A handscroll on the speed of sight

The Speed
of Seeing

From light touching your eye to your brain knowing what it saw: about 140 to 210 milliseconds. That is roughly one‑seventh to one‑fifth of a single second.

一秒

Unroll one second

seeing · 140–210 ms a neuron fires · 1 ms shortest useful glimpse · 13 ms
elapsed 1.000 s ◆ by now, a sight has been seen the brush crosses in exactly one real second
The whole scroll is one second. Seeing happens inside the pale green wash, still in the first fifth.
0.14–0.21 stime to see and know
⅐ – ⅕of one second
5 to 7separate sights could fit in one second
1 – 1.6×around the Earth: how far light travels in that time
試之

Feel it for yourself

Press a button.
A plum blossom will appear here,
then vanish.
Your screen redraws about every 16 ms, so each flash is accurate to roughly one frame.

Even the brief flashes are long enough to see the blossom. Your brain needs only that much time.

目至心

From eye to knowing

0 ms

Light lands

Photoreceptors in the retina turn light into electrical signals.

40–60 ms

Reaches the cortex

The signal arrives at the visual cortex at the back of your head.

100–150 ms

The brain knows

Brain activity already tells "animal" from "no animal" about 150 ms after an image appears.

up to 210 ms

The outer edge

Adding the stages end to end gives your full 140–210 ms range.

Each stage is measured from the moment the image appears.

The 100–150 ms for recognition already counts from the moment light hits the eye. It includes the trip to the cortex, so in practice recognition can finish within about 150 ms. That is even faster than the 140–210 ms you get by adding the stages.

長短

Against the length of a second

  • One neuron firinga single spike1 ms
  • A usable glimpseimage flashed, still understood13 ms
  • Seeing and knowingyour range140–210 ms
  • A blinkeyelid down and up100–400 ms
  • One secondthe whole scroll1000 ms
All bars use the same linear scale. The red hair is 1 ms; seeing takes 140 to 210 of them.
古之時

Old names for an instant

刹那

Chànà · kṣaṇa ≈ 13 ms

The Mahāvibhāṣā, a Buddhist treatise read in Song China, divides a day and night into 6,480,000 kṣaṇa, so one kṣaṇa is about 13 ms. That happens to match the shortest glimpse that modern experiments show people can still understand.

一瞬

Yī shùn · one blink ≈ 0.36 s

The Mahāsāṃghika Vinaya counts twenty thoughts to one shùn and divides a day into 4,800,000 thoughts, which makes one shùn about 0.36 s. That is long enough to see something about twice.