Chapter 1 — EEG Basics
Think of it like this: your brain is a huge stadium full of people (neurons) clapping (firing) at slightly different times. An EEG electrode is a microphone held outside the stadium — you don't hear any single clap, you hear the whole crowd's roar rising and falling. That roar is the squiggly line on an EEG trace. You'll never pick out one person's individual clap this way — there are billions of neurons and the microphone is, relatively speaking, very far away. What you get instead is the crowd's overall mood: is it roaring, murmuring, or dead silent?
The Seating Chart: the 10-20 System
A single microphone only tells you how loud the WHOLE stadium is. To know which section is cheering loudest, you place several microphones around the roof — that's exactly what EEG electrodes are, placed according to a standard layout called the 10-20 system (named for the fact that each electrode sits 10% or 20% of the way across the skull from its neighbors).
Each electrode has a letter + number, and the letters aren't random — they're a real seating chart: Frontal (front, behind your forehead), Central (top-middle), Parietal (back-middle), Occipital (very back, over your visual cortex), Temporal (sides, over your ears). Numbers tell you left or right: odd numbers = left side, even numbers = right side, and a "z" means dead center (like Fz, Cz, Oz). So "F4" is instantly readable once you know the code: frontal, right side.
Sampling Rate: the Flipbook Speed
EEG doesn't record a smooth continuous line — it takes rapid snapshots, like a flipbook or a stop-motion film. Sampling rate is how many snapshots per second. More snapshots per second (a higher sampling rate) means smoother playback and the ability to catch genuinely fast events. Too few snapshots and fast things get missed entirely, or worse, look like something else — the same reason old movies sometimes show a spinning wagon wheel appearing to rotate backwards: the camera's "sampling rate" was too slow for the wheel's real speed. Sleep EEG is usually sampled at 100–256 Hz (that's 100 to 256 snapshots every second) — plenty fast for brainwaves, which top out around 30–40 Hz.
Referencing: You Can't Measure Height Without a Floor
Voltage is always relative to something — you can't say "this electrode reads 10" without saying 10 compared to what. EEG systems pick a reference point (another electrode, or the average of several) and measure everything else against it, the same way you'd measure someone's height against the floor, not in some absolute sense. Different reference choices can make the exact same brain activity look slightly different on screen — worth knowing before you panic that two recordings of the same person look different.
Spotting the Seagulls: Artifacts
Sometimes a seagull flies past the microphone and goes SQUAWK — way louder than the crowd, and nothing like it. In EEG these loud, obviously-not-brain signals are called artifacts, and each one has its own distinctive "squawk":
- Eye blinks — a slow-moving spotlight swinging past the mic: one big, smooth, slow hump. Shows up strongest on frontal electrodes since the eyes sit right behind the forehead.
- Muscle tension (EMG) — like someone shaking the microphone stand: fast, jagged, chaotic scribble, nothing like a wave.
- Movement — someone actually bumping the mic: a sudden huge jump or offset in the whole trace.
- Electrical line noise — a nearby speaker humming at one constant pitch: a perfectly regular, too-clean, fast oscillation at exactly 50 or 60 Hz (depending on your country's power grid). Real brain signal is never this mechanically regular.
Once you've heard each seagull once, you never mistake it for the crowd again.
THE LAB
One click opens it directly in Google Colab (free, no install, no download). You'll download one real night of sleep EEG from inside the notebook, plot it, and go hunting for a real eyeblink artifact until you can spot one on sight.
DONE WHEN
You can look at a fresh 30-second raw trace and say "that's an artifact" vs. "that's signal," without anyone telling you which is which.
Up next: a dedicated game to drill exactly this skill before moving on.