Dropped frames but Window.callOnFlip() works fine?


Is it possible that Window.callOnFlip() is called without an actual buffer flip/frame shown?

I’m presenting flicker stimuli and sending triggers to an EEG system every 4th frame/flip. The triggers suggest that the timing is more or less precise (~15 Hz on a 60 Hz monitor) but for a (apparently random) subset of runs, my EEG data suggests that the flicker was actually 7.5 Hz (so 8 frames). However, the trigger latencies look okay, with triggers every ~0.06 s. To me, this suggests that we somehow miss a frame and wait for 4 more frames, while the triggers (using Window.callOnFlip()) are sent correctly every 4 frames.

I wonder if this is possible? Or if there is something else going on? I’d be happy to share code when I’m back in the office.

Unfortunately, I did not log any information on dropped frames, so I can’t gp back and check what PsychoPy has to say about this.

Any input welcome!