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
All oddities
143 things normal bodies (sometimes) do.
Results narrow the list below as you type.
143 shown
Organs Sound
Stomach growling
The rumble from your belly is gas and liquid being pushed through your gut by waves of muscle. It is louder when you are hungry because there is less food to muffle it.
Everyone
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 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
Skin Reflex
Blushing
When you are embarrassed, blood vessels in the face open wide and the skin turns red. You cannot stop it. Darwin called it the most human of all expressions, and no other animal is known to blush from embarrassment.
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
Organs Reflex
Cold diuresis
Being cold makes you need to urinate. The body pulls blood away from the skin into the core, the heart reads this as too much blood, and the kidneys are told to dump water.
Everyone
Heart Reflex
Diving reflex
Cold water on the face makes the heart slow down, sends blood to the brain, and holds the breath. Every mammal studied so far has it. You can trigger it with a bowl of cold water.
Everyone
Ears Sound
Ear clicking when swallowing
Swallowing opens a small tube between your throat and your middle ear for a fraction of a second. The faint click you can hear is that tube letting air through to balance the pressure.
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 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 one or two months after birth.
Almost 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
Skin Reflex
Goosebumps
Tiny muscles at the base of most hairs pull the hair upright when you are cold, scared, or moved. In a furry animal this traps warm air or makes it look bigger. In a human the raised hair does almost nothing.
Everyone
Organs Sound
Hiccups
A hiccup is a sudden jerk of the diaphragm followed by the voice box snapping shut. Everyone gets them, even before birth. The record case lasted 68 years.
Everyone
Nerves Reflex
Knee-jerk reflex
A tap on the tendon below the kneecap makes the lower leg kick. The signal goes to the spinal cord and straight back through one connection. It is a classic example of a one-synapse reflex.
Everyone
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
Nerves Reflex
Palmar grasp reflex
Put a finger in a newborn's palm and the fingers close around it, hard. The grip is strong enough to hold the baby's weight for a moment. It fades by about six months and is replaced by grasping on purpose.
All healthy newborns
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. Pressure on the eye makes the retina fire without any light.
Anyone with a working retina
Nerves Involuntary
Pins and needles
The prickling, buzzing feeling in a limb after you sit on it or lean on it. It happens when blood returns to a squeezed nerve and its sensory fibres briefly fire on their own.
Everyone
Skin Involuntary
Pruney fingers
Fingertips and toes wrinkle after several minutes in water. This is not just the skin soaking up water. It is the nervous system shrinking the fingertip on purpose, and the wrinkles may improve grip when wet.
Nearly everyone with intact nerves
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 pupils react together. 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 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
Nerves Reflex
Referred pain
Pain from an internal organ often shows up somewhere else, like the left arm during a heart attack or the shoulder blade from a gallbladder. The nerves from the organ and from that patch of skin share a line into the spinal cord.
Everyone
Ears Sound
Tensor tympani thump
The low thump or rumble in your ears when you yawn or shout is a tiny middle ear muscle tensing to protect your hearing from your own noise.
Everyone
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
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
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 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.
76% in one survey of 603 people
Skin Involuntary
Fordyce spots
Tiny pale yellow or white bumps, 1 to 3 mm across, on the edge of the lips or inside the cheeks. They are ordinary oil glands in a place without hair. Most adults have them.
70–80% of adults
Ears Sound
The sound of silence
In a room that absorbs almost all sound, many people hear their heartbeat, their blood, their breath, or a high ringing. True silence is not possible for a living person.
64–83% of people in studies
Mouth Trick
Tongue rolling
Between 65% and 84% of people can curl the sides of the tongue up into a tube. For decades it was taught as a simple one-gene trait. It is not.
65–84% of people
Nerves Reflex
Babinski reflex
Stroke the sole of a baby's foot and the big toe lifts while the other toes fan out. Do the same to an adult and the toes curl down. The baby's response is normal until about two years. In an adult it is a warning sign.
Most healthy newborns
Brain Involuntary
Hypnic jerks
A sudden jolt of the arms, legs, or whole body just as you fall asleep, sometimes with a feeling of falling. Most people get them, and they are harmless.
