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

  FT_VOLUMEDOWNSAMPLE downsamples an anatomical MRI or source reconstruction
  and optionally normalizes its coordinate axes, keeping the homogenous
  transformation matrix correct.
  Use as
    [volume] = ft_volumedownsample(cfg, mri)
  where the input mri should be a single anatomical volume that was
  for example read with FT_READ_MRI or should be a volumetric source
  reconstruction resulting from FT_SOURCEANALYSIS or FT_SOURCEINTERPOLATE.
  The configuration can contain
    cfg.downsample = integer number (default = 1, i.e. no downsampling)
    cfg.parameter  = string, data field to downsample (default = 'all')
    cfg.smooth     = 'no' or the FWHM of the gaussian kernel in voxels (default = 'no')
    cfg.keepinside = 'yes' or 'no', keep the inside/outside labeling (default = 'yes')
    cfg.spmversion = string, 'spm2', 'spm8', 'spm12' (default = 'spm8')
  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.