Note that this reference documentation is identical to the help that is displayed in MATLAB when you type “help ft_source2sparse”.

  FT_SOURCE2SPARSE removes the grid locations outside the brain from the source 
  reconstruction, thereby saving memory.
  This invalidates the fields that describe the grid, and also makes it
  more difficult to make a plot of each of the slices of the source volume.
  The original source structure can be recreated using FT_SOURCE2FULL.
  Use as
    [source] = ft_source2sparse(source)