Latency in stimulus onset of ERPs using OpenSesame synth + visual stimuli (?)
Hi, everyone!
I used OpenSesame (OpenSesame 4.0.13-py311-win64-1, psycho) to present visual stimuli in an EEG experiment. I set an audio trigger (a synth) before the visual stimulus presentation to mark the EEG data (by connecting the audio signal directly to the EEG system).
Now, I'm analyzing the EEG data by computing stimulus-locked ERPs. I noticed that the stimulus onset (extracted from the data of the audio signal) might not be properly adjusted to 0. Usually, the first negative component of the ERP (first uphill) appears some milliseconds (80-100 ms) after the stimulus onset, but not before (as it happens here).
This is how I determine the stimulus onset from the audio signal (the sample right before the signal pulse):
I read in the OpenSesame documentation that "the duration of the synth may be set to 0ms, in order to advance directly to the next item (e.g., a sketchpad), while the sound continues playing in the background." So, in this case, the audio should sound at the same time as the visual stimuli (i.e., both audio and visual stimuli should be synchronized, right?).
Am I missing something? Is there any delay in OpenSesame that I'm not aware of?
Any help would be wonderful!
Thank you in advance!
Irene