Unwanted triggers during ABR measurement

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NVanderStoep
Posts: 3
Joined: Fri Apr 13, 2018 10:56 am

Unwanted triggers during ABR measurement

Post by NVanderStoep » Fri Apr 13, 2018 11:19 am

Dear sir, madam,

recently we have acquired the ABR cord and input and have been able to record decent ABRs using a stimulus with fixed intensity and a single triggercode (sent synchronously with the click onset). We are now also able to present clicks with different amplitudes and different triggercodes. However, somehow we get additional triggers when checking marker quantities using Brain Vision Analyzer (BVA). The additional trigger codes (3 in screenshot) can often be found at the same time as a marker we actually want (10 or 11) or at the down slope a coupe of samples later (see attached picture). We always send a 0 after a stimulus trigger (e.g. 10, integer) and have a couple of milliseconds in between the 0 trigger and the next stimulus trigger. Using our setup (see below), trigger lines 9-16 are always high (rainbow colors in ActiABR) and stimulus triggers show up in triggerlines 1-8 (should all 16 trigger lines be 0 for triggers to work properly?).

I am not sure what we are doing wrong and how to solve the extra triggers. The extra triggers are problematic because they can overlap with other trigger codes we use. I hope you can help us out.

Kind regards,
Nathan

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Screenshot
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Ghost trigger.png
Ghost trigger.png (243.34 KiB) Viewed 203 times
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Some more information on our setup:
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Setup:
PC running Windows, Eprime 3.
PST Chronos box with IO expander for synchronization of sound onsets and trigger onset (works via USB, https://pstnet.com/products/chronos/).
BioSemi AD box and battery box with ABR Cord and ABR input (speedmode 9).
ActiABR for recording.
Brain Vision Analyzer for offline dataprocessing.
TDH-39 headphones connected to PST Chronos audio out.

Stimuli:
100 microsecond clicks (square wave) followed by 10 ms silence.
Interstimulus interval 27 ms, 37 Hz presentation rate.

Coen
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Posts: 898
Joined: Fri Mar 26, 2004 7:00 pm
Location: Amsterdam, Netherlands
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Re: Unwanted triggers during ABR measurement

Post by Coen » Fri Apr 13, 2018 5:01 pm

The problem may be that your trigger generator does not switch the separate lines (bits) within one sample period (62 microsecond). Try using trigger values that switch only one line, e.i. 1,2,4,8,16,32,64 and 128.

High (unconnected) lines 9-16 do not cause false triggers.

Best regards, Coen (BioSemi)

NVanderStoep
Posts: 3
Joined: Fri Apr 13, 2018 10:56 am

Re: Unwanted triggers during ABR measurement

Post by NVanderStoep » Fri Apr 13, 2018 9:35 pm

Thanks for the quick reply and suggestion. I will check whether this resolves the issue coming week and report back.

Kind regards,
Nathan

NVanderStoep
Posts: 3
Joined: Fri Apr 13, 2018 10:56 am

Re: Unwanted triggers during ABR measurement

Post by NVanderStoep » Tue Apr 24, 2018 5:04 pm

Hi Coen,

your suggestion resulted in zero unwanted triggers with 24000 samples. That is great. However, we need more than 8 trigger codes. Do you have any idea how we could make this happen?

Best,
Nathan

Coen
Site Admin
Posts: 898
Joined: Fri Mar 26, 2004 7:00 pm
Location: Amsterdam, Netherlands
Contact:

Re: Unwanted triggers during ABR measurement

Post by Coen » Wed Apr 25, 2018 3:14 pm

Use all 16 trigger lines or use D flip-flops to latch the triggers and sent them to the USB-receiver on the same moment (also see viewtopic.php?f=1&t=53&p=144 , sample clock is available on pin 32 of the trigger port)

Best regards, Coen (BioSemi)

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