Visualization with BESA Plot in FieldTrip

BESA Plot is a free tool created by Patrick Berg (BESA GmbH) for visualization of MEEG data. Initially it was meant to be complementary visualization tool to BESA Research, however, it could be used in more general context. In this document we demonstrate the possibility of using BESA Plot as a visualization tool in the Matlab based open source package for MEEG data analysis FieldTrip.

Since BESA Plot can be started with command line arguments, it can be called directly from Matlab and the required input parameters (e.g. data file, settings and plotting modalities) can be committed as such arguments. There are four files necessary for starting BESA Plot from Matlab and visualizing the corresponding data: 1) a data file (e.g. test1.avr), 2) a file containing the channel labels and coordinates (e.g. test1.elp), 3) a control file containing the plotting commands for BESA Plot (e.g. test1.bpctrl) and 4) a dataset file containing general information about the data file (e.g. test1.avr.dataset). These files should be generated dynamically in Matlab in order to be able to plot the Matlab data in BESA Plot.

Generating the data file

First of all we need the data which are going to be saved as an avr-file. This is usually a Matlab structure generated in FieldTrip with the function ft_timelockanalysis. Its general form is something like this:

      avg: [147x271 double]
      var: [147x271 double]
     time: [1x271 double]
      dof: [147x271 double]
    label: {147x1 cell}
   dimord: 'chan_time'
     grad: [1x1 struct]
      cfg: [1x1 struct]

In order to create a data file from that structure one needs the function besa_save2Avr which can be found in the FieldTrip installation in the folder external\besa or can be downloaded directly from BESA’s ftp-server here: [1]. Just unpack the archive and add the path to the Matlab path (e.g. addpath(‘D:\Path\to\MATLAB2BESA’)). Use the function like this:

status = besa_save2Avr(cfg.datapath, DataFile, data_matrix, ...
        time_samples, channel_labels, data_scale_factor, time_scale_factor);