About 70% of people
Brain Involuntary
Rubber hand illusion
Stroke a rubber hand and a person's hidden real hand at the same time, and within a minute or two the brain adopts the rubber hand as its own. It shows that your sense of your body is built moment by moment from sight and touch.
60–80% of participants
Brain Involuntary
Déjà vu
A strong feeling that you have lived the present moment before, even though you know you have not. About two in three people get it. It peaks between the ages of 15 and 25.
About 2 in 3 people
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
Mouth Reflex
Gag reflex
Touch the back of the tongue or the soft palate and the throat clamps shut. It keeps things out of the airway. About a third of healthy people have no gag reflex at all, and that is normal.
Present in about 2 of 3 people, absent in 37% in one study
Brain Involuntary
Phantom phone vibrations
You feel your phone buzz in your pocket, check it, and nothing happened. Most phone users get this. The cause is not known. The brain may misread ordinary sensations as a buzz.
30–89% of phone users
Brain Reflex
Contagious yawning
Seeing, hearing, or even reading about a yawn makes many adults yawn. The effect appears in early childhood, may be linked to empathy, and crosses over to dogs.
About 30–70% of adults
Mouth Trick
Pointed tongue
Making the tip of the tongue narrow and sharp, like a pencil point. About 55% of people in one Chinese survey can do it. It and tongue rolling are the two most common tongue tricks.
~55%
Brain Involuntary
Call of the void
A sudden, brief thought of jumping when you stand somewhere high, with no wish to do it. About half of people get it. It is probably the brain's safety system being misread, not a hidden wish.
43–60% of people
Brain Involuntary
Derealization and depersonalization
The world suddenly seems flat, distant, or like a film (derealization), or you feel like you are watching yourself from outside (depersonalization). Many people have a brief episode at some point, usually under stress.
26–74% of people, at least once
Ears Trick
Ear rumbling
Up to about half of all people can make a low roar inside their own head by tensing a tiny muscle in the middle ear.
43–55% of people
Nerves Involuntary
Brain freeze
A short, sharp headache in the forehead a few seconds after cold food or drink touches the roof of the mouth. It usually passes in under a minute.
About 40–50% of people
Bones Trick
Knuckle cracking
Pulling or bending a finger joint until it pops. Between 25% and 54% of people do it out of habit. The pop comes from a gas bubble in the joint fluid, and it does not cause arthritis.
25–54% crack habitually
Bones Variation
Accessory ossicles
Small extra bones, most often in the foot, that form when a growth centre never fuses with its main bone. They are usually harmless but can be mistaken for fractures on an X-ray.
About 2 in 5 feet
Brain Involuntary
ASMR
A pleasant tingling that starts on the scalp and moves down the neck and spine, set off by whispering, tapping, soft sounds, or gentle personal attention. Not everyone gets it. A large online community is built around it.
Unclear; estimates 20–60%
Heart Reflex
Vasovagal syncope
The most common reason people faint. A trigger like blood, pain, heat, or standing too long makes the vagus nerve slow the heart, and blood pressure drops. The brain briefly runs short of blood and you go down.
About 1 in 3 people faint at least once
Muscle Variation
Diastasis recti
A widening of the gap between the two halves of the six-pack muscle, seen in nearly every first pregnancy and in some men. A bulge down the midline when you sit up is the usual sign.
Almost all first pregnancies; about 1 in 3 women a year after birth
Muscle Trick
Raising one eyebrow
Lifting a single eyebrow while the other stays down. Informal estimates say 30% to 35% of people can do it without training. No study has measured how many of the rest can learn it.
30–35%
Ears Variation
Darwin's tubercle
A small bump on the folded rim of the ear, which Darwin thought was the leftover tip of a pointed ancestral ear. Between 10% and 58% of people have one, depending on the population.
10–58% of people
Brain Involuntary
Exploding head syndrome
A loud bang, crash, or explosion heard inside the head while falling asleep or waking up. Nothing actually happens, there is no pain, and it is harmless.
