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

  FT_FREQINTERPOLATE interpolates frequencies by looking at neighbouring
  values or simply replaces a piece in the spectrum by NaN.
  Use as
    freq = ft_freqinterpolate(cfg, freq)
  where freq is the output of FT_FREQANALYSIS or FT_FREQDESCRIPTIVES and the
  configuration may contain
    cfg.method   = 'nan', 'linear' (default = 'nan')
    cfg.foilim   = Nx2 matrix with begin and end of each interval to be
                   interpolated (default = [49 51; 99 101; 149 151])
  To facilitate data-handling and distributed computing you can use
    cfg.inputfile   =  ...
    cfg.outputfile  =  ...
  If you specify one of these (or both) the input data will be read from a *.mat
  file on disk and/or the output data will be written to a *.mat file. These mat
  files should contain only a single variable, corresponding with the
  input/output structure.