- EEG records electrical activity and brain waves using electrodes placed on the scalp. For a faster application, EEG electrodes are mounted in elastic caps, meshes or rigid grids, ensuring that the data can be collected from identical scalp positions across sessions or respondents.
- EEG is a non-expensive, non-invasive and completely passive recording technique.
- The most central benefit of EEG is its excellent time resolution, it can take hundreds to thousands of snapshots of electrical activity across multiple sensors within a single second. This renders EEG an ideal technology to study the precise time-course of cognitive and emotional processing underlying behaviour.
- Since the electrical signals are very small, the recorded data is digitized and sent to an amplifier. Price differences in EEG systems are typically due to the number and quality of the electrodes, the quality of the digitization as well as the quality and sampling rate of the amplifier. Usually, EEG systems with higher sampling rates typically are more expensive than devices with lower sampling rate.
- Once the data is amplified, it can be displayed as a time series of voltage values. 100 years ago the EEG time course was a plot on paper. While EEG recordings in clinical environments are often still paper-based, EEG systems in academic and commercial research typically show the data as a continuous flow of voltages on a computer screen.
Recording EEG data
There is no algorithm that is able to decontaminate poorly recorded data. So, you need to record clean data and not hope that you can clean them later.
Gathering data takes place using electrodes attached to the scalp to pick up electric potentials generated by the brain. There is important to check electrodes before recording. Some types of electrodes require gel that in time can corrode the metal.
How to get optimal electrode positioning
- With a band tape, measure the vertical distance from nasion to inion (for example 38 cm). Mark down the center (here, 38/2 = 19 cm) with a skin-friendly, water-based and sterile marker pen.
- Measure the horizontal distance from right to left pre-auricular points (for example 30 cm). Mark down where the marked position intersects with half of the distance (30/2 = 15 cm). This establishes the center (Cz) of the 10-20 system.
- Properly place the electrode array (cap, strip, headband etc.) on the respondent using Cz and make sure that the other electrodes are placed in the expected location.
The following aspects can influence the EGG data, and you should consider them when you interpret your recordings:
- The muscles activity, especially the facial muscles but also the heart beating.
- Eye movement and blinks
- Movement of an electrode or the whole cap (stable connection means clean data)
- Swaying and swinging, especially the head ones
For more information about EEG see Pocked Guide – EEG.
Protocol EEG experiment
- Make sure the battery is charged
- Ask the participant the circumference of the head (it would be better to already ask it the day before the experiment)
- Place the electrodes on the cap of the right size
- Start up the EEG and test computer and open Brain Vision Recorder
- Make sure the syringe is clean when clean fill it with the gel and lay it down
- Make sure the alcohol, the pencil, the towels, the wads of cotton wool, cotton swabs, stickers and the measuring tape are laid down
- Prepare the EOG:
- Red: left and above à in plus
- Blue: right and beneath à in min
- Vertical: above and beneath the left eye
- Ground EOG: above the right eye
- Aux 1: horizontal
- Aux 2: vertical
- Turn on the red light above the door
- Welcome the participant.
- Hand over the informed consent form to the participant to be filled in.
- Let the participant perform the left-right handedness test, write down the results.
- Perform the Acuity test, one eye at a time! Write down the results.
- If necessary, perform the colour blindness test, write down the results.
- Check the head circumference:
- from nasion (between eyebrows) to inion (protrusion at the back of the head) over the top of the head and write down the exact distance as well as the middle of the distance
- from earlobe to earlobe over the top of the head and write down the exact distance as well as the middle of the distance
- There an intersection of the two measurements, mark this on the scalp as the center of the head.
- Mark at 10% of the nasion-inion distance (from nasion).
- Mark two points to the left and right of this point at 5% of the earlobe-earlobe distance.
- Place the cap on the head and check twice if the cap is correctly placed.
- Make sure the Cz is in the middle of the head, is the cap symmetrical?
- Attach the electrodes to the clothing of the participant.
- Clean the skin at the places where the electrodes will be placed.
- Attach the stickers onto the electrodes and then stick them on the face.
- Place the EOG electrodes,
- Aux 1: horizontal, Aux 2: vertical
- Red: left and above, Blue: right and beneath
- Place ground electrode.
- Take the charger out of the EEG battery, and connect this on top of the EEG computer (grey in EEG battery, red in the computer).
- Put the brain vision recorder on impedance check, red lights turn on.
- Take the syringe with gel, explain what you are going to do, tell them it will not hurt and they have to say it when they feel discomfort.
- Start with the ground electrode. To make sure that all the lights turn green, put gel in the electrodes and use cotton swabs to move the gel around in the electrode.
- If this does not work, remove the electrode, clean the skin with alcohol and try again.
- Click on the eye icon in the programme and check if all the electrodes give good signals.
- Fill in the data on the participant registration form.
- Start the experiment and check if the participant understands everything.
- Press play.
- Name the file: participant number, man/woman, left/right handed, age
- Pause: P, continue with R.
- Check during the experiment if the electrodes still give a good signal.
- Write down the number the participant gives you each break for how focused the participant feels.
- When the task is finished, turn off the EEG, press stop and exit.
- Remove the cap, give the participant shampoo and a towel.
- Thank the participant for their participation and answer possible questions. Ask if the participant wants to receive further information about the purpose of the and the results of the research.
- Clean everything:
- Electrodes with a toothbrush and distilled water
- Cap with shampoo and water, and cotton swabs for the gel
- Syringe with water and put them in the tray
- Clean up the rest of the used items
- Recharge the battery