[Freesurfer] mris_convert to gifti

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Dear Freesurfer experts,

A quick question. I am trying to convert my cortical thickness analysis results into a gifti file preferably combining left and right, like so:

mris_convert --combinesurfs lh.gamma.mgh rh.gamma.mgh both.gamma.gii

I get this error:

error: ERROR: MRISread: file 'lh.gamma.mgh' has 0 vertices!

Probably trying to use a scalar data file as a surface!

It then try to convert a single hemi:

mris_convert lh.gamma.mgh test.gii but I get the same error.

The gamma file looks fine to me, so I am not sure what the issue is.

I am running this version of freesurfer: freesurfer-darwin-macOS-7.2.0-20210713-aa8f76b

Could someone provide an example how this is done?

Thank you so much! Lauri Tuominen