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Solutions for sending triggers in an AEP (EEG) experiment?



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

--
Dr. Daniel Oberfeld-Twistel
Johannes Gutenberg - Universitaet Mainz
Department of Psychology
Experimental Psychology

Staudingerweg 9
55128 Mainz
Germany

Phone ++49 (0) 6131 39 22423
Fax   ++49 (0) 6131 39 22480
http://www.staff.uni-mainz.de/oberfeld/