different between lh and rh. So I started looking at fsaverage_sym, which has already registrations lh-rh. I thought this would be a nice shortcut to avoid registering every subject again. But there are a couple of confusing points:

1. Why isn't lh.inflated well aligned between fsaverage and fsaverage_sym (see figure below) ?

2. I found an old post in 2012, in which Donald advises to not run analyses on a simple left-right registration because one hemisphere would be more processed than the other. Since I might want to run some analyses, can you please explain what is the best way to get asymmetry surfaces (possibly by avoiding new registrations)? Why wouldn't a simple L-R registration of fsaverage be ok?

I run the analyses in R and don't necessarily need the glm tools within FS.

Thank you in advance.

Dorian

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