Eyes Reflex
Accommodation reflex
When you look at something close, three things happen at once. The eyes turn inward, the pupils shrink, and the lens in each eye gets fatter. You do not decide any of it.
Everyone
Eyes
Things you can see that are not out there, and structural quirks of the eye.
38 oddities. 27 have this as their primary region.
Eyes Perception
Afterimages
An image that stays in your vision after the thing you looked at is gone. Bright things leave a same-colour copy for a moment. Long stares leave a copy in opposite colours.
Everyone
Brain Perception
Alice in Wonderland syndrome
Objects, body parts, or the whole room suddenly look far too big, far too small, or far away. It mostly happens to children, often during a fever or infection, and it passes on its own. In adults it is usually linked to migraine.
6–7% of teenagers (size distortion)
Eyes Perception
Blue arcs of the retina
Look at a small red light in a dark room and two faint blue arcs curve away from it towards your blind spot. You are seeing the nerve fibres of your own retina light up.
Most people, in the right conditions
Eyes Perception
Blue field entoptic phenomenon
Tiny bright dots that dart along wiggly paths when you look at a clear blue sky. They are your own white blood cells moving through the capillaries of the retina.
Everyone
Brain Perception
Charles Bonnet syndrome
People who lose a lot of their sight often start to see things that are not there: patterns, faces, small people in costume, whole landscapes. They know the images are not real. It is caused by the eyes, not the mind.
10–40% of people with serious sight loss
Brain Perception
Chronostasis
Glance at a clock and the second hand seems to freeze for a moment before it moves. The brain hides your eye movement and stretches the first thing you see to cover the gap.
Everyone
Eyes Perception
Closed-eye hallucinations
Close your eyes and you do not see nothing. You see speckle, drifting colour, and, as you fall asleep, moving patterns and even scenes. Everyone has them. Almost nobody talks about them.
Everyone
Eyes Trick
Crossing your eyes
Turning both eyes inward toward the nose on purpose. Nearly everyone with normal two-eyed vision can do it. Your eyes will not get stuck that way.
Nearly everyone
Eyes Variation
Distichiasis
A second row of eyelashes growing from the oil-gland openings along the inner edge of the eyelid. It is rare, mostly inherited, and Elizabeth Taylor was born with it.
Rare
Eyes Perception
Eigengrau
The colour you see in total darkness is not black. It is a very dark grey, because the light-sensing cells in your eyes fire a little even when no light reaches them.
Everyone
Eyes Reflex
Emotional tears
Humans are the only animals known to weep from feeling. Emotional tears have a different chemistry from the tears that wash the eye, and babies cannot make them until a few months after birth.
Everyone
Eyes Trick
Eyelid flipping
Turning the upper eyelid inside out so the pink inner surface shows. Most people with reasonably loose lids can learn it. Eye doctors do it to patients to look for grit.
No formal count Icky
Eyes Involuntary
Floaters
Small grey threads, dots, or cobwebs that drift across your vision and dart away when you look at them. They are shadows of tiny clumps inside the eye. They are common and usually harmless.
About 70% of people, at some point
Eyes Perception
Foveal tritanopia
The very centre of your vision cannot see blue. The middle of the fovea has no blue-sensitive cones at all. You never notice, because the brain fills the colour in from the surrounding area.
Everyone
Brain Perception
Ganzfeld effect
Fill your whole field of view with a featureless colour and, within minutes, your brain starts inventing shapes, lights, and scenes. Two halved ping-pong balls and a lamp are enough to do it at home.
Most people who try it
Eyes Perception
Haidinger's brush
Humans can see the polarization of light without any tool. It appears as a faint yellow bow-tie in the middle of your vision when you look at a white screen or a blue sky.
Most people, with practice
Eyes Variation
Heterochromia
Eyes of two different colours, or one eye with two colours, in fewer than 1 in 100 people. When it is present from birth it is almost always harmless.
Under 1% of people
Eyes Trick
Independent eye movement
Moving one eye while the other stays still, or moving the two eyes in different directions. Very rare in humans. Chameleons do it all day.
