A cap of listeners
Imagine slipping on a soft cap dotted with many little metal buttons. Each button is an electrode, and together they form an EEG — short for electroencephalography, which simply means "writing down the brain's electricity." The cap doesn't poke or shine anything into you; it just rests on your scalp and *listens*.
Here's the lovely part. Beneath the cap, billions of brain cells are buzzing, like a stadium full of people murmuring at once. A single electrode can't hear any one person. Instead, it picks up the chorus — the faint, blended hum of many neurons firing together in the patch of brain below it.
What's a brainwave?
When many neurons fire in step, their combined hum rises and falls in steady rhythms, like waves rolling onto a beach. We call these brainwaves. A faster wave wobbles up and down many times a second; a slower one takes its time. The EEG sees these rhythms as gentle squiggles dancing across the screen.
Scientists gave the common rhythms friendly Greek-letter names by how fast they go. Delta and theta are slow and sleepy; alpha shows up when you relax with your eyes closed; beta appears when you're alert and focused; and gamma is the quickest flicker. You don't need to memorize the numbers — just remember that slower waves feel restful and faster waves feel busy.
Naming the spots: the 10–20 system
If a lab in Tokyo and a lab in Toronto both put electrodes "on the scalp," how do they make sure they mean the same spots? They use a shared map called the 10–20 system. It measures even fractions of the distance across your head and gives each position a short name — like Cz at the very top or O1 near the back over the vision area. Think of it like numbered seats in a stadium: say the seat name, and everyone knows exactly where you mean.
Why it's noisy
EEG is wonderfully easy to wear, but it pays a price for staying outside your head. Between the neurons and the electrode sit your skull and skin, and they blur the signal — like listening to a conversation through a thick wall. So the brain's whisper arrives smeared and softened, and a single electrode hears a smudged blend rather than a crisp voice.
On top of the blur come artifacts — bursts of electrical clutter that aren't really brain activity. A single eye blink, a clenched jaw, or a shrugged shoulder can swamp the delicate brainwaves. We call this unwanted clutter a signal artifact, and it's often far louder than the brain itself.