I'm not sure it is 500Hz, just that it's somewhere in that range (trying to look it up I'm finding a lot more on comprehension than just literally noticing that something changed at all), and that comes from having read about studies that did shit like insert a single wrong frame in a video and attempt to determine the threshold at which people no longer notice that there was an incongruous frame. Perhaps to put it another way, around 2 milliseconds is ostensibly the shortest time something can be visible for someone to notice that there was something there, since as someone else pointed out we don't actually see in "frames" there's just a limit to how quickly information can by absorbed and processed.

Reading further, just below 5ms of having an image visible is the minimum time at which trained professionals start identifying outlines correctly, and ostensibly around 13ms is where the average person starts being able to see and recall content