ft_denoise_prewhiten.m

function [dataout] = ft_denoise_prewhiten(cfg, datain, noise)

% FT_DENOISE_PREWHITEN applies a spatial prewhitening operation to the data using the
% inverse noise covariance matrix. The consequence is that all channels are expressed
% in singnal-to-noise units, causing different channel types to be comparable. This
% ensures equal weighting in source estimation on data with different channel types.
%
% Use as
%   dataout = ft_denoise_prewhiten(cfg, datain, noise)
% where the datain is the original data from FT_PREPROCESSING and
% noise should contain the estimated noise covariance from
% FT_TIMELOCKANALYSIS.
%
% The configuration structure can contain
%   cfg.channel     = cell-array, see FT_CHANNELSELECTION (default = 'all')
%   cfg.split       = cell-array of channel types between which covariance is split, it can also be 'all' or 'no'
%   cfg.lambda      = scalar, or string, regularization parameter for the inverse
%   cfg.kappa       = scalar, truncation parameter for the inverse
%
% The channel selection relates to the channels that are pre-whitened using the same
% selection of channels in the noise covariance. All channels present in the input
% data structure will be present in the output, including trigger and other auxiliary
% channels.
%
% See also FT_DENOISE_SYNTHETIC, FT_DENOISE_PCA, FT_DENOISE_DSSP, FT_DENOISE_TSP

% Copyright (C) 2018-2019, Robert Oostenveld and Jan-Mathijs Schoffelen
%
% This file is part of FieldTrip, see http://www.fieldtriptoolbox.org
% for the documentation and details.
%
%    FieldTrip is free software: you can redistribute it and/or modify
%    it under the terms of the GNU General Public License as published by
%    the Free Software Foundation, either version 3 of the License, or
%    (at your option) any later version.
%
%    FieldTrip is distributed in the hope that it will be useful,
%    but WITHOUT ANY WARRANTY; without even the implied warranty of
%    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
%    GNU General Public License for more details.
%
%    You should have received a copy of the GNU General Public License
%    along with FieldTrip. If not, see <http://www.gnu.org/licenses/>.
%
% $Id$

% these are used by the ft_preamble/ft_postamble function and scripts
ft_revision = '$Id$';
ft_nargin   = nargin;
ft_nargout  = nargout;

ft_defaults
ft_preamble init
ft_preamble debug
ft_preamble loadvar    datain
ft_preamble provenance datain
ft_preamble trackconfig

% the ft_abort variable is set to true or false in ft_preamble_init
if ft_abort
  % do not continue function execution in case the outputfile is present and the user indicated to keep it
  return
end

% ensure that the input data is correct, the next line is needed for a
% attempt correct detection of the data chanunit (with a hdr-field it fails
% for meggrad data)
if isfield(datain, 'hdr'), datain = rmfield(datain, 'hdr'); end

% check if the input data is valid for this function
datain = ft_checkdata(datain, 'datatype', {'raw' 'timelock' 'freq'}, 'haschantype', 'yes', 'haschanunit', 'yes');
noise  = ft_checkdata(noise,  'datatype', {      'timelock' 'freq'}, 'haschantype', 'yes', 'haschanunit', 'yes');

% check if the input cfg is valid for this function
cfg = ft_checkconfig(cfg, 'forbidden',  {'channels'}); % prevent accidental typos, see issue 1729

% set the defaults
cfg.channel   = ft_getopt(cfg, 'channel', 'all');
cfg.split     = ft_getopt(cfg, 'split',   'all');
cfg.lambda    = ft_getopt(cfg, 'lambda',  0);
cfg.kappa     = ft_getopt(cfg, 'kappa',   []);
cfg.tol       = ft_getopt(cfg, 'tol',     []);
cfg.realflag  = ft_getopt(cfg, 'realflag', true); % for complex-valued crsspctrm
cfg.invmethod = ft_getopt(cfg, 'invmethod', 'tikhonov');

dtype_datain = ft_datatype(datain);

% check for allowed input combinations
switch dtype_datain
  case 'raw'
    assert(ft_datatype(noise, 'timelock'), 'noise data should be of datatype ''timelock''');
  case 'timelock'
    assert(ft_datatype(noise, 'timelock'), 'noise data should be of datatype ''timelock''');
  case 'freq'
    if ft_datatype(noise, 'freq')
      % this is only allowed if both structures have the same singleton frequency
      assert(numel(noise.freq)==1 && numel(datain.freq)==1 && isequal(noise.freq,datain.freq), ...
        'with both datain and noise of datatype ''freq'', only singleton and equal frequency bins are allowed');
    elseif ft_datatype(noise, 'timelock')
      % this is OK
    end
  otherwise
    ft_error('unsupported input data');
end