2–37% of people, lifetime
Bones Variation
Hitchhiker's thumb
A thumb whose top joint bends back past 50 degrees, once taught as a simple recessive gene. Thumb angle is really a smooth range, and the genetics are not that simple.
24–36% of people in the US
Brain Involuntary
Hypnagogic hallucinations
Vivid images, sounds, or touches that appear while you are falling asleep or waking up. A face, a voice saying your name, a hand on your shoulder. They may be dream fragments arriving early or late, and they are usually normal.
About 25–37% of people
Nerves Reflex
Photic sneeze reflex
About one person in four sneezes when they step into bright sunlight. It runs in families, and the leading idea is a wiring mix-up between the nerve for the eyes and the nerve for the nose.
About 18–35% of people
Mouth Variation
Supertasters
About a quarter of people taste bitter compounds far more strongly than everyone else. They tend to have more taste buds, and they often dislike coffee, dark greens, and grapefruit.
About 25% of people
Mouth Trick
Tongue twisting
Turning the tongue on its side so the edges face up and down, or all the way over. Between 12% and 36% of people can twist it at least halfway.
12–36%
Ears Trick
Ear wiggling
Moving the outer ears using three small muscles that most mammals use to point their ears at sounds. About 1 in 5 people can wiggle their ears. Some people can learn.
About 1 in 5
Muscle Variation
Linburg–Comstock variation
An extra tendon link in the forearm that ties the thumb to the index finger. When the thumb bends into the palm, the index fingertip bends with it. Roughly 1 in 5 people have it, but counts vary widely.
About 1 in 5 people. Studies range from 5% to 60%.
Brain Involuntary
Sleep paralysis
You wake up, or start to fall asleep, and cannot move or speak for a few seconds to minutes. Sometimes a threatening presence seems to be in the room. It is the normal paralysis of dreaming sleep running a little late.
8–30% of people
Bones Variation
Torus palatinus
A hard bony lump along the midline of the roof of the mouth that grows slowly and is completely harmless. Many people find it by chance and worry that it is a tumour.
About 20% of people
Muscle Variation
Palmaris longus
A thin tendon that stands out in the wrist when you press your thumb and little finger together. About 1 in 7 people do not have it at all, and they lose nothing.
Missing in about 15–20% of people
Bones Variation
Joint hypermobility
About 1 in 5 people have joints that bend further than usual, measured with a nine-point test called the Beighton score. For most people it causes no trouble.
10–25% of people
Mouth Trick
Cloverleaf tongue
Folding the tongue into three lobes, like a clover leaf. In one Dutch study, about 15% of adults could do it. It is usually called the hardest tongue shape.
About 15% (one Dutch sample)
Ears Involuntary
Tinnitus
A ringing, hissing, buzzing, or roaring sound with no source outside your head. Between 10% and 15% of people have it. It is mostly the hearing system filling in for damaged or tired inner-ear cells.
About 10–15% of people
Mouth Trick
Tongue folding
Folding the tip of the tongue backward over itself inside the mouth. Studies find that between 1.5% and 27.5% of people can do it.
1.5–27.5%
Mouth Trick
Gorlin's sign
Touching the tip of your nose with your tongue. About 10% of people can do it. About half of people with Ehlers-Danlos syndrome can, so doctors use it as a quick check for stretchy connective tissue.
~10% of people Icky
Bones Variation
Morton's toe
A second toe that looks longer than the big toe because the first metatarsal bone in the foot is short. It is common, and it is the foot shape seen on classical Greek statues.
3–22% of people
Muscle Variation
Plantaris muscle
A small calf muscle with a very long, thin tendon that medical students mistake for a nerve. Between 7% and 20% of people do not have it.
Missing in 7–20% of people
Mouth Involuntary
Tonsil stones
Small white or yellow lumps that form in the folds of the tonsils and sometimes come out when you cough. They smell bad and feel like something is stuck in the throat. They are usually harmless.
Up to 10% of people; up to 46% on CT scans Icky
Bones Trick
Midtarsal break
A foot that bends in the middle, between the heel and the ball, as well as at the toes. About 1 in 12 people have it. It looks like the midfoot bend of apes.
