Programming Tutorial:Working with the FieldTrip buffer

Getting the data in Matlab

The FieldTrip buffer is a multi-threaded and network transparent buffer that allows data to be streamed to it by BCI2000, while at the same time allowing a seperate MATLAB session on the same or another computer to read data from the buffer for analysis. Besides writing the data, BCI2000 also writes the changed status variables as events.

To read from the FieldTrip buffer, you should have a copy of the FieldTrip toolbox, or at least a copy of the FieldTrip fileio module. Subsequently you can do something like this

% read the header for the first time to determine number of channels and sampling rate
hdr = read_header(filename, 'cache', true);

count      = 0;
prevSample = 0
blocksize  = hdr.Fs;
chanindx   = 1:hdr.nChans;

while true
  % determine number of samples available in buffer
  hdr = read_header(filename, 'cache', true);

  % see whether new samples are available
  newsamples = (hdr.nSamples*hdr.nTrials-prevSample);

  if newsamples>=blocksize

    % determine the samples to process
    begsample  = prevSample+1;
    endsample  = prevSample+blocksize ;

    % remember up to where the data was read
    prevSample  = endsample;
    count       = count + 1;
    fprintf('processing segment %d from sample %d to %d\n', count, begsample, endsample);

    % read data segment from buffer
    dat = read_data(filename, 'header', hdr, 'begsample', begsample, 'endsample', endsample, 'chanindx', chanindx);

    % create a matching time-axis
    time = (begsample:endsample)/hdr.Fs;

    % plot the data just like a standard FieldTrip raw data strucute
    plot(time, dat);

    % ensure tight axes
    xlim([time(1) time(end)]);

    % force Matlab to update the figure
    drawnow

  end % if new samples available
end % while true

Closing the loop, writing a control signal from Matlab to BCI2000

To close the loop, you have to write teh control signal back to BCI2000. Since the FieldTrip buffer can only hold raw data, the control signal cannot be written as data. Instead, the control signal is written as an event. This is easily demonstrated if you run the Feedback Demo Task.

event_up.type  = 'Signal';
event_up.value = 1;

event_down.type  = 'Signal';
event_down.value = -1;

event_null.type  = 'Signal';
event_null.value = 0;

You can write the events to the buffer according to the following example code:

filename = 'buffer://localhost:1972';
write_event(filename, event_up);
% ... the cursor will move up
write_event(filename, event_down);
% ... the cursor will move down
write_event(filename, event_null);
% ... the cursor will stay at a constant value

The control signal in BCI2000 remains at a constant value as long as you don't write another event with another control signal.

Getting the data in Matlab

Closing the loop, writing a control signal from Matlab to BCI2000

See also