Biosemi photocell analysis

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Andria
Posts: 1
Joined: Fri Apr 07, 2017 10:01 am

Biosemi photocell analysis

Post by Andria » Fri Apr 07, 2017 10:18 am

Hello, I would like to test for any potential delays between my E-prime script and Biosemi trigger recording. I have used the Biosemi's photocell using the Ergo channel and have recorded a session. Can you please explain the steps required next in order to check the timing between E-prime and Biosemi using a third-party EEG software, e.g., EEGLab, as I understand that checking the timing cannot be done within the Actiview? Basically, I would like to check whether there is a delay between the two systems, and if so what is this delay in ms (in order to take it into account when analysing offline my EEG data) and whether it is constant across trials or drifts over time.

NTS
Posts: 1
Joined: Wed May 11, 2016 10:54 am

Re: Biosemi photocell analysis

Post by NTS » Mon Apr 10, 2017 2:40 pm

Hi Andria (and everyone else!),

Funnily enough, we are in a very similar position! We have run an Eprime script in which the display changes from a black to white screen at the same time as sending a trigger to the USB receiver. With the photo-diode connected to the Ergo port, we've got a data-file which we can open in BESA, with triggers visible (as expected), and a dramatic change in the output from the photodiode.
Due to current lacking in BESA skills, we are having difficulty in finding a precise way in determining the difference in the offset between the trigger, and the change in light at the photodiode. Does anyone have any advice?

Furthermore, can I ask anyone with expertise how this lag is generally rectified? Is it usual practice to code triggers to be sent prior to the onset of stimuli to account for the delay, is it a step that is usually done in post-processing with the data, or is there some way of altering acti-view to account for these differences?
Apologies for my naivety, and thank you very much in advance for any advice.

NTS

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