About 1 in 12 (8%)
Eyes Trick
Voluntary nystagmus
About 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.
About 8% of people
Muscle Involuntary
Proctalgia fugax
A sudden, cramping pain deep in the rectum that lasts seconds to minutes and then goes away completely. It has no known cause, and it is harmless.
About 7% of adults. Studies range from 4% to 18%.
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)
Mouth Variation
Tongue tie
A short or tight band of tissue under the tongue that holds the tip down, present in about 1 in 14 babies. Some cannot breastfeed well, but many grow up with no trouble at all.
About 7% of babies
Bones Variation
Clinodactyly
A finger that curves sideways toward its neighbours, most often the little finger leaning toward the ring finger. Between 1% and 20% of people have it, and on its own it is harmless.
1–20% of people
Brain Rare ability
Eidetic memory
Some children can keep a detailed picture of an image in view for a short time after it is taken away. The ability fades by the teenage years. A true photographic memory in adults has never been confirmed.
2–10% of children aged 6–12
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 5% of people
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
Skin Reflex
Dermatographia
In a few people, a firm stroke on the skin raises a red welt within minutes, clear enough to write words with a fingernail. It is the skin's hive response set on a hair trigger.
About 2–5% of people
Ears Sound
Autophony
Your own voice, breathing, or heartbeat sounds far too loud inside your head. It can mean a tube or a thin patch of bone is letting sound take a shortcut into the ear.
About 0.3–6.6% for patulous Eustachian tube
Skin Variation
Supernumerary nipples
An extra nipple, usually small and flat, on a line running from the armpit to the groin. Between 1 in 500 and 1 in 18 people have one, and it is often mistaken for a mole.
0.2–5.6% of people
Ears Involuntary
BPPV
A short, intense spinning feeling set off by turning over in bed or tipping your head back. Tiny crystals in the inner ear have drifted into the wrong place. It is the most common cause of vertigo and it is very treatable.
About 2.4% of people, lifetime
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% of people have it. The eyes are fine. The visual part of the brain is over-active.
About 2% of people
Mouth Variation
Geographic tongue
Smooth red patches with pale borders that form a map on the tongue and move around over days and weeks. About 2 to 3% of people have it, and it is harmless.
2–3% of people
Brain Rare ability
Mirror-touch synesthesia
About 1 in 60 people feel a touch on their own body when they watch someone else being touched. The brain system that mirrors other people's actions appears to be turned up too high.
About 1.6% of people
Mouth Variation
Bifid uvula
The uvula, the dangling flap at the back of the throat, is split into two lobes, in about 1 to 2% of people. On its own it is harmless, but in a baby it can be a clue to a hidden cleft of the palate.
1–2% of people
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
Bones Variation
Cervical rib
An extra rib above the normal top rib, growing from the lowest bone in the neck. About 1 in 100 people have one, and most never know.
About 1% of people
Brain Trick
Ambidexterity
Using both hands equally well for every task. About 1% of people are truly ambidextrous. Many people who say they are ambidextrous are mixed-handed: right hand for some jobs, left for others.
About 1%
Body Rare ability
CCR5-delta32
About 1% of people of Northern European descent carry two copies of a broken gene that makes them almost immune to the most common form of HIV. A bone marrow transplant from one such donor produced the first cure of HIV.
About 1% of Northern Europeans (two copies)
Ears Variation
Preauricular pit
A tiny hole in the skin just in front of the ear, present from birth, that can lead to a tunnel under the skin. It is harmless unless it gets infected.
0.1–10% of people
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.
Ears Sound
Pulsatile tinnitus
A whooshing or thumping in one ear that keeps time with your pulse. It is usually the sound of blood flowing near the ear. It often has a treatable cause, so get it checked.
Under 10% of people with tinnitus
Bones Variation
Camptodactyly
A finger that stays bent forward at its middle joint and cannot fully straighten, usually the little finger. Under 1% of people have it. Mild cases rarely cause trouble, but the bend can get worse while a child grows.
Under 1% of people
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 usually harmless.