% select channels and trials of interest, by default this will select all channels and trials
tmpcfg = keepfields(cfg, {'trials', 'channel', 'tolerance', 'showcallinfo', 'trackcallinfo', 'trackconfig', 'trackusage', 'trackdatainfo', 'trackmeminfo', 'tracktimeinfo'});
datain = ft_selectdata(tmpcfg, datain);
noise  = ft_selectdata(tmpcfg, noise);

% restore the provenance information
[cfg, datain] = rollback_provenance(cfg, datain);
[cfg, noise]  = rollback_provenance(cfg, noise);

if ft_datatype(noise, 'timelock')
  if ~isfield(noise, 'cov')
    ft_error('noise covariance is not present');
  else
    noisecov = noise.cov;
  end
elseif ft_datatype(noise, 'freq')
  if ~isfield(noise, 'crsspctrm')
    ft_error('noise cross-spectrum is not present');
  else
    if istrue(cfg.realflag)
      noisecov = real(noise.crsspctrm);
    else
      noisecov = noise.crsspctrm;
    end
  end
end

% determine whether it is EEG and/or MEG data
hasgrad = isfield(datain, 'grad');
haselec = isfield(datain, 'elec');
hasopto = isfield(datain, 'opto');

if isequal(cfg.split, 'no')
  chantype = {};
elseif isequal(cfg.split, 'all')
  chantype = unique(noise.chantype);
else
  chantype = cfg.split;
end

% zero out the off-diagonal elements for the specified channel types
if numel(chantype)>0
  invnoise = zeros(size(noisecov));
  tra      = zeros(size(noisecov));
  for i=1:numel(chantype)
    sel = strcmp(noise.chantype, chantype{i});
    %noisecov(sel,~sel) = 0;
    %noisecov(~sel,sel) = 0;
    invnoise(sel,sel) = ft_inv(noisecov(sel,sel), 'lambda', cfg.lambda, 'kappa', cfg.kappa, 'tolerance', cfg.tol, 'method', cfg.invmethod);
    [U,S,V]           = svd(invnoise(sel,sel), 'econ');
    diagS             = diag(S)./numel(chantype);
    selS              = 1:rank(invnoise(sel,sel));
    tra(sel,sel)      = U(:,selS)*diag(sqrt(diagS(selS)))*U(:,selS)';
  end
  %invnoise = ft_inv(noisecov, 'lambda', cfg.lambda, 'kappa', cfg.kappa, 'tolerance', cfg.tol, 'method', cfg.invmethod);
  
else
  % invert the noise covariance matrix
  invnoise = ft_inv(noisecov, 'lambda', cfg.lambda, 'kappa', cfg.kappa, 'tolerance', cfg.tol, 'method', cfg.invmethod);
  [U,S,V]  = svd(invnoise,'econ');
  diagS    = diag(S);
  %sel     = diagS./diagS(1)>1e-12;
  sel      = 1:rank(invnoise);
  
  % the prewhitening projection first rotates to orthogonal channels,
  % then scales, and then rotates the channels back to (more or less)
  % their original MEG-channel representation
  tra      = U(:,sel)*diag(sqrt(diagS(sel)))*U(:,sel)';
end

prewhiten             = [];
prewhiten.tra         = tra;
prewhiten.labelold    = noise.label;
prewhiten.labelnew    = noise.label;
prewhiten.chantypeold = noise.chantype;
prewhiten.chantypenew = noise.chantype;
prewhiten.chanunitold = noise.chanunit;
prewhiten.chanunitnew = repmat({'snr'}, size(noise.chantype));

% apply the projection to the data
dataout = ft_apply_montage(removefields(datain, {'grad', 'elec', 'opto'}), prewhiten, 'keepunused', 'yes');

if hasgrad
  % the gradiometer structure needs to be updated to ensure that the forward model remains consistent with the data
  dataout.grad = ft_apply_montage(datain.grad, prewhiten, 'balancename', 'prewhiten');
end

if haselec
  % the electrode structure needs to be updated to ensure that the forward model remains consistent
  dataout.elec = ft_apply_montage(datain.elec, prewhiten, 'balancename', 'prewhiten');
end

if hasopto
  % the electrode structure needs to be updated to ensure that the forward model remains consistent
  dataout.opto = ft_apply_montage(datain.opto, prewhiten, 'balancename', 'prewhiten');
end

ft_postamble debug
ft_postamble trackconfig
ft_postamble previous   datain
ft_postamble provenance dataout
ft_postamble history    dataout
ft_postamble savevar    dataout