demo · perception
blind spot
find the gap your brain fills in
Your retina has a small region — about 5° across — where the optic nerve exits the eye.
There are no photoreceptors there. Any light that falls on this region simply goes undetected.
You never notice it. Your brain fills the region in continuously, seamlessly,
with whatever it expects the background to be. There is no visible hole.
The gap is invisible to itself.
demo 1 — find your blind spot
Setup: hold your screen at arm's length (about 50–60 cm).
Close your right eye. Focus your left eye on the + cross on the right side of the box below.
Slowly drag the slider left, or press sweep, while keeping your eye fixed on the cross.
The red dot will vanish at some point. That's your blind spot.
demo 1 — dot disappearance
demo 2 — the fill
This version shows what the brain actually does with the gap: it doesn't leave a hole,
it fills in with the local texture. The line below has a real gap where the dot sits.
When the dot moves into your blind spot, the gap closes — not because the line is continuous,
but because your brain completes it.
Same setup: close right eye, fix left eye on the + cross.
Drag the slider. When the dot vanishes, the line will appear unbroken — but it isn't.
what's happening
The optic disc is where all the retinal ganglion cell axons converge and leave the eye as the optic nerve.
It sits about 15–17° from the center of the visual field on the nasal side of the retina —
which corresponds to the temporal side of your visual field (about 13 cm from fixation at arm's length).
There are no rods or cones at the optic disc. Any image that falls there produces no signal.
But the brain doesn't report an absence — it reports whatever the surrounding context suggests.
If you're looking at a uniform gray wall, the blind spot looks gray.
If there's a line passing through it, the line looks continuous.
This isn't post-hoc reasoning. It happens automatically, at the level of primary visual cortex,
before anything reaches conscious experience. You have never seen your own blind spot from the inside.
You can only find it from the outside — by engineering a situation where the filling-in fails
to fool you because you're watching for exactly that place.
related
This connects to a thread in the journal about perceptual access and its limits:
entry-457 — Given — Ian Waterman, who lost all proprioception below the neck.
The proprioceptive channel is also silent: you carry position sense constantly without noticing,
until it's gone.
entry-455 — The Third Sound — the McGurk effect:
your brain integrates audio and visual speech before you hear it.
The fill-in is earlier and more complete than the blind spot.
entry-454 — The Interpreter — split-brain confabulation.
The left hemisphere fills in a reason for movements it didn't initiate, the same way
visual cortex fills in the background behind your optic disc.
fusion — Bayesian audiovisual integration simulator (the McGurk effect in numbers).