The nitty-gritty on timing

Pixel Mode provides microsecond-precise triggering based on when the video signal reaches your display. As we have seen, the trigger begins 6 ms before the top left pixel is illuminated. 

On our CRT replacement monitors, which use a scanning backlight, exact timing of pixel illumination is location-specific. The backlight scans down the screen, creating a temporal offset between when the top left pixel and the bottom right pixel of the display are illuminated. This is the same behaviour you would observe in a true CRT monitor. Of course, this delay is always less than the screen refresh rate (8.33 ms/frame at 120Hz).

If you want to characterize the exact offset between your trigger (based on the start of the video frame), and the illumination of the specific patch of pixels where your stimulus appears, you will need to perform a one-time measurement using a photodiode placed over that stimulus’ position on the screen. This will give you the offset between the trigger and the spatially-dependent onset of your stimulus. 

If you choose to perform this measurement, ensure you are using the same refresh rate that is used in your experiment.