Reaction Time Tester

Wait for the colour to change, click as fast as you can, repeat. The delay before each go is randomised between about 1.5 and 5 seconds so you cannot learn the rhythm and anticipate it, clicking early is caught and voided rather than rewarded, and every attempt is kept so you can see your average and your best across a session rather than just the last lucky one.

What is being measured is a chain, not a moment. Light reaches your retina, the signal travels to visual cortex and then to motor cortex, a command goes down your arm, and a switch closes under your finger — and only then does the browser hear about it. Typical adult results land somewhere around a quarter of a second, and almost all of that is biology, but not quite all of it.

The rest is your hardware, and it is larger than most people assume. The clock here starts when the code changes the colour, but the pixels do not change until the next frame is drawn and pushed to the panel: roughly 17 milliseconds of waiting at 60 Hz, about 8 at 120 Hz, plus whatever processing the display itself adds before the light actually leaves it. A wireless mouse polls on its own schedule and adds a few more. The result is that the same person, on the same day, scores measurably better on a fast monitor with a wired mouse than on a laptop panel with Bluetooth — and the gap can be larger than months of genuine improvement. Compare against yourself on the same machine, or the comparison means nothing.

Two more caveats worth having. This is simple reaction time, one stimulus and one response, which is the least interesting kind: choice reaction time, where you have to decide which of several things just happened, runs substantially slower and correlates better with anything you would actually care about. And a single attempt is nearly meaningless — lapses of attention produce occasional very slow trials that drag an average around, so take the median of ten rather than the mean of three, and treat the fastest score of the day as a curiosity rather than a measurement.