Department of Psychiatry

SOPs

Dept. data last updated on :23/11/2021

Standard Operating Procedure for Electroencephalography (EEG)

Introduction to using the EEG lab

The most important thing to remember when recording EEG is that there is no substitute

for clean data. While filters and other transformations can be applied to data after

recording, these can have adverse effects on the quality of the signal. Therefore, every effort should be made to ensure that the data that is collected is as clean, reliable and artefact-free as possible.



In the lab


In general, if you use the EEG laboratory, you should leave it as you would like to find it. With an increasing amount of users and experiments being run, time in the lab can be at a premium. Thus, time spent before a session having to clean up or rearrange after the last user is time wasted, especially when a participant is waiting. EEG sessions are time consuming for participants so having them waiting while the lab is being prepared is to be avoided. Having your participant in the chair for any time longer than is necessary can increase the likelihood of fatigue, with concurrent lapses in concentration and potential decrease in motivation and performance. Besides reducing the risk of these adverse confounds, an efficient manner in the laboratory with the participant serves to promote the air of professionalism and aptitude.

Most experiments can be run using the same hardware set-up (i.e., computers, monitors, amplifier, electrodes, etc.). In the recording booth, there is a monitor for users to see the recording computer output, allowing the impedance meter to be seen, as well as allowing the participants to see their own EEG signal. Do not switch cables from computers/monitors unless it is absolutely necessary. An already-confusing array of cables can be difficult to fathom, especially when a participant is waiting. If you must do this, remember to reconnect them to their original positions after your session.

Computers in the lab


The computers in the lab are to be used for EEG related activities only. Only use the internet for experiment- or analysis-related activity.



Before your session


Before your session, make sure that the amplifier is working by taking the following steps


  1. Turn on the amplifier using the switch at the back.

  2. On the EEG recording computer, open Neuropage link. Within this program, open the appropriate workspace by clicking ‘File’ ‘Open Workspace’.

  3. Click ‘Monitor’.

  4. A blank signal should appear. This ensures that the recording software is recognising the amplifier.

  5. Click ‘Stop Monitoring’ to end.

  6. Turn on the computer which will present the tasks. Have your task(s) ready.


  1. Have EEG caps ready. Make sure that the caps are clean and dry from their last use. This is very important as water or excess gel can cause the smearing or ‘bridging’ of the signal across electrode sites, leading to artefacts in the data.

  2. Prepare the other consumables for the recording if they are not already laid out.


This includes ensuring adequate supplies of tissues, cotton-tipped buds, EOG ring stickers and holders, alcohol solution and syringes filled with electrode gel. Lay these out on the table for easy access during the session.


When the participant arrives


When the participant arrives, seat them on the chair in front of the monitor. Put the ‘Testing in Progress’ sign on the door. Measure the circumference of their head using the measuring tape which is hung on the handle of the door. This dictates which electrode cap is to be used.



Attach the electrodes to this cap (Note: you may have met the participant at a pre-testing session. If so, that is a good time to measure the head circumference allowing you to prepare the right cap in advance of the session). Ask the participant which form of strapping they would prefer; the chin strap or chest strap. Remember to measure the distance from the nasion (i.e., where the nose meets the face) to the inion (i.e., the projection of the occipital bone at the rear part of the skull) before putting the cap on (see Figure 1). This allows you to position the cap correctly. To decide where the cap sits, calculate 10% of the nasion-inion distance, and measure this distance from the nasion up along the forehead. This is the place where the front of the cap should lie.


If this is done correctly, all the electrodes on the cap should be located at the correct sites. When they have decided which strap they prefer, put the cap on and fasten it with this strap. When the cap is in place, the participant is ready to be gelled up.





Gelling the participant up


  1. With the electrode cap in place, use a cotton swab to part the hair in the centre of each electrode so that scalp is visible.


2. Clean the scalp/skin in the centre of each electrode using a cotton swab and the alcohol solution.
3. Fill the centre of each electrode with gel using a syringe, making sure to start injecting the gel at the skin and withdrawing the syringe as you push the gel out.

When the participant is ready

When the impedance at each electrode is low enough and EOG electrodes are in place, recording can begin. Since movement artefact negatively impacts on signal quality in a manner that is difficult to describe to participants, it is good practice to show participants their own EEG signal. This can be done by exiting from the impedance mode in Recorder

and clicking ‘Monitor’

The EEG signal should now be visible both on the main recording monitor in the lab and the monitor in the recording cubicle. To illustrate the extent of interference caused by eye blinks and movement, ask the participant to watch their signal as they blink, cough, grind their teeth etc. This method normally proves very useful in minimising movement artefact in the data.

Blinking:



Coughing:



Grinding Teeth:





General Movement: