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Delay between stimulus presentation and trigger sent to port

Hello community

I am currently finding a delay between the time that the visual stimulus is presented (based on a photodiode) and when Psychopy writes a trigger to the EEG recording. I am comparing the difference between the Psychopy trigger and when the photodiode writes a trigger to the EEG recording.

I am using a simple experiment built in PsychoPy version v3.2.4 using the builder with the code components to send the triggers. The experiment is run using an Apple Mac Mini (2018), version 10.14.6.

The experiment displays a white fixation cross for 1 second, then a black screen for 1 second (defined using frames). When the white fixation cross is displayed Psychopy will send a trigger, through a USB2TTL8 Adapter made by Labhackers (

The photodiode used is part of the BlackBox Toolkit (BBTK; The photodiode is placed in the centre of the monitor (where the white fixation cross appears). The BBTK is programmed to respond to the white fixation cross, sending a trigger. This is accomplished by using the Digital Stimulus Capture And Response (DSCAR) option, the BBTK will generate a trigger (TTL1 out) with an RT of 0mS after detecting a visual stimulus on Opto 1 (the photodiode).

The PsychoPy trigger precedes the BBTK trigger, with the difference between the two is one of three values, 7.8, 9.7, and 11.7ms.

The monitor being used is the Dell S2419HGF at 120 Hz and is set to standard presets (Powerclock = off, response time = normal, game enhance mode = off and free sync = off).

I know that since MacOs 10.13 there is a delay, however, I am finding the same delay when using my Laptop which uses Windows.

When I run the same experiment on a windows device and the mini mac (with the same monitor but set to 60 Hz), both produce a delay between the two triggers. The mini mac delay is between 23 – 30 ms and the windows delay is between 25 – 39 ms.

If any has the advice to reduce this delay, it would be appreciated. Thanks in advance.