| Term | What it covers | Typical scale |
|---|---|---|
| Reaction time | Signal appears until your response begins | 200–300 ms for a visual signal |
| Movement time | Response begins until the movement is complete | Depends entirely on the movement |
| Response time | Reaction time and movement time added together | Always longer than reaction time alone |
| Reflex | Involuntary, routed through the spinal cord without the brain | 20–50 ms |
A reaction is voluntary, a reflex is not
The confusion worth clearing up first is that people call fast responses reflexes when almost none of them are. A true reflex, the knee-jerk your doctor tests with a rubber hammer, runs through a monosynaptic arc in the spinal cord. The signal never reaches your brain, no decision is made, and the whole loop closes in roughly 20 to 50 ms.
A reaction takes the long way round. Light hits the retina, the signal climbs to the visual cortex, something up there recognises it as the thing you were waiting for, a motor command is composed and sent back down. That detour is why a voluntary reaction to a light lands nearer 250 ms, five to ten times slower than the reflex.
This is also why the ceiling is where it is. You cannot train a reaction into a reflex, because they are different circuits, so no amount of practice takes a genuine reaction below roughly 100 ms. Anything quicker on a test is anticipation, which is why sub-100 ms starts are ruled false in athletics and why this site voids rounds under 80 ms rather than scoring them.
Where the measurement stops
The second confusion is more technical and it changes the number. Reaction time ends the moment your response begins, not when it finishes. Everything after that point is movement time, a separate interval, and the two added together are response time.
The distinction decides what a test is actually measuring. A sprinter's reaction time is the gap between the gun and the first force against the blocks; how long they then take to cover 100 m is movement time. Both matter, but only the first is a reaction, and quoting one for the other compares different things.
Researchers using electromyography split the interval finer still, into premotor time (signal until electrical activity appears in the muscle) and motor time (that activity until the limb actually moves). The premotor portion is the purely neural part. Our tests stop the clock on `pointerdown`, at the earliest instant the hardware can report, which keeps the measurement as close to reaction time as a browser allows: see the methodology for what that costs in practice.
Donders named the three types in 1868
The categories still in use were set out by the Dutch physiologist Franciscus Donders over 150 years ago, in a paper on the speed of mental processes. He built three tasks that differ only in what the brain has to do in the middle, then subtracted one from another to price each mental step. Discrimination cost him about 36 ms and choosing a response about 47 ms.
His three response types are the same three you can take on this site today. That is not a coincidence of naming: the tasks were designed to isolate exactly these steps, and nobody has improved on the design.
Where the types differ is in how much the middle step costs, and that cost is lawful rather than arbitrary. The guide covers Hick's law and the full picture in the ultimate guide to human reaction time.
| Donders' task | What it adds | Test here |
|---|---|---|
| A-task (simple) | Detection only: one signal, one response | Classic reaction time test |
| C-task (go/no-go) | Discrimination, but the response is predetermined | Go/No-Go test |
| B-task (choice) | Discrimination plus choosing between responses | 5-Choice reaction time test |
- 01Donders, F. C. (1868/1969). On the speed of mental processes. Translated by W. G. Koster, Acta Psychologica, 30, 412-431. Donders, F. C. (1868/1969). On the speed of mental processes. Translated by W. G. Koster, Acta Psychologica, 30, 412-431.
- 02Schmidt, R. A., & Lee, T. D. Motor Control and Learning: A Behavioral Emphasis. Human Kinetics. (Reaction time, movement time and response time as distinct intervals; premotor and motor fractionation.)
- 03Whitmer, E. Patellar and Achilles Reflexes, in A Mixed Course-Based Research Approach to Human Physiology. Medicine LibreTexts. (Monosynaptic stretch reflex latency.) Whitmer, E. Patellar and Achilles Reflexes, in A Mixed Course-Based Research Approach to Human Physiology. Medicine LibreTexts. (Monosynaptic stretch reflex latency.)