The Complete Electroencephalography Guide [Part 1]

Electroencephalography, or EEG, is a way of measuring brain activity.

EEG involves placing small, conductive electrodes on the scalp to measure electrical activity in the brain. A device, called an amplifier, records the electrical activity for use by a doctor or scientist.

Here is everything you need to know about EEG (part 1).

What does EEG stand for?

EEG stands for electroencephalography.

Electroencephalography can be broken down into three parts: electro from electric; encephalon from the Greek word for brain (enkephalos); and graphy from the Greek word graphein, meaning to write.

Put together: write down the electrical activity of the brain – which is exactly what EEG does!

How does electroencephalography work?

The reason EEG works is because neurons in the brain process information using electrical currents. Even when you are asleep, neurons communicate with one another using electricity.

Groups of neurons near the surface of the scalp produce so much electrical current that we can detect them using electrodes on the scalp. In fact, each EEG electrode will collect the activity of thousands of neurons over a 6cm² area (Olejniczak, 2006).

These neurons tend to point vertically upwards near the surface of the scalp. However, neurons that extend into deeper regions of the brain also affect EEG recordings.

For instance, recent studies have demonstrated that EEG can be used to detect activity in the cerebellum, a deep brain region involved in movement (Andersen et al., 2020).

For more information about the neurophysiology of EEG, see Holmes and Khazipov (2007).

Why do electroencephalography?

EEG is used for the diagnosis of epilepsy, stroke, dementia, brain tumours, concussion, sleep disorders and encephalitis. EEG can also detect the brain activity of coma patients.

Scientists use EEG to investigate brain function. For instance, in BCI research, a scientist might use EEG signals to control a computer. For more on this, check out our article on SSVEPs.

The biggest benefit of EEG is that it can read brain activity in real time. This is why EEG is so widely adopted. Few other procedures can register brain activity as it is happening.

This is important, because EEG is known to have relatively poor spatial resolution, which it makes up for with almost unparalleled temporal resolution.

In fact, high frequency EEG recordings have been used to detect activity at 2000 Hz – that’s 2000 oscillations a second (Zijlmans et al., 2012).

Who performs EEG?

For medical diagnosis, EEG is performed by highly trained professionals, generally clinical neurophysiologists.

However, for a polysomnography, which is a comprehensive assessment of your bodily activity during sleep, trained nurses will apply the EEG electrodes and connect them to your EEG device.

If you are part of a scientific experiment, a researcher may set you up with their EEG equipment. This is perfectly fine – EEG is very safe (see below), what matters most is the position and conductance of the electrodes.

In all cases, you will be told what is happening and why, and before the EEG is performed, the researcher or doctor will ask for your consent.

To perform the EEG procedure, your scalp may be cleaned, or you may be asked to wear a special cap, which holds the EEG electrodes in place.

Then the doctor or researcher will place the electrodes on your head, usually with a conductive paste, and connect the electrodes to the EEG recorder.

Is electroencephalography safe?

An EEG recording is generally comfortable and very safe. In essence, the procedure involves reading electrical activity from your brain. Remember: EEG is reading from your brain, not acting on it.

In fact, if there are any side effects to an EEG procedure, it is likely due to what is happening alongside the EEG recording, rather than the recording itself.

Where are EEG electrodes placed?

EEG electrodes are placed on the scalp. Researchers and doctors place the electrodes in strategic locations to maximise the information they obtain from your brain.

For instance, one researcher might use 4 electrodes just above your forehead to capture activity in the brain’s frontal lobe (that’s the most modern part of the brain, located at the front).

While another researcher might use over 200 electrodes placed all over the head to improve their recording’s spatial resolution.

If you’d like to learn more about this, check out our article that discusses how many electrodes to use in research.

One of the most popular ways to place electrodes, however, is the “10-20” system. The 10-20 is an international standard that maps EEG electrodes onto the brain so that researchers all work to the same, reproducible method.

In essence, the 10-20 system places adjacent electrodes 10% away from one another when measuring from the front to the back of the head, and 20% away from one another when measuring from the left side to the right side of the head.

For more information about the 10-20 system, it is worth looking at the international guidelines, written by Klem et al. in 1999. They were first laid out by Herbert Jasper in 1949 after he was asked to standardize EEG at an International Congress.

When is an EEG recording normal?

Whether an EEG recording is normal or not depends on several factors. These factors include the patient’s age, health, and state of consciousness, as well as several characteristics of the recording itself.

A doctor will look at a brain wave’s amplitude, frequency, symmetry, and continuity amongst other things to determine whether it is normal (Nayak, 2021).

What is more, we can split brain waves into frequency bands. Typically, the lower bands accept 0.1-1 Hz, and the higher bands accept 40-100 Hz (Newson & Thiagarajan, 2019).

In order to identify non-normal brain waves, a doctor will look at a person’s phenotype, the signal as a whole, and the characteristics of waves in each frequency band.

Because each of these factors is highly variable, describing a normal EEG recording is very difficult; identifying problems in EEG is as much an art as a science.

For instance, consider polysomnography. Here, a sleep technician will use EEG to ascertain when you are in deep sleep during the night. That is, they will read the EEG signals you produce while you are sleeping and “score” your brain waves.

However, research shows that no two sleep technicians will score brain waves in exactly the same way (Lee et al., 2022). This is because deciphering the cause of a brain wave, the so-called inverse problem, is very challenging.

This doesn’t mean that identifying problems in an EEG recording is impossible, far from it, but describing a normal EEG recording in text is tricky.