01 / 08
How Reaction Time Works — The Three Stages of Perception, Decision and Movement
From the moment a signal reaches your eyes or ears until your finger moves: the stages that make up reaction time and the factors that lengthen each one.
Reaction time is the time it takes from the moment a stimulus appears to the moment a response begins. Even though pressing a single button looks like a simple action, it involves a chain of processing stages.
Stage 1: Perception — until the signal reaches the brain
Light is converted into an electrical signal through chemical reactions in the retina's photoreceptors and travels along the optic nerve to the brain's visual areas. For sound, the hair cells of the cochlea convert mechanical vibration directly into an electrical signal. Because of this difference in conversion, sound signals generally reach the brain faster.
The perception stage is also affected by stimulus intensity. Bright, large signals and loud, clear sounds are detected faster. That's why this site flips the entire board and uses a clear 1,000 Hz beep.
Stage 2: Decision — choosing what to do
After detecting the signal, the brain decides "Should I press now?" In a simple reaction task there is only one signal, so this stage is very short. But when signals must be distinguished, as in Go/No-go, or when there are multiple options, decision time increases. The relationship between the number of options and decision time is described by Hick's law.
Stage 3: Movement — until the command reaches your finger
A command from the motor cortex travels through the spinal cord and motor nerves to the muscles, which contract and actually press the button. This stage is shorter when your muscles are primed, so during the test it's best to rest your finger lightly on the button or spacebar.
Device time is mixed into the measurement
The reaction time shown on screen is the three human stages plus device latency. It includes the time your monitor takes to draw a new frame, the time your mouse or keyboard takes to deliver input to the computer, and the time the operating system and browser take to process the event. That's why the same person gets different results on different devices.
In short
- Reaction time = perception + decision + movement (+ latency of the measuring device)
- The stronger the signal, the fewer the options and the better prepared you are, the faster you react.
- Measuring several times on the same device and comparing is the fairest approach.