Correct Positioning & Usage of EEG Caps

CHBH MEG uses 64 Channel BrainCap EASYCAPs purchased from Brain Products UK, and have 6 caps available in 4 different sizes. The caps use the 10/20 system of electrode placement.

• 1 x 54cm, 2 x 56cm, 2 x 58cm, 1 x 60cm

The International 10/20-System

• This system employs connecting lines of anatomical landmarks, with the length of these lines defined as 100%.
• Electrodes are placed at distances of 10% or 20% in either the longitudinal or the lateral lines, resulting in approximately equidistant points.
  
  
  
  

Cap Sizes

• The cap size denotes the circumference of the head in centimeters, with the size labels found at the back of the cap.

• Measure the participant's head circumference at the "hat line" (10% higher than the Nasion, Inion, and ear openings), and choose a corresponding cap size (or one slightly larger).

Correct Positioning of the Cap

• Caps are placed on the head in relation to the Nasion and Inion anatomical landmarks.

• Measure the distance from Nasion to Inion centrally over the head with a tape measure.
  

• Then put the cap on the participant, making sure the Cz electrode position is centered/halfway between the Nasion and Inion.

• Make sure the cap is left-right symmetric and then close the chin strap.

• The cap is fitted on the participant correctly when Fp1/Fp2 - O1/O2 - T7/T8, viewed from the side, are all in the same plane (with Fp1/Fp2 close to the eyebrows).
  
  

Achieving good signal quality

Good contact between the skin and the electrode, using an electrolyte gel or paste, is essential to maintain low impedance and ensure high signal quality.

• With the wooden end of a cotton swab, push aside the hair in the electrode opening until the skin is clearly visible.
• Clean the skin with 70% Isopropyl Alcohol. Decant some alcohol into the provided stainless steel pot. Rinse out/clean the pot after use.
  Dip the cotton end of the swab in the pot and then, by twirling the swab between thumb and index/forefinger, degrease the skin in the electrode opening by rotating on the spot. This should reduce the initial impedance to below 30 kOhm.

If possible, ask participants to arrive with their hair washed, without using any conditioner.

• Abrade the skin with NuPrep paste. Squeeze some paste into the provided stainless steel pot. Rinse out/clean the pot after use.
  Dip another cotton swab into the paste and, again, rotate the swab on the spot in the electrode opening. This should further reduce the impedance to below 5 kOhm.

• EasyCap EEG caps have large openings in their ring-shaped electrodes, enabling abrasion of the skin by means of a cotton swab.
• Best results are achieved when rotating the swab FAST (4-5 fast twirls) but with MINIMAL PRESSURE.
• Using more pressure will only result in reddening of the skin/possible bleeding.
• "Quick, but gentle, is the order of the day!".
• With low pressure, this procedure is painless and very effective.
  
  
  
  

• After abrading using NuPrep (removing as much as paste as possible as it is non-conducting), apply electrode gel through each electrode opening, using the filled syringe and a blunt needle.

Check needle is firmly luer-locked in place, and gel is being extruded, BEFORE using the syringe.

• Touch the skin with the blunt needle and start to press the plunger. This is to establish contact between the skin and the gel. Only after the gel starts to extrude is the syringe pulled back, filling the electrode opening with gel.

DO NOT re-use the same needle on a second participant. Throw the needle away, in the provided yellow Sharps box, and open a new blunt needle.

• If more electrode gel is required, reverse fill a syringe using the provided caulking gun and the 300g gel cartridge.

Check impedances

• As most electrolytes permeate the skin, impedances improve after a few minutes 'magically' by themselves. Thus it's best to fill all the electrodes first, starting with REF and GND, and check the impedances afterward.
• If needed, repeat the abrasive step by gently twirling with a cotton bud.
  • Most impedance problems result from insufficient contact between skin and gel or gel and electrode.

SIGGI-II Impedance Meter

• Although the EEG impedances are checked by the Acquisition software, after plugging the cap into the side of the gantry, it is worth checking them whilst the participant is outside the MSR.

SIGGI-II User Manual

• The SIGGI-II can measure either single electrodes (Single-Ch-In) or multiple electrodes (Multi-Channel-In).

• A customised adaptor (Brain Products UK) allows 32 of the 64 EEG cap electrodes to be checked at the same time, with the Ground electrode plugged into the BLACK GND socket, and the Reference electrode plugged into the BLUE REF socket.

• When testing a single electrode e.g. an EOG or ECG electrode ...

  • Plug a Ground-Electrode into "N"
  • Plug a Reference-Electrode into "-"
  • Plug the to-be-measured electrode into "+"

Switching the SIGGI II On

• Press the white button named Enter/Esc on the jog-shuttle.
• After a short message the Main Menu will appear on the LCD-display.
  • SIGGI II starts with the settings stored last within the Setup Menu.

Impedance Measure

• Select Impedance Meter from the Main Menu (turn the jog-shuttle) and press Enter/Esc.
• The menu Impedance Measurement appears. The electrode impedances of the to-be-measured electrode, the reference- and the ground-electrode are displayed simultaneously.
• Impedance is shown as a bar graph as well as a numerical value. The range of the bar graph can be adjusted in the Setup menu, and also the Impedance measurement frequency if so desired.
• The electrode potential of the to-be-measured electrode compared to the reference electrode is shown beneath the selected input channel.
  • If all the electrodes are the same material => high potentials indicate faulty electrodes.
  • If the electrode materials are different => some amplifiers may be unable to process the signal if the electrode potentials are high.
• To select the channel for impedance measurement select CH menu and press Enter/Esc.
  By turning the jog-shuttle, Single-Ch-In with its three safety sockets, or one of 32 channels of Multi-Channel-In, is selected.
  Single-Ch-In is depicted by III, whereas Multi-Channel-In is shown by the channel numbers 1–32.
• To quit channel selection press Enter/Esc.
• To quit Impedance Measurement select the "up arrow" (on top row) and press Enter/Esc.
• To switch off the SIGGI-II, select Power Off from the Main Menu and press Enter/Esc

• When checking EEG Cap electrodes 1-32 (yellow connector), SIGGI-II displays numbers 1-32 as the channels under test.
• When checking EEG Cap electrodes 33-64 (green connector), SIGGI-II STILL displays numbers 1-32, NOT 33-64 e.g.
  • Electrode #33 => SIGGI-II channel #1
  • Electrode #64 => SIGGI-II channel #32