Letters presented for the first few trials are faster than expected

Hi All!

Currently, we are trying to display some letters (say “K”) for 150 ms on top of a 1-second black dot in the center of the screen (following is an example block routine, and the block routine is wrapped in a loop to go through an excel file of stimuli). We are using time (s) rather than frame (N) for timing.

Everything worked fine locally (when it’s running in Python). But when we converted to online JS scripts and piloted it on Pavlovia, the first several letters were displayed for only around 100 ms (basically, it was flashing faster than expected, and when using codes to quantify the duration, it was indeed faster).

And this problem is NOT consistent across different web browsers. When we use Chrome and Firefox, the issue appears. But when we switch of Safari, the stimuli are presented with the correct timing. We figured that the refresh rate shouldn’t be the issue, because we are using time (s) rather than frame (N)? But we are not sure.

We’ve exhausted options, and I wonder if anyone has encountered similar issues before, where the duration of stimuli presented online is sometimes shorter than specified.

Thanks a lot in advance!