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
Nerves
The peripheral nervous system: signals that go wrong, cross over, or fire when they should not.
26 oddities. 13 have this as their primary region.
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.
All healthy infants
Muscle Involuntary
Benign fasciculations
Small visible twitches in the eyelid, calf, thumb, or arm that come and go with no weakness. About 70% of healthy people get them. They are not a sign of nerve disease.
About 70% of healthy 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 passes in under a minute.
About 30–40% of people
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 not dangerous.
Common when stopping antidepressants
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
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
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.
Works on most people tested
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 has it. You can trigger it with a bowl of cold water.
Everyone
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. Up to a third of healthy people barely have it, and that is normal.
Present in about 2 of 3 people, weak or absent in 10–37%
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 the simplest reflex in the body.
Everyone
Brain Perception
Meditation body dissolution
During long, still meditation the brain loses track of the arms and the edges of the body. Hands can feel close to the shoulders and far away at the same time. Flotation tanks produce the same effect. It is common, harmless, and well studied.
Common in long-term meditators
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
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
Nerves Involuntary
Pee shivers
A brief involuntary shudder during or just after urinating. Most men and many women get it. It is widely known and almost completely unstudied.
About 83% of men, 58% of women
Brain Involuntary
Phantom phone vibrations
You feel your phone buzz in your pocket, check it, and nothing happened. Most phone users get this. It is a normal learning effect in the brain, not a fault.
68–89% of phone users
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
Nerves Perception
Pinocchio illusion
Vibrate the biceps tendon while a person touches their own nose with their eyes closed, and their nose feels as if it is growing. It shows how the brain works out where the body parts are.
Most people, in lab conditions
Nerves Involuntary
Pins and needles
The prickling, buzzing feeling in a limb after you sit on it or lean on it. It happens when pressure comes off a nerve and the nerve fibres restart at different speeds.
Everyone
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. It is the most common cause of chest pain in children and teenagers, and it is harmless.
Most chest pain in ages 6–20
Skin Involuntary
Pruney fingers
Fingertips and toes wrinkle after 5 to 15 minutes in water. This is not the skin soaking up water. It is the nervous system shrinking the fingertip on purpose, and the wrinkles may improve grip when wet.
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 eyes always match. You can watch it in a mirror.
Everyone
Nerves Involuntary
Random shivers
A single brief shudder that runs down the spine or through the whole body when you are not cold. It is very common, and nobody has studied it formally.
Very common, no formal count
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 from a gallbladder. The nerves from the organ and from that patch of skin share a line into the spinal cord.
Everyone
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 drop blood pressure. The brain briefly runs short of blood and you go down.
About 4 in 10 people faint at least once
Skin Trick
Voluntary piloerection
Giving yourself goosebumps on command, without music, cold, or fear. About 1 in 1,500 people can do it. The muscles involved are not supposed to take orders.
About 1 in 1,500