Is it possible to move event label locations in EEGLAB after data has already been collected?

Question

I was recently added onto a project to analyse EEG data, only to discover that data collection had been faulty.
The experiment was run using EPrime for stimuli presentation, with a BioSemi Active 2 system for recording of EEG. Triggers were sent from E-Prime at stimulus onset, and were supposed to have been sent at response. However, due to the nature of the experiment, stimuli did not disappear at response, which somehow affected trigger timing. Triggers for response only registered after the stimulus disappeared from the screen. This means that every response event label in the EEG data is postponed by a couple hundred milliseconds, differing on a trial-by-trial basis. RT data WAS recorded accurately, however, and we have all that data in an .edat file (which can be extracted as excel or whatever else).
My question now would be: is it possible to adjust the event label locations in the EEG data? We use the EEGLAB toolbox in Matlab for analysis. I was thinking that it may be possible to 'sync' an excel file of RTs with the corresponding event in the EEG and run a script to do all the processing. Not sure how to go about it though, if it is possible in the first place. Help is greatly appreciated, thank you! (and if this is not the correct forum to ask, let me know and I will delete)

Answer 1

It is definetely possible to edit event field values. You can try to do this by using the function pop_editeventvals (either via command line or GUI > Edit > event values) which requires an EEG structure and 'key' 'value' argument pairs, eg.:

EEG = pop_editeventvals(EEG,'changefield',{34 'latency' 320.4});

will change latency of event 34 to 320.4 msec. Alternatively, loop through or index the corresponding events and directly change absolute latency within the event field or the relative-to-epoch-locking eventlatency within the epoch field.

Answer 2

Just for record, you can import E-Prime logfile into EEGLAB and match your timing that way.

If you use LPT triggers in Biosemi the timing of you triggers is best from this source. But when in trouble, you can try this out. Just make sure your timing accuracy.