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I'm stopped at a stoplight and, with nothing to do, my thoughts wander to the timed don't-walk sign that governs any pedestrians who might wish to cross the street I'm driving along. I can see two such signs, one on my side of the cross street, and one across the way. They are synchronized: each counts down the seconds at exactly the same time as the other, as far as I can tell, lighting up as "5", then a second later as "4", etc. Nothing's wrong with my ocular nerve or brain, as far as I know. Can you fill in the blanks please? I can be …% sure that the lights are synchronized within … seconds of one another. (Not something trivial like "100" and "1", of course.)

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I'm only familiar with human perception of audio-visual asynchronies, but in that case it is highly context-dependent and varies by individual. Under best-case scenarios, humans can detect an audio delay of 100 milliseconds. Under more normal conditions, 200-300 milliseconds is the threshold for perception. I would imagine we'd be better at detecting asynchrony between two visual stimuli in general, but the fact that you can't look at both lights at once probably makes it harder to detect asynchrony. So it's probably hard to know what the result is on balance without testing it specifically. –  seaotternerd Dec 9 '13 at 23:10
    
Thanks for the info, @seaotternerd. I figured it varies widely by individual, which is why I asked for a confidence interval rather than a straight answer. –  msh210 Dec 11 '13 at 7:55

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