Recording Brain Activity with Ear-Electroencephalography

Hi, I'm Martin Bleichner from the Neurophysiology of Everyday Life Group at the University of Oldenburg. We use the cEEGrid to study brain processes beyond the lab. The large advantage of the cEEGrid is that you can record EEG in everyday life.

The goal of this protocol is to help you to obtain high quality ear-EEG data. We show how to use the cEEGrid to obtain optimal results, how the cEEGrids are prepared, how the participant is prepared, and how the recording can be done. The cEEGrid has two sides.

There is black text on the outside. The conductive surfaces of the electrodes are on the inside and they face the skin of the participant during the recording. Handle the cEEGrids with care.

Do not touch the conductive surfaces, do not excessively bend the cEEGrids, and avoid pulling on them. To ensure that your cEEGrids are functional, we recommend that you test them before the recording. Already used cEEGrids can also be reused again.

To check each electrode, you can use a multi-meter. Start by setting the multimeter to measure resistance, then attach one pin of the multimeter to the cEEGrid electrode, and the other pin to the corresponding contact on the cEEGrid. Check if you measure a resistance.

Alternatively, you can use electrode gel to bridge all of the cEEGrid electrodes. Make sure that there are no gaps between the electrodes. After bridging the electrodes, attach the cEEGrid to the connector of the amplifier.

To see a signal, you need to attach the cEEGrid to the side where you have your reference and ground electrode according to the connector layout. Check whether you see low impedance for all of the electrodes. You should see eight recording electrodes lighting up.

Then wipe off the gel. For optimal recordings, participants should have clean and dry hair without any product. To prepare the participant, you need access to the area behind and around the ear.

For longer hair, use hair clips for easier access. Place a cEEGrid around the participant's ear to see how it fits. Also, check if it can be positioned around the ear without touching the ear.

The cEEGrid should not touch the back of the ear or the ear lobe, as this can be uncomfortable after some time. This refitting also gives an indication of the area that will be covered by the cEEGrid, and needs to be cleaned afterward. cEEGrids come in one size and may not fit all ear sizes.

For larger ears, cut some of the plastic around the electrodes on the inside of the C with a small pair of scissors. Pay special attention to neither cut into the electrodes nor the conductive paths. Use abrasive electrode gel to clean the skin around the participant's ear with a little bit of pressure, but make sure it remains comfortable for the participant.

Make sure to clean the whole area, which will be covered by the cEEGrid, generously. Ask your participants to indicate if any of the preparatory steps become uncomfortable for them. After applying the gel, clean the area behind the ears with alcohol.

Then dry it off with a clean towel. The skin cleaning can make the difference between a successful and an unsuccessful recording. As an additional option, you can also place a small stripe of medical tape behind the ear for higher levels of comfort.

Attach the double-sided adhesive stickers around each electrode. Make sure that the stickers do not cover the connective surface of the electrodes. Then put small amounts of electrode gel approximately the size of a lentil on each electrode.

Avoid using too much gel, as this might spill onto the adhesive material and reduce adhesion to the skin. Too much gel could also create bridges between the electrodes. Remove the adhesive stickers.

Reapply the gel in places where it was removed during this step. Ask the participant to hold their hair away from the ears, so that it does not obstruct the fitting. Move any hair out of the way as much as possible, so that the stickers touch the skin directly.

Depending on the hairline, this is not always possible for the hair directly above the ear. Position the cEEGrid around the ear, and when in place, press it into the skin. Make sure to not place the cEEGrid too close to the ear, as this may become uncomfortable for the participant.

Leave some space, one or two millimeters, between the cEEGrid and the backside of the ear. Also, ask the participant to press onto the electrodes. Remove any hair clips.

You can now also carefully place glasses or the strips of facial masks onto the ears, if necessary. Repeat this procedure for the other ear. To summarize, first, clean the skin with abrasive gel.

Second, clean the skin with alcohol. Third, rub dry with a clean towel. Some cEEGrids also come with pre-attached C-shaped stickers, as shown here.

The procedure remains the same. Add small drops of electrode gel to each electrode and remove the C-shaped sticker. Again, position the cEEGrid around their ear, and when in place, press it onto the skin.

If the cEEGrid does not stick well, or the signal quality is poor, we recommend to remove the cEEGrid, wipe off residual gel around the participant's ear, and fit a new cEEGrid. In the next step, we connect the cEEGrids to the amplifier. Connect the connector to the mobile EEG amplifier.

Then connect the cEEGrids to the connector. cEEGrid can be connected to any amplifier with the right connector. Make sure that you know the layout of your connector, including the position of the ground and reference electrodes.

To hold the amplifier in place, you can use a headband, for example. Alternatively, we use here an amplifier that is built into a neck speaker, which we call the nEEGlace. The nEEGlace makes the setup more comfortable and faster.

While connecting, avoid excessively bending the cEEGrids or pulling on the cEEGrid. Pay attention that you plug the right side of the cEEGrids into the connector. Once you're connected to your smartphone or laptop, check the impedance of the electrodes.

Impedance will generally improve over time and does not need to be below 10 kiloohms for each electrode in the beginning. Next, briefly check the EEG signal. Ask your participant to clench their jaws, blink, and close their eyes while watching the respective EEG signals.

If everything works up to this point, you are ready to start your experiment and collect data. After finishing the data recording, disconnect the cEEGrids from the amplifier and remove the amplifier. Gently remove the cEEGrids from the participant.

Make sure to avoid bending the cEEGrid too much or pulling out the hair of your participant. To clean the cEEGrids, soak them in water for a few minutes, then carefully detach the adhesive stickers, and rinse off any remaining electrode gel. Again, take care not to scratch the material.

After the cEEGrids have been dried, they can be reused again. There are some mistakes that experimenters can make when using the cEEGrids. Here we show some exemplary data of them.

Here we illustrate what the data looks like when no gel is used The cEEGrids has wet electrodes and does not work without using gel. If too much gel is used, the electrodes of the cEEGrid can become bridged. Data for this scenario is shown here.

The bridged electrodes show exactly the same signal. When the preparation and fitting is carried out carefully, you can expect high quality data, which we display here. Lastly, we recorded a piano played oddball task, where we played one tone frequently and another tone infrequently.

The participant was instructed to count the infrequent tones. The data from this experiment is shown here. Here we see the expected ERP responses, such as the P3 component for the infrequent tone.

If you follow these steps carefully, you will obtain high-quality ear-EEG data. We hope that this protocol helps you to advance your research.