01 / RECORDwait, how does that work?

What does EEG
measure?

EEG, or electroencephalography, records tiny voltage differences at the scalp, produced largely by the activity of groups of brain cells.

1. Place electrodesSensors touch the scalp.

2. Compare voltageEach channel records a difference against a reference.

3. Follow the changesA line shows voltage over time.

What can we learn from these signals?

Those changes unfold in milliseconds, so EEG lets us follow sleep through the night, look for seizure-related activity, or measure the brain’s response to a sound.

Finding exactly where a signal began takes more work, because electrical activity spreads and overlaps before reaching the scalp.

Who decided where the electrodes go?

To compare recordings, we need to know where each electrode sat. Herbert Jasper described the 10–20 system in 1958, giving researchers a shared map of the scalp.

We measure across the head using landmarks near the nose, ears and back of the skull, then divide those distances into set proportions, so the positions remain comparable across different head sizes.

10–20 means spacing.

The gaps are 10% or 20% of a measured distance. The 10–10 and 10–5 maps add positions in between, giving us more places to record from.

one measured arc, divided into percentages

Sources & a little more detail

Placement maps: FieldTrip’s standard layouts, used here as 2D teaching projections. History: FieldTrip’s guide above. Denser layouts: Oostenveld & Praamstra, 2001. Names and clinical placement: Seeck et al., 2017 (IFCN). What EEG records and its clinical uses: American Epilepsy Society. Recording basics: American Epilepsy Society EEG atlas.

Viewer inspiration: Aref Pariz / PARSEC Neuro and EEG Electrode Visualization. This notebook is an independent implementation.

The displayed templates include extra named sites; they are not a prescription for a clinical cap. Ear/reference/auxiliary channels and individual anatomy vary. Data and attribution notes.

Once we start recording, the brain won’t be the only thing leaving a trace. That’s what we’ll look at next.

THE ELECTRODE MAPviewed from above

Which layout would you use?

Select an electrode to decode its location, or switch layouts to compare scalp coverage.

nose / fronttheir lefttheir rightback of the head

Right down the middle.

C refers to the central row, while z places this electrode along the middle of the head.

Odd = leftEven = rightz = midline

Decoding the alphabet soup

Fp: Frontopolar (the very front of the head)
F: Frontal (named for the frontal lobe, at the front)
C: Central (across the top of the head; there’s no “central lobe”)
T: Temporal (named for the temporal lobe, at the sides)
P: Parietal (named for the parietal lobe, toward the upper back)
O: Occipital (named for the occipital lobe, at the back)

You’ll also see letters paired together, so FC, for instance, places an electrode between the frontal and central rows on the scalp.

A study may use just some of these positions, depending on what the researchers need to record.