[Freesurfer] Monte Carlo Simulation w/ custom design matrix

Hello,

I have a question regarding performing Monte Carlo simulations when a custom design matrix is being used. The wiki says that "For both MC simulations, you must supply the smoothness of your data as Full-Width/Half-Max (FWHM) of the residual. This can be obtained from the ResidualFWHM value in y.fsgd in the glm output directory." If I understand correctly, this means that the --fwhm flag should be specified when running mri_glmfit to generate CSD's... However, when running a custom design matrix, the y.fsgd file is not generated (since the --fsgd flag cannot be specified at the same time as --X). So, my question is: how do I get the required FWHM value?

Thanks, - Jerry