Reaction Time

What reaction time measures

Simple reaction time — the time between a visual stimulus appearing and your finger moving in response — is one of the most basic measures in cognitive psychology. The neural pathway is well understood: light hits the retina, signals travel through the optic nerve to the visual cortex, are processed, and a motor command is sent through the spinal cord to the muscles of your hand. The total time from stimulus to movement is what this test measures.

Typical reaction times

For a simple visual stimulus like the one used here, the average reaction time for healthy adults is approximately 200-250 milliseconds, with most people falling between 180 and 300 ms. The fastest recorded human reaction times in controlled lab conditions are around 100-120 ms; times below 100 ms almost always indicate that the participant anticipated the stimulus (i.e., guessed) rather than genuinely reacted to it.

Reaction time tends to be slightly faster in your dominant hand, in the morning, and when you are well-rested. It slows with age (a typical 70-year-old is ~30-50 ms slower than a typical 20-year-old), with fatigue, with alcohol, and with many medications. Caffeine, in moderate doses, can speed reaction time slightly.

Worked example

If you click in 215 ms on trial 1, 232 ms on trial 2, 198 ms on trial 3, 240 ms on trial 4, and 207 ms on trial 5, your average is (215 + 232 + 198 + 240 + 207) / 5 = 218.4 ms, and your best single trial is 198 ms. Both numbers are within the typical adult range.

Factors that affect your score

• Display latency: Cheap displays add 10-50 ms of input lag. A 60 Hz monitor refreshes every ~16.7 ms; a 144 Hz monitor every ~7 ms. For a "true" measurement you'd need a calibrated CRT or a high-refresh-rate monitor.
• Input device: A wired mouse is fastest (~5 ms); a wireless mouse adds 5-15 ms; a touchscreen adds 10-30 ms.
• Anticipation: If the stimulus always appears after exactly 3 seconds, you'll start to anticipate and click at ~2.9 s — measuring expectation, not reaction. We randomize the delay (2-5 seconds) to prevent this.
• False starts: If you click before the green appears, it counts as a false start. Real reaction times below 100 ms are physiologically implausible.

Why this matters

Simple reaction time correlates weakly but consistently with general cognitive function and is one of the few cognitive measures that declines steadily across the adult lifespan. It is also a sensitive (though non-specific) marker of fatigue, intoxication, and certain neurological conditions. As with everything on this site, the test is informational, not diagnostic.

Practicing this specific test will improve your score by maybe 10-30 ms over a few sessions as you calibrate to the timing — this is a practice effect on the task itself, not a general improvement in reaction speed.