Seeing Delayed Components

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frio007
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Joined: Fri Dec 08, 2017 1:04 am

Seeing Delayed Components

Post by frio007 »

Last week I exported some of our ERP data from a large randomized control trial we are conducting. This is the first study we have used this new Bio Semi Active2 recording box. We collect brain data during a simple dot probe task time-locked to the presentation of both the cues (faces up for 500 ms ) and the probes (arrows up for 500 ms). We have triggers for both the faces and the probes. When I was looking at grand average waveforms (we reference to all 64 channels) and heat maps, I was able to observe the expected early visual ERP components (P100, N170, N2), but they seem to be delayed by approximately 50 to 100 ms. For example, the P100 is maximal at a time window of 170 to 200 ms instead of the typical 80 to 120 ms presentation that is observed using this same task across many labs including our own lab in previous studies.

We use E-Prime to send the triggers to the box and from those output files, the timing of triggers all seems correct with an offset delay of either 0 or 1 ms for both faces and probes as indicated by the E-Prime output files. What would I look for in EEG data acquisition that would indicate a problem or delay in triggers being sent? Otherwise, I do not know additional steps to troubleshoot. Could this be a delay is the communication between E-Prime and BioSemi? How would I check this?

Coen
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Re: Seeing Delayed Components

Post by Coen »

Check the timing between the visual stimuli and the triggers with the additional Photo Sensor (and ERGO input) connected to the AD-box. Also see https://www.biosemi.com/accessoires.htm and https://www.biosemi.com/faq/review_trigger_timing.htm.

If you find a timing difference between the Photo Sensor response and the received triggers, then this difference is caused by the E-prime setup (should be corrected). If you do not find a significant timing difference, then the EEG data measured with the BioSemi setup is correct.

Best regards, Coen (BioSemi)

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