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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
The lights are probably working off the same controller, so actually are synchronous to within maybe the few nanoseconds it takes electrical impulses to travel through different length wires. – jamesqf Mar 20 '15 at 5:10
Can I add something to the question? What about audio delay in audio visual clip? – Devashish Das Apr 25 '15 at 5:03
@DevashishDas - your added question is about intersensory asynchrony; for the answer see the answer on this question:… – Christiaan Apr 25 '15 at 11:16

Short answer
I can be 75% sure that the lights are synchronized within 0.02 - 0.2 seconds of one another.

Interesting question, but hard to answer! Because it has been unanswered for quite a while I will try to formulate an answer that may provide an approximate value for the minimal asynchrony perceivable.

The temporal resolution of the visual system can be quantified in a number of ways. As you are referring to relatively simple stimuli, the critical flicker fusion frequency is probably the most relevant. At a certain critical frequency, a flickering stimulus will appear as a continuous stimulus. For example, the good old CRT screens can sometimes seem to be flickering. The mains is 50 Hz (US) or 60 Hz (Europe, Australia), and indeed the critical flicker fusion frequency is 5 - 50 Hz, dependent on the lighting conditions (Kalloniatis & Luu). Hence, stimuli separated by 200 ms (low lighting) to 20 ms (high lighting conditions) can be temporally resolved.

I am aware that your question is different, as you ask whether two stimuli on the retina can be perceived as being asycnchronous. Flicker fusion testing is mainly done with flash stimuli, i.e., a single stimulus per image, not two. However, from the various measures of temporal resolution of the visual system that have been used to date, the critical flicker fusion has the most conservative value, in that it provides the top-value of temporal resolution. Higher-level processing is much slower and values of around 4 Hz are obtained. For example, when subjects are tested on a stimulus of dots that are white when moving right and black when moving left, and they change directions/colors at a certain frequency, it becomes hard to tell whether the color change and direction change are synchronized at alteration rates of ~5 Hz and higher (Kalloniatis & Luu).

As to your question what the percentage chance is that stimuli separate by 20 -200 ms are separable, I can say that in general, psychophysical procedures determine the threshold defined as halfway between chance and 100% correct. Assuming a typical yes/no tasK (are the traffic lights synchronous yes/no?), chance would be 50% (guessing), and threshold would thus be defined as halfway between 50% and 100%, i.e., 75% correct.

- Holcomb, Trends Cog Sci 2009; 13(5): 216-21
- Kalloniatis & Luu, WebVision, chapter "Temporal Resolution" 2007

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