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

Classification:  UNCLASSIFIED 
Caveats: NONE

This is almost exactly how I am setting up a lab right now.
We will have a line from the soundcard (and pre-amp) going into a digital
filter (likely an oscilloscope) and converting to TTL, which will go into
the interface box. Meanwhile, also send a data code via parallel port to
designate the trial type. They will not arrive together, but (a) it give you
both things in the trial, and (b) also tells the delay related to

One additional option (also considering myself) is to "steal" one or some of
the data lines on that parallel port for use by the auditory trigger.
The "data" sent by the parallel port is just a 5v high swing on one of the
data pins, according to the bit logic of the value sent; there are 8
involved, providing 255 possible values. In truth, you probably only need
about 4-5 of them (32 different stimulus combinations is plenty for me!). 
So what one could do is to get 2 DB25-to-8 BNC adapter cables (or make a box
for cheap). Imagine taking the 2 connectors and pinning them together, so it
goes DB25 to BNC - BNC to DB 25. Now take the last 3 (corresponding to data
pins 6,7,8), and don't connect them together; rather, on the side going into
the Synamps box, connect these to each of audio-to-TTL converters as you
mentioned. Voila, now you have the capability of several direct-feed audio
triggers and still retain parallel digital coding.


Dr. W. David Hairston
Research Fellow
Oak Ridge Affiliated Universities

Auditory Research Team, VAP, HRED
Army Research Lab
Aberdeen Proving Ground, MD 21005

Office: (410) 278-5925
Fax:    (410) 278-3587

-----Original Message-----
From: AUDITORY - Research in Auditory Perception
[mailto:AUDITORY@xxxxxxxxxxxxxxx] On Behalf Of Pádraig Kitterick
Sent: Friday, September 05, 2008 11:19 AM
To: AUDITORY@xxxxxxxxxxxxxxx
Subject: Re: [AUDITORY] Solutions for sending triggers in an AEP (EEG)

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

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.


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
> 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
York YO10 5DD

Tel: +44 (0) 1904 43 3170
Email: p.kitterick@xxxxxxxxxxxxxxxx
Classification:  UNCLASSIFIED 
Caveats: NONE

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