Very rare; no formal count
Brain Involuntary
Migraine aura
A shimmering zigzag or C-shaped patch of light that grows across your vision over 5 to 60 minutes and then fades. It can come before a migraine headache or, in some people, with no headache at all.
About 3% of people
Brain Perception
Motion aftereffect
Watch something move in one direction for half a minute, then look at something still. The still thing appears to drift the opposite way for a few seconds.
Everyone
Eyes Perception
Palinopsia
An image that keeps appearing after the thing you saw is gone. Brief trails behind moving objects are common and harmless. Copies that persist or return minutes later need a doctor.
Brief trailing is common. Persistent forms are rare.
Eyes Involuntary
Phosphenes
The flashes, rings, and patterns of light you see when you rub or press your closed eyes, or after a knock on the head. The retina fires from pressure instead of light.
Everyone
Nerves Reflex
Photic sneeze reflex
About one person in four sneezes when they step into bright sunlight. It runs in families, and it is probably a wiring mix-up between the nerve for the eyes and the nerve for the nose.
About 18–35% of people
Brain Perception
Prisoner's cinema
After hours or days in darkness, people start to see light shows: colours, geometric patterns, and sometimes figures or scenes. Prisoners, miners, polar explorers, and meditators all report it.
Up to 77% of people after 4 days blindfolded
Eyes Reflex
Pupillary light reflex
Shine a light into one eye and both pupils shrink, even the eye in the dark. The wiring crosses in the midbrain so that the two eyes always match. You can watch it in a mirror.
Everyone
Eyes Perception
Purkinje effect
As the light fades at dusk, red things go dark first and blue things stay bright. Your eyes switch from colour-sensing cones to blue-favouring rods, and the whole world shifts towards blue.
Everyone
Eyes Perception
Purkinje images
A candle held near someone's eye produces four tiny reflections, one from each surface of the cornea and lens. Eye trackers and eye doctors use them. In the right conditions you can see one of them inside your own eye.
Everyone
Eyes Perception
Purkinje tree
A branching shadow of your own retinal blood vessels. It is always on your retina, but you only see it when light enters the eye from an unusual angle and the shadow moves.
Everyone
Heart Involuntary
Seeing stars when standing up
You stand up fast, your vision goes grey or sparkly, and your head feels light for a few seconds. Blood has briefly pooled in your legs. It passes on its own.
Almost everyone, at some time
Brain Perception
Strange-face illusion
Stare at your own face in a mirror in dim light for a few minutes and it starts to distort. Most people see their features warp. Some see a stranger, an animal, or a monster.
66% see their face distort
Eyes Perception
Tetrachromacy
Most people have three kinds of colour-sensing cone. A few people have four. In theory they can tell apart colours that look identical to everyone else. Confirmed cases are extremely rare, and online tests cannot detect it.
A handful of confirmed cases
Eyes Perception
The blind spot
Each eye has a hole in its field of view where the optic nerve leaves the retina. You never notice it, because the brain paints over the gap with whatever surrounds it.
Everyone
Eyes Perception
Troxler effect
When you hold your gaze on one point, things in the edge of your vision fade out and disappear. The brain stops reporting anything that does not change.
Everyone
Brain Involuntary
Visual snow
Tiny flickering dots across the whole field of vision, like static on an old TV, present with eyes open or closed. About 2–3% of people have it. The eyes are fine. The visual part of the brain is over-active.
About 2–3% of people
Eyes Trick
Voluntary nystagmus
Between 5% and 8% of people can make both eyes shake rapidly from side to side on command. It looks alarming and is harmless. It is tiring and lasts only seconds.
5–8% of people
Eyes Trick
Voluntary pupil control
Changing the size of your pupils by will alone, without imagining a light or doing sums. Only a handful of cases are documented. Indirect control by thinking hard is possible for almost everyone.
A handful of documented cases
Eyes Trick
Wall-eyed divergence
Turning both eyes outward, away from the nose, on purpose. It is much rarer than crossing the eyes. Most people can learn a mild version through Magic Eye pictures.
Rarer than crossing; no formal count