Inserting Triggers Relative to Existing Events

Module information
Modules	BESA Research Basic or higher
Version	5.2 or higher

Say you are interested in studying neural activity that precedes an existing trigger. This can be addressed in the following ways:

1. You are interested in a certain epoch that precedes the trigger event that is already coded. So, instead of entering a new trigger code, you could just add a new condition that works on the existing trigger code, and merely changes the epoch of interest. Presumably you have defined a condition where you are looking at the activity around the trigger event: some baseline before and then the activity that follows the trigger. Now, you can use the ERP-> Edit Paradigm to insert a new condition (CURRENT.code IS ... ). Then click on the “Epoch” tab and modify the epoch of this new condition – e.g. -1000 ms...-200 ms, also change the baseline epoch accordingly, then highlight the new condition in the list, and assign the epoch to the new condition. This condition will now average over the defined epoch that precedes the trigger.
2. If you really need a new trigger code, you can export the trigger codes as event file, using the menu ERP->Save Events As... This will save events as a text file. Then open this “*.evt” file in a text editor or e.g. in Excel. The file lists the elapsed time in microseconds, and the event type. Now, for all event types that you want to replace, subtract 1000000 from the number in the first column. Change the code of the trigger to a new value, e.g. 511. Get rid of the events that you don’t want to change. Save this file under a different name in text format, and open it again in BESA using ERP->Open Event File... (For example, you can use Excel in the following way: Open the file. Select all columns, and sort by the “Comnt” column. Delete all rows that contain trigger events that you don’t want to change. In cell E2, type the formula “=A2-1000000” and apply to all rows. Then copy this and paste values back into the A column. Delete column E. Change column C values (TriNo). Then save as text file, and re-load in BESA.)