User Tutorial:EEG Measurement Setup

This page describes the physical setup required for EEG measurements. EEG utilizes metal electrodes attached to a human subject's scalp, measuring tiny electrical potentials which reflect the brain's electrical activity. Although setting up amplifiers and electrodes appears simple and straightforward, a successful, good quality EEG recording requires attention to not-so obvious details, and some practice.

Electrodes

• materials: Ag/AgCl, Sn, others
• inconsistent electrode materials obstruct measurement due to contact potentials
• ratio of electrode impedance to amplifier input impedance determines SNR; should be around 5-10kOhms
• measuring electrode impedance
• attaching electrodes to the head: skin preparation, gel

The 10-20 International System

The 10-20 international system is the standard naming and positioning procedure for EEG applications. It is based on the iterative subdivision of arcs determined on the scalp starting from craniometric reference points: Nasion (Ns), Inion (In), Left (PAL) and Right (PAR) pre-auricular points. The intersection of the longitudinal (Ns-In) and lateral (PAL-PAR) is named the Vertex.

The original 10-20 system included only 19 electrodes (see panel B of the figure). Later on, extensions were proposed so that now you can place over 70 electrodes in standard positions (see panel C of the figure). This extension also renamed four electrodes (marked in black in the figure); the original names were: T3, T5, T4, and T6 for T7, P7, T8, and P8, respectively.

Sometimes, one of the electrodes mounted in these positions is used as reference channel. More often, ear lobe or mastoid (i.e. bony outgrowth behind the ear) are used.

Montage Instructions

Following the flow of information, the biological (ionic) currents must be transduced into electronic currents. This task is carried on by the electrodeelectrolyte interface. In general, the electrode picks up the scalp potential, but if you go a little deeper, you will realize the importance of the gel and how it can be source of artifacts.

For the reasons that we will cover later on (see determining the most relevant channel, artifact detection and spatial filtering) it is mosten often not sufficient to only acquire one channel. In this tutorial we will acquire from 16 channels, but numbers between four and 64 are all reasonable. Two more electrodes (named Reference (Ref) – and Ground (Gnd)) are needed for the electronics of the amplifiers to work properly (i.e with high CMRR)

Sticking 16 (or worse 64) individual electrodes on the scalp of a subject is not a simple task. And doing it with high geometrical accuracy is a task for very experienced technicians. This is why we use EEG caps. They are made of elastic fabric (available in different sizes), and electrodes are already fixed in the proper configuration. One proven technique to perform an accurate montage is the following:

• find the vertex on the scalp of the subject and mark it
• find the position for Fpz and Oz and mark them
• find the Cz electrode on the EEG cap and place on the vertex
• keep Cz still, and slip the cap onto the head.
• paying attention that Cz does not shift, adjust the cap so that:
  • the Fz, Cz Pz line is on the midline
  • the Fp1-Fp2 line is horizontal and at the level of the Fpz mark
  • the O1-O2 line is horizontal and at the level of the Oz mark

You can now fix the Ref and Gnd electrodes (usually on the earlobes, with a specific electrode or on the mastoid, with cup electrode and collodion; some caps have electrodes for Gnd embedded on a scalp position).

Reasonable EEG traces should now be visible on the screen as soon as gel is injected. Visual inspection can detect the effetct of poor contact - excessive mains disturbance, frequent loss of contact, etc.). You may also want to use an impedance meter to make sure that electrodes have good contact (i.e. that their impedance is below $5-10k\mathrm{\Omega }$).

EEG Artifacts

Mains Interference

Electrical power lines use sinusoidal voltages with a frequency of 50 or 60 Hz, depending on your country. Generally, 50Hz are used in Europe, Asia, Africa, and parts of South America; 60Hz are used in North America, and parts of South America.

Mains voltage is typically 110 or 230 Volts, and thus exceeds the EEG's 50 to 100 Microvolts by a factor of $2*10^6$. Therefore, mains interference is ubiquitous in EEG recordings, especially if taken outside specially equipped, shielded rooms, and EEG amplifiers usually provide a so-called notch filter that suppresses signals in a narrow band around the mains frequency in question.

When mains interference is still present in the signal after activating the amplifier's notch filter, this is often due to high electrode impedance.

Blink Artifacts

Blink artifacts are generated by the dipole layer that is physiologically present on the cornea and on the retina. Closing the eyelids produces a fast movement that is detected as a positive peak lasting a few tenths of a second, mainly visible on the frontopolar derivations, but often perceivable on all the electrodes of the montage.

Their frequency content is negligible in the alpha band, so they produce no apparent effect on the Mu data analysis (if their number is reasonable). At the same time their amplitude is quite large so that time domain analyses (as P300 detection) can be strongly influenced by the presence of this kind of artifact.

Eye Movement (EOG) Artifacts

EOG artifacts are generated with similar mechanism to those underlying blink artifacts and are typically produced by eye movements. The effect on frontopolar and frontotemporal electrodes can be symmetric or antisymmetric, depending whether the movement was vertical or horizontal, respectively.

Their effect on frequency- or time-domain analysis is of the same kind of the effect of blink artifact except that the affected frequency band is at even lower frequencies, while amplitude can even be higher.