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

  FT_PREPROC_RECTIFY rectifies the data, i.e. converts all samples with a
  negative value into the similar magnitude positive value
 
  Use as
    [dat] = ft_preproc_rectify(dat)
  where
    dat        data matrix (Nchans X Ntime)
 
  See also PREPROC