🧬 Reaction Time

What does a reaction time test measure?

A reaction time test measures the complete interval from stimulus onset (the moment a signal appears) to motor response (the moment you click or press a key). It captures the entire neural processing chain.

The neural reaction chain

~5ms
Photoreceptor activation — Light hits the retina
~20ms
Retinal processing — Signals travel to optic nerve
~50ms
Visual cortex (V1) — Primary visual processing
~80ms
Conscious perception — You become aware of the stimulus
~100ms
Decision & motor planning — Brain selects and prepares response
~30ms
Nerve signal to hand — Motor cortex → muscle
~5ms
Mechanical click — Finger depresses button
Total ~290ms (typical)

The Human Benchmark Reaction Time test uses performance.now() for sub-millisecond precision. It also includes a randomised delay before the stimulus appears to prevent anticipatory responses (clicking before the signal).

Reaction time is clinically used as a cognitive biomarker. Slowing RT is often the earliest detectable symptom of cognitive aging and appears before memory problems in longitudinal studies. It is used in neurological assessments for ADHD, concussion, and early dementia screening.

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