Creating ERPs with BESA

Averaging event related potentials is easily done with BESA using either trigger channels or a log file to define event times. Events on trigger channels are detected by defining Schmitt triggers. First select an event from the channels display:

then choose ERP->Create Triggers From MEG/EEG... which brings up the following dialog:

Hitting the View Current Settings button brings up a graph of the selected trigger and allows for interactive setting of the trigger parameters:

The green line indicates how long the voltage needs to stay above the indicated level in order to be considered a trigger. Thus it needs to extend beyond where the voltage drops back to zero. The values which can be interactively set from this view settings graph are listed in the Trigger Settings frame of the Schmitt Trigger settings dialog (shown above) and are Threshold: 0.5; Limit during dead time: 0.5; Dead Time (prior to trigger): 190; and Return Time: (Below Limit) 3175.  Hitting the 'Add to List' button adds these values to the triggers list. Next hitting the 'Search Selected' starts the search through the data for the defined trigger which can take a couple of minutes and results in the total number of trigger events found being listed in the last column under the # sign and with a narrow black line in the events bar of the channel display corresponding to each event. These events can be saved/recalled into/from an ascii file by choosing ERP->Save Events As.../Open Event File.  By creating or modifying one of these text event files one can transfer events form a log file. 

Averaging data segments to create  ERPs can use multiple triggers and complex relationships between them.  Use menu item ERP->Edit Paradigm to create and edit how ERPs are computed. The following Paradigm dialog is brought up by this menu choice:

There are several tabs used to set up the paradigm. The trigger was set up as discussed above so the Condition, Epoch, Filter , Artifact rejection, and Average settings must be made.  Conditions are given names and can be as simple as a single trigger event  as shown above or complex boolean combinations of triggers (see BESA manual or help file for details). The Epoch tab sets up the times relative to the event which will be used in computing the ERPs:

Here the ERP is defined to extend from 100 ms prior to the event to 1000 ms post event.  The Artifact Rejection column indicates that the entire 1.1 seconds will be examined for artifacts when deciding whether to use that epoch in the average.  Next the filter tab can be used to enable and set high and low cut filters if desired:

The artifact tab which comes next is where the weeding out of bad channels and epochs is done. Artifact rejection can be turned off, set to fixed thresholds or interactive scanning can be done to determine which channels/epochs to reject:

The interactive Artifact Scan Tool is shown above and indicates that only 31 out of 79 epochs pass the thresholds. Noisy channels can be removed by using the mouse to move the horizontal line down. When this is done the accepted number of epochs is updated so that a decision can be made about removing these channels. The top right corner shows the epochs with the largest amplitude (or gradient depending on the choice of  Sort by button) and which of these epochs to include/exclude can be made by sliding the vertical line right/left.  After these adjustments click on the Average tab:

The range for computing the average ERP can be selected along with a constraint on which events to use.  Hit the Average button and the record is scanned and the accepted epochs are averaged as shown below: