Reaction time breaks into sensory conduction, central processing, motor conduction, and muscle activation

What it is

Reaction time (RT) is the interval from stimulus onset to the start of the response (not its completion), and it decomposes into four sequential stages: sensory conduction (the stimulus travels from receptor to central nervous system), central processing (identifying the stimulus and selecting a response), motor conduction (the decision signal travels outward to the effector muscle), and muscle activation (the muscle builds enough tension, after receiving the command, to produce an observable movement). This decomposition comes from the classic information-processing model of RT. Card, Moran, and Newell's Model Human Processor offers a widely cited reference for the magnitudes involved: roughly a 100ms cycle for the perceptual processor, 70ms for the cognitive processor, and 70ms for the motor processor — but these are reference cycle times, not a fixed delay every reaction is guaranteed to incur.

Why it happens

The decomposition holds because each stage corresponds to a distinct physiological or cognitive process that can be manipulated independently: increasing stimulus intensity mainly shortens sensory conduction, adding more response alternatives mainly lengthens central processing, and changing the responding limb or movement amplitude mainly affects motor conduction and muscle activation. Whether the four stages run strictly in series or overlap (for instance, whether the motor system can begin pre-activating before sensory input is fully processed) varies across tasks and modalities — but as an analytic tool, the four-stage split turns the vague observation "the response got slower" into a diagnosable question of which stage got slower, pointing to a specific point of intervention.

Studying it

The four stages are hard to measure directly — most experimental setups can only record the total interval from stimulus onset to the observable response (a keypress, a vocalization), not how many milliseconds sensory conduction alone took. Researchers typically manipulate one variable at a time (only stimulus intensity, or only the number of alternatives) and observe how much the total RT shifts, inferring which stage that manipulation mainly targets. Electromyography (EMG) can resolve the boundary between motor conduction and muscle activation more finely, separating the moment central processing finishes from the moment the muscle actually starts generating force.

Where it stops holding

The four-stage model is a discretized simplification of a continuous neural process; real sensory, cognitive, and motor stages overlap and can pre-activate ahead of schedule (the motor system may begin pre-activating before a decision is fully settled). Treating the four stages as a strictly serial, non-overlapping pipeline overstates how much a single variable should shift the total RT. The decomposition also only covers the interval up to the start of the action — the execution of the movement itself and the time needed to actually reach the target fall outside these four stages.

Applying it

When you need to shorten a user's response to a prompt, first diagnose which stage the bottleneck most likely sits in before deciding what to change: if the meaning of the prompt itself is hard to identify (an ambiguous icon), the problem is in central processing, so simplify the meaning rather than making it appear faster; if the prompt's physical intensity is too weak (low contrast, low volume), the problem is in sensory conduction, so increase the stimulus's physical salience; if the response action itself is cumbersome (a long arm movement), the problem is in motor conduction and muscle activation, so shorten the movement distance or shift the action to a smaller muscle group. How to check: vary stimulus intensity, task complexity, and movement amplitude one at a time, and compare how much each shift moves the total RT, to confirm the change actually hit the stage you intended to target.

Related

• Same group: A8.01.2 Reaction time and movement time are two independent stages that must be measured separately · A8.01.3 Simple reaction time: one stimulus, one response · A8.01.4 Choice reaction time: RT grows with the number of alternatives · A8.01.5 Discrimination reaction time: the stimulus category must be judged before deciding whether to respond · A8.01.6 There is a floor on reaction time; anything faster is anticipation, not a genuine reaction · A8.01.7 Individual differences in reaction time exceed what most interface optimizations can gain
• Nearby: A8.03 Expectancy and preparatory signals · A8.04 Psychological refractory period
• Search terms: reaction time · information-processing model · Model Human Processor