Odd Visual P300 morphology

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mckallister
Posts: 1
Joined: Thu Jun 08, 2006 8:25 pm
Location: North Dakota State University

Odd Visual P300 morphology

Post by mckallister »

We recorded a couple people with our new 168 channel ActiveTwo, Mark II system. We are executing data pre-processing in BESA 5.1.6. Our P300 paradigms are very simple and have elicited strong and reliable visual and auditory oddball responses in the past.

However, with this system, when the ERPs are averaged in BESA, we end up with what appears to be an extension of the baseline for about 100ms past the trigger event around which we are epoching, in all visual conditions only, across all channels.

This effect is persistent across other visual stimulus paradigms we have piloted with the system, including a Posner-style cueing task, a scene analysis task, and an IOR task. Data files opened in BESA, either collected from different systems or manually generated, do not demonstrate this phenomenon.

We have extensively tested trigger:stimulus variability comparative analysis of our Biosemi data against an oscilloscope and have found there to be a variation of no more than +/-1ms at our 2048Hz sampling rate. We collect data in a new Lindgren EM shielded enclosure, which has demonstrated strong attenuation of noise. We have also verified all of our electrode locations between our cap and the BESA montage.

We are at a loss for why our Biosemi data shows the extended baseline effect across visual paradigms, but not auditory.

Do you have any suggestions as to why the data would be treated differently, as to why the effect is occurring, and as to how we can continue troubleshooting it?

We're going to also begin coordinating with Cortech Solutions and BESA Support for assitance.

Thanks,
Jake

Lloyd
Posts: 26
Joined: Fri Mar 26, 2004 7:46 pm
Location: Wilmington, NC, USA
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ActiveTwo trigger port timing is extremely reliable

Post by Lloyd »

Hello, McKallister. I am Lloyd Smith, and I installed the NDSU system, so I am responding to your post.

It is difficult to speculate about the cause of the problem without knowing more about the problem and the scenario in which the data were collected.

Can you send me a sample average data file or a screen shot of real data illustrating the problem?

The fact that you say auditory stimuli do produce responses at the times you would expect leads me to suspect the visual stimulus apparatus. You mention that you know the variability of trigger timing to be low, but how about absolute timing delay of visual images with respect to trigger events?

I would suggest using a scope or your ActiveTwo system with the special photocell interface to record some test data with one channel (the photocell) plus the trigger on the trigger input port. Be sure to perform this test on the actual computer and monitor used to deliver the stimuli to the subject. At least some dual output video cards demonstrate significant time delays on secondary displays. Also, I would strongly suggest performing the timing test with the actual experiment / stimulation paradigm used to generate the P300 data, since the time delay could be a function of the way the code is written. If you can send a sample data file or a screen capture including a photocell signal and trigger events, that would be helpful in narrowing down the problem.

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