[open] Triggers for EEG
edited December 2013 in OpenSesame
I need some help to create my experiment: I would like to present on a computer screen a cross, followed by a picture, followed by a cross etc (80 pictures in total). For each picture I need a trigg (I have already downloaded the parallel port trigger) which have to appear at the begining of each picture. Here is the details of what I have done and I'd like to have your advices and opinion of whether it is correct or not:
- Because of the recommandations I have read on your website I have put a trigger at the begining of my experiment to "have all the pins set to zero" with a value = 0; duration = 500 and port adress = 0 x 378.
- Then I have created a loop with the trial sequence: cross fixation; picture; parallel port trigger with a value of the code associated for each picture I have created; a duration at 0 and a port adress of 0 x 378.
Are these two steps correct?
Here are some more questions:
- I am not sure whether it is usefull to put the trigger after the pictures in my loop (or can I put it before?) and the other problem I have is that when two pictures with the same code followed one after the other the trigg appears only for one picture and not the other.
- I don't understand wich duration I should enter to make the trigger appears at the begining of each picture because for now my trigger appears after the picture.
Thanks a lot for your answer and help.
Welcome on board, I had similar questions when I started using OpenSesame for EEG experiments.
I think you'll find the most detailed explanation in Lotje's answers in a thread I started, see here.
You should send the trigger for the pictures after the picture/sketchpad item to avoid screen refresh issues (see cited post). However, you want the trigger to appear at the beginning of the picture presentation. To achieve this, you set the sketchpad duration to 0 and the trigger duration to the value of the actual presentation time of the picture. An item of 0 length takes the duration of the item following next in the trial, which would be the trigger in your case.
The thing to get behind is that triggers also have a length, although in the EEG they appear as an atomic event. But in reality a trigger is just a state (certain pins in the parallel port are at 5V or not) and that state can last some time. What the EEG recording device notices is only a change of that state. And exactly at that point the visual trigger line in the EEG appears. Therefore it is also a good advice to send a zero trigger at the beginning of the trial. That way you can be sure that all previous states are nulled. Internally, the parallel port trigger item does this as well after each trigger sending to avoid ghost triggers (see my previous post in the cited thread). Before we had this great plugin (thanks to the developers!), we needed to script an additional 0 trigger as well but with that plugin you do not need to do this any more.
Johannes (a.k.a. absurd)
Many thanks ++++++++ absurd!!