hl1_to_events.m

%
% Insert events based on transitions on a high-level input channel.
%
% As with all code from the internet, verify that it works for your unique situation.
%
function EEG = hl1_to_events(EEG)
  % Find the index of the high-level (trigger) channel.
  hl1_index = eeg_chaninds(EEG, 'HL1');
  hl1 = EEG.data(hl1_index,:);

  % My high-level channel is in microvolts. Divide by 1M to make it volts, and easier to work with.
  hl1 = hl1 / 1000000;
  % Values above 2.5v are considered 'high'.
  thresholded = hl1 > 2.5;

  % Take the derivative to find transitions in the signal.
  % High to low transitions will be -1, low to high will be +1.
  d_th = diff(thresholded);

  % Find the indexes of the transitions, and converts to seconds
  % based on sampling rate of the EEG signal.
  % For high-to-low transitions, use find(d_th < -0.5)
  % For low-to-high transitions, use find(d_th > 0.5)
  hl1_onset_indexes = find(d_th < -0.5);
  hl1_onsets = hl1_onset_indexes / EEG.srate;

  % Insert an event for each high-level onset.
  % This can be slow if there are a lot of events.
  for onset_ndx = 1 : length(hl1_onsets)
    latency = hl1_onsets(onset_ndx);
    EEG = pop_editeventvals(EEG, 'insert', { 1 'EVENT' latency 0 });
  end

  % Have EEGLAB check that the events are consistent.
  EEG = eeg_checkset(EEG, 'eventconsistency');
  EEG = eeg_checkset(EEG, 'checkur');

  eeglab redraw;
end