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Biology · No. 90 of the first 100

Human senses

Between the light hitting your eye and your finger moving lies a quarter of a second — and you just lived it without noticing.

Plate XXIX — Measure your wiring eye → brain → finger · ~250 ms
Wait for green, tap fast, and time your own nervous system.
tap to startwait for green, then tap as fast as you can 150 · world-class 200 · F1 driver 250 · typical adult 350 · distracted
FIG. XXIX — MEASURE YOUR WIRING
Your average
–ms
Your best
–ms
In your ~250 milliseconds, a lot happened: your eye caught the green, sent it up the optic nerve, your brain decided, and a command raced down your arm to the finger. That whole relay is what you just timed. Try ten rounds — and notice you can't get near zero, no matter what.
The short answer

When you see something and react, the news has to travel: eye → brain → decision → arm → finger. Nerves are fast, but not instant — the whole relay takes about a quarter of a second, and no amount of trying gets it to zero.

What's actually happening

Your senses feel instantaneous, but they are a postal service, not telepathy. Light striking your retina triggers a chemical cascade that takes tens of milliseconds before a signal even leaves the eye. It then travels to the visual cortex at the back of your head, gets recognised ("green!"), routes through decision circuits, and a command races back down your spinal cord and arm to the finger muscles. Every leg of the relay is fast; none is free. The total — around a quarter of a second — is your reaction time, and the tester above measures yours.

The wiring itself has a speed limit. Nerve signals aren't electricity in a wire (300,000 km/s); they're waves of chemical gates flipping open along the fibre, topping out around 120 metres per second in the best-insulated nerves. That's a million times slower than copper. Evolution's workaround is myelin — fatty insulation that lets the signal leap between gaps — and brute prioritisation: reflex arcs for hot stoves route through the spinal cord and fire your arm muscles before the pain report has even reached your brain.

Because the lag is unfixable, your brain cheats: it predicts. A batter facing a 145 km/h ball starts the swing before the ball's flight could possibly be fully perceived; your brain even edits the lag out of consciousness, backdating events so the world feels live. That's why the tester punishes early taps — a tap before the green isn't fast perception, it's a guess. And it's why athletics disqualifies any sprinter who leaves within 100 ms of the gun: human biology simply cannot hear and act that fast.

Try it at home The falling-ruler test
  1. 1A friend holds a 30 cm ruler vertically; you hover your open thumb and finger at the zero mark, not touching.
  2. 2They drop it without warning; you catch. Read the centimetre mark where you caught it.
  3. 3Distance → time: 10 cm ≈ 143 ms, 15 cm ≈ 175 ms, 20 cm ≈ 202 ms. Compare with your tap result above — rulers don't lie, and neither do nerves.