Hi Tim,
Please note that the OnShow function is called AFTER the display stimulus is intended to be shown. This means that very heavy computations in the OnShow function can significantly alter the accuracy of the RT (as this is based on the current time against the Stimulus OnsetTime).
As a rule of thumb, I would say that a calculation that takes more than 8 ms is unwanted, anything below should be fine in 99.9% of the cases. You can measure how long your function takes to see if it would alter the accuracy very much.
Best regards,
Erwin