Fewer than 200,000 people in the US
Bones Variation
Webbed toes
Two toes joined by skin, most often the second and third, in about 1 in 2,000 to 2,500 babies. The toes usually work normally and many people leave them as they are.
About 1 in 2,000–2,500 births
Eyes Variation
Distichiasis
A second row of eyelashes growing from the oil-gland openings along the inner edge of the eyelid. It is rare. It can be inherited or acquired. Elizabeth Taylor was born with it.
About 1 in 10,000 people
Brain Rare ability
Perfect pitch
An often-quoted estimate is that 1 in 10,000 people can name or sing any musical note without hearing a reference note first. It runs in families and is far more common in people who started music lessons very young.
Often quoted as 1 in 10,000 people
Body Variation
Situs inversus
All the organs in the chest and belly sit on the opposite side from usual, as in a mirror. About 1 in 10,000 people are built this way, and most are healthy.
About 1 in 10,000 people
Nerves Rare ability
Congenital insensitivity to pain
A tiny number of people are born unable to feel pain. Single-gene changes switch off their pain nerves. It is not a superpower. Injuries go unnoticed, and most people with it are badly hurt in childhood.
About 1 in 1,000,000
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
Muscle Involuntary
Benign fasciculations
Small visible twitches in the eyelid, calf, thumb, or arm that come and go with no weakness. One survey found them in about 70% of healthy clinicians. They are very rarely a sign of nerve disease.
About 70% of clinicians in one survey
Eyes Trick
Crossing your eyes
Turning both eyes inward toward the nose on purpose. Most people with normal two-eyed vision can do it. Your eyes will not get stuck that way.
Most people (no formal count)
Heart Reflex
Oculocardiac reflex
Pressure on the eyeball, or a pull on an eye muscle, makes the heart slow down. It matters most during eye surgery, especially in children. It is not a safe way to slow your own heart.
Found in 14–90% of eye muscle operations. Its strength varies a lot.
Bones Trick
Voluntary joint dislocation
Pushing a joint partly or fully out of its socket on purpose, then back in. Between 10% and 25% of people have some joint hypermobility. One survey of students and their siblings found 0.5% to 2.6% could dislocate a shoulder at will.
0.5–2.6% (shoulder, one survey) Icky
Skin Trick
Voluntary piloerection
Giving yourself goosebumps on command, without music, cold, or fear. Nobody knows how many people can do it, but it seems rare. The muscles involved are not supposed to take orders.
Unknown, but rare
Body Rare ability
Chimerism
A chimera is one person made of two sets of cells with different DNA. It can happen when two embryos fuse in the first days of pregnancy. Most chimeras never find out. A few discovered it when a DNA test said they were not the mother of their own children.
Unknown. Fewer than 100 documented cases
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
Mouth Trick
Gleeking
Shooting a thin jet of saliva from under the tongue. It can happen by accident, often during a yawn, or on purpose. No survey has counted how many people can do it.
No formal count Icky
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
Nerves Involuntary
Precordial catch syndrome
A sudden, sharp, stabbing pain near the heart that lasts seconds to a few minutes and gets worse when you breathe in. Doctors count it among the most common causes of chest pain in children and teenagers, and it is harmless.
80–90% of chest pain in young people, once injury is ruled out
Ears Variation
Attached and free earlobes
Some earlobes hang free and others join the head directly. Textbooks taught this as one gene, but a 2017 study found at least 49 genetic regions involved, and the two shapes blend into each other.
About 61% attached in one Korean sample
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
Brain Involuntary
Brain zaps
A brief feeling like a small electric shock inside the head, often set off by moving the eyes. It is most common when stopping or lowering an antidepressant. It is unpleasant, but usually not a problem.
15–50% of people stopping an antidepressant get withdrawal symptoms
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
Nerves Reflex
Cutaneous rabbit illusion
A few quick taps at the wrist followed by taps at the elbow feel like a small animal hopping up the arm. The brain fills in taps between the two spots that never happened.
