`FreeSurfer`_ is software for analysis and visualization of MRI data. In the MNE ecosystem, freesurfer is used to convert structural MRI scans into models of the scalp, inner/outer skull, and cortical surfaces, which are used to
- model how changes in the electrical and magnetic field caused by neural activity propagate to the sensor locations (part of computing the "forward solution"), and
- constrain the estimates of where brain activity may have occurred (in the "inverse imaging" step of source localization).
System requirements, setup instructions, and test scripts are provided on the
`FreeSurfer download page`_. Note that if you don't already have it, you will
need to install tcsh for FreeSurfer to work; tcsh is usually
pre-installed with macOS, and is available in the package repositories for
Linux-based systems (e.g., sudo apt install tcsh on Ubuntu-like systems).
Next: :doc:`install_mne_c`