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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 the stimulus.

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 alone.

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.

Padraig

Daniel Oberfeld wrote:
Dear list,

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 condition...

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 this problem?

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 74HC75.


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 about yet...?


All the best,

Daniel


-- Pádraig Kitterick Graduate Student Department of Psychology University of York Heslington York YO10 5DD UK

Tel: +44 (0) 1904 43 3170
Email: p.kitterick@xxxxxxxxxxxxxxxx