Felt on 90% of trials in one brain-scan study
Organs Rare ability
Extraordinary breath-holding
Untrained adults usually stop within a minute or two. The best freedivers manage over 11 minutes, and over 29 minutes after breathing pure oxygen. One group of sea nomads has evolved larger spleens for diving.
Trained divers only; no count
Eyes Trick
Eyelid flipping
Turning the upper eyelid inside out so the pink inner surface shows. Nobody has counted how many people can do it. Eye doctors do it to patients to look for grit.
No formal count Icky
Muscle Trick
Flaring your nostrils
Widening the nostrils on purpose. No study has counted how many people can do it on command. It also happens on its own during hard breathing and in strong emotion such as anger.
No formal count
Brain Involuntary
Frisson
A wave of chills up the back, sometimes with goosebumps, at a powerful moment in music, a film, or a speech. It is thought to come from the brain's reward system reacting to an expectation being met or broken.
Reported rates vary between studies
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
Heart Trick
Heart rate control
Slowing or speeding up your own heartbeat on purpose, using breath, attention, or training. Breathing can shift it a little within minutes. A few trained people shift it a lot.
No formal count
Brain Rare ability
Highly superior autobiographical memory
Fewer than a hundred people worldwide have been confirmed to recall what happened on almost any date since about age 10, in detail and without effort. Their memory for other things, such as lists or numbers, is ordinary.
Fewer than 100 confirmed cases
Brain Rare ability
Human echolocation
Some blind people find their way by making sharp tongue clicks and listening to the echoes. Brain scans show the echoes are processed in the visual cortex. Sighted people can learn a basic version in ten weeks of training.
Nobody has counted it
Eyes Trick
Independent eye movement
Moving one eye while the other stays still, or moving the two eyes in different directions. Few people seem to manage it, and nobody has counted them. Chameleons do it all day.
No formal count
Brain Involuntary
Jamais vu
A familiar word, face or place suddenly feels strange, as if you are meeting it for the first time. It is the opposite of déjà vu, and researchers think it is rarer. In lab studies, more than half of people felt a common word turn strange after copying it about 30 times.
Unknown. Thought to be rarer than déjà vu.
Brain Sound
Musical ear syndrome
Some people with hearing loss hear clear, complete music that no one is playing. The brain replays stored songs to fill the silence. Most people with it know the music is not real.
3.6% of people sent for a hearing test
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 Involuntary
Pee shivers
A brief involuntary shudder during or just after urinating. Many people seem to get it, men more often than women. It is widely known and almost completely unstudied.
Nobody has counted it
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
Muscle Trick
Platysma display
Tensing the thin sheet of muscle under the skin of the neck so that cords stand out from jaw to collarbone. Everyone has the muscle. Nobody has counted how many can flex it on its own.
No formal count
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
Nerves Involuntary
Random shivers
A single brief shudder that runs down the spine or through the whole body when you are not cold. Many people report them. Nobody has counted them.
Not known
Brain Rare ability
Short sleeper syndrome
A small number of people need only 4 to 6 hours of sleep a night, wake without an alarm, and feel fine. Several gene variants have been linked to it. Most people who sleep that little are probably sleep-deprived.
Nobody has counted it
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
Organs Trick
Voluntary burping
Swallowing air on purpose and letting it back up as a belch. Nobody has counted how many people can do it. The loudest recorded burp reached 112.4 decibels.
No formal count
Ears Trick
Voluntary Eustachian tube opening
Opening the tubes that link the middle ear to the throat on command, without swallowing or pinching your nose. You hear a click, and your own voice gets loud. Some divers and freedivers learn it, but most never do.
No formal count
Eyes Trick
Voluntary pupil control
Changing the size of your pupils by will alone, without imagining a light or doing sums. One case has been studied with modern methods. Older reports exist, but most describe indirect tricks. Indirect control by thinking hard is possible for almost everyone.
One modern case study. No rate known
Eyes Trick
Wall-eyed divergence
Turning both eyes outward, away from the nose, on purpose. Nobody has counted how many people can do it. Most people can learn the mild version, parallel viewing, from Magic Eye pictures.
Not known
Missing something your body does? Tell me about it.