What an EEG Actually Sees in Your Head

The first thing that happens has nothing to do with electricity. A technician sits you down and takes out a cloth tape measure.

She measures from the bridge of your nose to the small bump at the back of your skull. Then ear to ear. Then she does arithmetic on a scrap of paper and marks your scalp in wax pencil at ten percent of that distance, then twenty, then twenty again. Every electrode sits at a percentage of your own head. That’s where the name comes from, the 10–20 system, and it’s why one map fits a toddler and a heavyweight boxer.

Tape measure, wax pencil, a tube of gritty paste that smells of salt, twenty-one little metal cups glued to a scalp. It looks like a craft project. What it’s about to do is read voltages measured in millionths of a volt off the outside of a sealed skull, which I still think is one of the odder things medicine gets to do.

§ The Man Who Was Looking for Telepathy

In 1893 a German cavalry recruit named Hans Berger was thrown from his horse into the path of a gun carriage. He wasn’t hurt. That evening his father sent him a telegram, the only one he ever sent, because Berger’s sister — many miles away, with no way of knowing anything — had become suddenly and unshakably certain that something had happened to her brother.

Berger spent the rest of his life trying to explain that telegram. He became a psychiatrist at Jena and went looking for the physical energy he assumed must carry thought from one mind to another. He tried blood flow. He tried temperature. Eventually he tried electricity, and in 1924 he recorded the first electroencephalogram from a living person: a young man with a skull defect left over from an earlier operation, which let the signal out more easily.

Then he sat on it for five years, checking and rechecking, convinced he’d be laughed at. He published in 1929 and was laughed at anyway. The rhythm he’d found — a steady 8–13 Hz oscillation that appears when you shut your eyes and disappears when you open them — was written off as a fault in his equipment until Edgar Adrian and Brian Matthews reproduced it at Cambridge in 1934. People still sometimes call it the Berger wave. He never found telepathy. He found something better, and by all accounts he didn’t much enjoy it.

§ What the Squiggles Are Made Of

Almost everyone assumes an EEG records neurons firing. It doesn’t. Action potentials last about a thousandth of a second, they point in every direction at once, and by the time they reach the scalp they’ve cancelled each other out into nothing at all.

What survives is slower and better organised: the summed postsynaptic potentials of cortical pyramidal cells. These neurons are stacked with their long apical dendrites all pointing the same way, perpendicular to the surface, like grass standing up in a field. Nudge a patch of them at the same moment and their tiny voltage shifts point the same way too. They add.

So picture standing outside a football stadium. You can’t hear any one person in there, not the loudest, not the one nearest the wall. But when forty thousand of them chant the same two syllables together, you hear it straight through the concrete. Synchrony is the only thing that gets out. Individual voices are gone for good. And the walls — bone, cerebrospinal fluid, scalp — smear whatever does escape.

An EEG isn’t a thought detector, then. It’s a synchrony detector. Estimates vary, but it takes something on the order of several square centimeters of cortex acting in unison to leave a visible mark on the page. Alpha rhythm is a large patch of visual cortex idling in step. A seizure, which I’ve written about before, is the whole crowd in lockstep, and that’s exactly why the machine sees it so well.

§ Why They Ask You to Breathe Like That

A routine EEG runs twenty to thirty minutes. If your brain does the interesting thing once a month, your chances of it happening inside that window are terrible. So the technician cheats. Three provocations, all of which feel from the chair like being messed with:

• Hyperventilation: three minutes of fast deep breathing, which drops carbon dioxide, constricts cerebral blood vessels, and is very good at bringing out absence seizures in children who have them.

• Photic stimulation: a strobe lamp stepped through a range of flash rates, hunting for the small minority of brains that answer flicker with an epileptiform discharge.

• Sleep deprivation: the reason you were told to come in tired. Drowsiness lowers the threshold for abnormal discharges, and some findings only show up as a person drifts off.

None of it is theater. Each one is an attempt to make a rare event happen on cue, in a short window, while somebody is watching.

§ What It Cannot See

Here’s the part that matters most, and the part nobody says clearly enough in the room:

• A normal EEG doesn’t rule out epilepsy. One routine study picks up epileptiform activity in roughly half of people who genuinely have it, and the figure moves around a lot depending on the study and how the recording was done. Repeat studies push it higher. A clean tracing is reassuring about the tracing.

• An abnormal EEG doesn’t confirm it. A small percentage of people who’ll never have a seizure in their lives throw sharp waves anyway. Epilepsy gets diagnosed from the story — what the person was doing, what happened, what it looked like to whoever was there — and the EEG is a supporting witness.

• Deep sources are nearly invisible. A seizure starting in the mesial temporal lobe, or low in the frontal lobe, can be an unmistakable clinical event and almost silent on the scalp.

• A lot of what’s on the page isn’t brain. Blinks, chewing, sweat, heartbeat, an electrode coming loose, the sixty-hertz hum of the building’s own wiring. Learning to read an EEG is largely learning what to throw out.

Which is the honest answer to the question in the title. It sees synchrony, in the top few millimeters of cortex, through bone, for half an hour, buried in noise. And it means nothing on its own, outside the hands of whoever took the history. Nothing here is guidance about anybody’s own result.

Berger wanted a machine that could tell him what was in his sister’s mind. What he built can tell you the crowd is roaring. Not one word of what they’re shouting.

§ Neuro Note

• 10–20 system: electrode positions are set as percentages of your own skull measurements, so one map fits any head.

• What it records: summed postsynaptic potentials in aligned cortical pyramidal cells, not individual neurons firing.

• Synchrony, not content: it takes several square centimeters of cortex acting together to register at all.

• Provocation: hyperventilation, strobe and sleep deprivation are all attempts to catch a rare event inside a short recording.

• Limits: a normal EEG doesn’t exclude epilepsy, an abnormal one doesn’t confirm it, and deep temporal or frontal seizures can be clinically obvious yet electrically quiet.