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Screen freezes after response at random trials

OS: Win10
PsychoPy version : 2020.1.3.
Browser: Mozilla
I have an experiment that I will be putting online. It is a modified Posner paradigm, nothing too complicated. I have a video as a central cue and grating appearing in the 4 corners of the screen (I use images, instead of the grating component as the online version does not support it yet. All works great when I tried it both in builder and online with 3 blocks, i.e. 36 trials, it all works fine. However, both online and offline, when I increase the set to the proper number of blocks I need, the experiment crashes at random points; sometimes at the end, sometimes directly at the beginning. Because it’s not isolated to the online version, I am posting this here. The location of the grating changes every repeat, along with the video cue. The freezing seems to happen either after a response or before even the response is allowed. It is not clear to me which one, as I have to press Esc to exit.

What did you try to make it work?: I read that the online version has issues with videos that repeat, so I added the code components that reset and stop the videos. So that all worked fine. But I still get the random freezing. (on the builder version on PsychoPy I do not have the code components, because there it is handled properly). Memory on Mozilla does not overload, I checked, and there are no errors anywhere. Neither in the js console on the browser nor when I run it in PsychoPy either coder or builder. It all just seems like it gave up on life.

I have worked with Psychopy before, both using the builder and the coder, but this behavior I have not seen. Any help will be appreciated.

Any other information I can give to help, let me know.

Are you using H.264 encoded MP4 files with AAC audio encoding. d.govan?

So no errors in the runner or online?

Hi. thanks for getting back to me. The video doesn’t have audio, and it was created with MatLab, and as far as I can tell it is H.264 encoded MP4.

No errors anywhere. Unless I am missing something. I am new to js so I asked a colleague to run it with me and he was not able to see anything unusual in the online console. and the runner just says Experiment has ended as I had to press Esc to leave. Absolutely nothing else, as if it has run properly. I am not sure, but I don’t think it is the videos, as once I increase the number of blocks, it sometimes even crashes before it has run through the 3 blocks I was testing with at the beginning when it worked fine. I even asked a friend to run it on her laptop through the Builder, just in case it was my laptop, and she had the same issue, even saying that the second time she tried to run it, it crashed faster than the first time.

Difficult to say what’s going on. Share it with me if you’d like me to pilot it. A crash without an error message seems strange.

Best wishes,


We were able to figure this out in case someone else is in my place. While I have the keyboard response to have an end time of max 2s even if there was no response, I had not told that to my fixation screen, which just never ends. Thank you so much to @wakecarter for looking at my code ! I do not know how I didn’t realise this!