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Re: Solutions for sending triggers in an AEP (EEG) experiment?
We send an identifier code using the parallel port (whose timing is not
critical) prior to the stimulus and then use a method identical to what
you describe to generate TTL triggers to accurately mark the onset of
We can then link each trigger pulse to a stimulus type by examining the
code value that occurred immediately before the trigger. In software,
the value of the trigger pulses can then be updated to reflect the
stimulus type so that each stimulus can be identified from the trigger
What software are you using to analyse the EEG data? If you're using
BESA to do the analysis, you can create conditional rules to use both
numerical codes in conjunction with trigger pulses to identify stimuli
without needing to post-process the data.
Daniel Oberfeld wrote:
I would like to know whether anyone here knows a 'better' method for
synchronizing acoustic stimulation and EEG recordings, more
specifically, for sending triggers accurately time-locked to the audio
signal to a multi-channel EEG system.
We use a NeuroScan SynAmps system (32 channels) for recording auditory
evoked potentials (AEPs).
As we have a MATLAB-based stimulus generation system, we currently use a
very simple triggering scheme. On one channel of our soundcard we create
a short 10 kHz tone burst coinciding with the onset of the auditory
event, and use a small circuit to convert this tone burst to a TTL
pulse, which is then sent to one pin of the parallel-port-like trigger
input of our EEG system.
This way we achieve submillisecond synchronicity between trigger and
sound, but we have only *one single* trigger code appearing in the EEG data.
Thus, some rather complex post-processing is needed to tell the EEG
analysis software which trigger event corresponded to which experimental
In principle, the NeuroScan system could receive an 8-bit trigger
allowing to code 256 different events. So it is tempting to use the
parallel port for this. But I'm afraid it is impossible to synchronize
the parallel port to audio under Windows XP...
Thus, my question is whether someone knows (and/or uses) a solution to
One way to do this would be via a "latched" type of "trigger box":
1) From Matlab, the parallel port is set to a specific code prior to
sound generation (i.e., before the trial starts)
2) The trigger box sits between the parport and the EEG system
3) The trigger code is routed to the EEG system only when an audio
trigger (like the one we use currently) is received by the "trigger box".
This would allow sending time-locked 8-bit trigger events. And it should
be possible to construct such a box using some standard components like
So did anyone ever come across such a device, either commercially
available or custom-made?
Or maybe there are also different solutions which I have not thought
All the best,
Department of Psychology
University of York
York YO10 5DD
Tel: +44 (0) 1904 43 3170