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

[Pádraig Kitterick <p.kitterick@xxxxxxxxxxxxxxxx>]

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