Reaction Time Test: How It Works
Reaction time is how long your nervous system takes to detect a stimulus and produce a response. A typical visual reaction is 200 to 250 milliseconds, and understanding where that quarter of a second goes explains most of what a test can and cannot tell you.
Where the time goes
| Stage | Approximate duration |
|---|---|
| Light reaches the retina and is transduced | 20–40 ms |
| Signal travels to the visual cortex | 30–50 ms |
| Recognition and decision | 50–100 ms |
| Motor command to the muscles | 20–40 ms |
| Muscle contraction and key press | 30–50 ms |
Roughly half the total is processing and half is transmission and mechanics. Only the middle stage is meaningfully trainable, which is why practice improves reaction time modestly rather than dramatically.
Typical figures
| Stimulus | Typical adult |
|---|---|
| Visual | 200–250 ms |
| Auditory | 140–160 ms |
| Touch | 150–170 ms |
Sound is faster than sight because auditory signals reach the brain through a shorter pathway with less preprocessing. This is why races start with a gun rather than a light, and why a start under about 100 ms after the gun is ruled a false start — it is faster than a human can genuinely react.
What affects your score
- Age. Reaction time is fastest in the early twenties and declines gradually thereafter.
- Fatigue. Sleep deprivation has a large effect and, importantly, one people underestimate in themselves.
- Alcohol. Slows reaction measurably well below any legal driving limit.
- Caffeine. A small improvement in alert people, a larger one in tired ones.
- Anticipation. Knowing roughly when the stimulus is coming shortens it; genuine unpredictability lengthens it.
- Input device. Screen refresh rate, input latency and connection type can add 20–80 ms that has nothing to do with you.
That last point matters for interpreting any online test: a measurable part of what you are measuring is your hardware.
Simple and choice reaction
A test with one stimulus and one response measures simple reaction time. Add options — respond only to green, or press a different key per colour — and it becomes choice reaction time, which is considerably slower and rises with the number of alternatives. Hick's law describes this: response time grows with the logarithm of the number of choices, which is why simplifying an interface speeds people up more than making them practise it.
Reaction time and driving
Perception-reaction time in driving is conventionally taken as about 1.5 seconds, far longer than a laboratory measurement, because a real driver must first notice something among many other things, recognise it as a hazard, and decide what to do. At 100 km/h a vehicle covers about 42 metres in that time, before braking begins to have any effect.
Reading your result
Take several attempts and use the median rather than the best. A single fast result may be an anticipated guess, and a single slow one a lapse of attention. Variability between trials is itself informative — high variability usually indicates fatigue or distraction rather than slow reflexes.