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
Perception
Things your senses and brain construct that are not out there, or are more real than you think.
Showing 27 oddities.
Perception entries are things your senses and brain construct, that are not fully out there in the world. The blind spot, afterimages, and the Purkinje effect all come from how the visual system processes light, not from anything a doctor would call a symptom.
This kind sits apart from involuntary because nothing is happening to the body. The eye and brain are simply doing their ordinary job of building an image, and the entry describes what that job looks like from the inside.
The clearer boundary is with involuntary: Charles Bonnet syndrome, seeing detailed images after vision loss, could read as a hallucination, but it belongs here because the cause is a known, mechanical gap in visual input, not an event happening to the body.
Results narrow the list below as you type.
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)
Brain Perception
Aphantasia
Most people see a picture in their mind when they think of a friend's face. People with aphantasia see only a faint image or none. About 4 in 100 people are like this, and about 1 in 100 see nothing at all. A few people have images almost as vivid as real sight.
About 1% have no mental images. About 4% have faint or none.
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. The arcs follow the paths of the nerve fibres in your own retina.
94% of healthy eyes in one clinic test
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.
Most people
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 sight loss, not by mental illness.
10–40% of older people with serious sight loss
Brain Perception
Chronostasis
Glance at a clock and the second hand can seem 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. Almost everyone sees the speckle, and many people see more. Almost nobody talks about them.
Almost everyone
Eyes Perception
Eigengrau
The colour you see in total darkness is not black. It is a very dark grey, most likely because the light-sensing cells in your eyes fire a little even when no light reaches them.
Everyone
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, after some minutes, your brain may start 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, in lab tests
Brain Perception
Geometric nightmares
A kind of dream, often during fever in childhood, filled with huge or tiny shapes, infinite space, crushing pressure, and dread. The name comes from an online forum, not a medical text, but each piece of it is well known to neuroscience.
Not known
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
Brain Perception
Meditation body dissolution
During long, still meditation the brain loses track of the edges of the body. The body can feel indistinct or seem to expand into space. Flotation tanks produce the same effect. It is usually harmless.
About 29% of recent meditators
Brain Perception
Motion aftereffect
Watch something move in one direction for about 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. A faint afterimage after a bright light is normal and harmless. Copies that persist or return later need a doctor.
Not counted in the general population
Nerves Perception
Pinocchio illusion
Vibrate the biceps tendon while a person touches their own nose with their eyes closed, and their nose can feel as if it is growing. It shows how the brain works out where the body parts are.
About 30% of students in one lab test
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, truck drivers, pilots, and meditators all report it.
77% of sighted people blindfolded for four days
Brain Perception
Prosopagnosia
People with prosopagnosia, or face blindness, find it hard to recognise faces, even those of family and friends. Most have had it all their lives. It can also start after brain damage. Estimates run from about 1 in 100 people to about 1 in 40, depending on how it is tested.
About 1–2.5% of people
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 two 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
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
Brain Perception
Synesthesia
In some people, one sense triggers another. Letters have colours, sounds have shapes, or days of the week sit in fixed places in space. The pairings are automatic and stay largely the same over many years.
About 4% of people
Eyes Perception
Tetrachromacy
Most people have three kinds of colour-sensing cone. About 12% of women carry a fourth. In theory they can tell apart colours that look identical to everyone else. Clear cases are extremely rare, and online tests cannot detect it.
About 12% of women carry a fourth cone type
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