"Simple reaction time: one stimulus, one response"

What it is

Simple reaction time (SRT) measures the most basic response task: a single stimulus mapped to a single required response, with the participant knowing in advance exactly what the stimulus is and what to do about it — the only uncertainty is when it will appear. Typical values run around 190–200ms for visual stimuli and somewhat shorter, around 150–160ms, for auditory and tactile stimuli, a gap that mainly traces back to differences in conduction speed across sensory modalities.

Why it happens

Because there is only one possible stimulus-response mapping, the participant can have the entire response fully prepared — the motor program pre-assembled — before the stimulus even appears. Once it appears, central processing only needs to complete the minimal judgment "it happened" to trigger the already-prepared action, with essentially no discrimination or selection involved. This compresses the central-processing stage in SRT down close to its physiological floor, so sensory conduction and motor conduction make up a proportionally larger share of the measured total.

Studying it

The standard paradigm has participants fixate a constant location and, after a randomized interval, presents a single stimulus (a flash, a tone), requiring the fastest possible fixed response (pressing the same key every time). Because the only variable is the unpredictable timing of the stimulus, SRT is commonly used as the baseline in reaction-time research — subtracted from choice or discrimination reaction time to isolate how much time the extra discrimination or selection process costs on its own.

Where it stops holding

SRT captures a floor value under idealized conditions. A single, fully-expected, discrimination-free stimulus is rare in real interfaces — most interactions involve at least a basic judgment of "is this the thing I'm looking for" — so directly applying laboratory SRT figures to expected interface response speed will systematically underestimate the time actually required.

Applying it

Only when a design genuinely achieves "one stimulus, one pre-set response" should the SRT range (roughly 150–200ms) be used to set a reasonable response window for interface feedback — for instance, a physical switch that always leads to one certain consequence. As soon as an interface presents multiple possible prompts, or requires the user to first identify which category a prompt belongs to, SRT figures are no longer the right benchmark — use discrimination or choice reaction time instead. How to check: verify that, at the moment in question, there really is only one possible stimulus and one acceptable response; if any step requires judging "which one is it," the premise for SRT does not hold and that benchmark should not be applied.

Related

• Same group: A8.01.1 Reaction time breaks into sensory conduction, central processing, motor conduction, and muscle activation · A8.01.2 Reaction time and movement time are two independent stages that must be measured separately · 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: A8.04 Psychological refractory period
• Search terms: simple reaction time · SRT · sensory modality reaction time