Recording Brain Activity with Ear-Electroencephalography

Provided here is a protocol for ear-EEG recordings with the c-grids. Following the steps of this protocol ensures high-quality recordings. In the following paragraphs, a comparison is made with cap-EEG, the most critical steps in the protocol along with some best practice recommendations are discussed, and some modifications are discussed.

Comparison of c-grids to cap-EEG and in-ear EEG
The c-grid allows the unobtrusive recording of brain activity in everyday life settings and is well suited for longer recordings. It has several advantages compared to cap-EEG. First, due to its weight, comfort, and low visibility, it barely restricts participants in their everyday activities¹. Second, it can be worn for extended periods of time – more than 11 hours in one study⁶– without the electrodes falling off^1,3,6, since they are sealed by the adhesive stickers. On the downside, the c-grid covers only a fraction of the surface of cap-EEG and, thus, cannot replace cap-EEG for all purposes. However, in cases when a lightweight, unobtrusive, quick-to-setup, minimally restricting solution is necessary (e.g., in the workplace), c-grids can provide relevant neural information.

The comparison of results across participants is potentially more difficult for c-grids compared to cap-EEG. For cap-EEG, often the international 10-20 system is used to facilitate the comparison of results across studies and across participants with different head sizes. In this system, the electrodes are positioned relative to specific anatomical landmarks (i.e., the nasion and inion for front to back and the ears for left to right). In practice, different cap sizes are used to account for different head sizes and thereby approximate the optimal electrode positioning. The c-grid cannot be easily integrated into that system for two reasons. First, these are currently available in one size and, thus, cover more or less space depending on the head size. Second, the shape of the ear influences the positioning of the grids. In general, the two top-most electrodes will be directly above the ear, but depending on the ear shape, they might be tilted more to the front or to the back. We are not aware of any study that has investigated whether these shifts in electrode positions are large enough to be of relevance.

Another approach to measuring ear-EEG is to place the electrodes inside the ear, for instance, in the outer ear canal or the concha^23,24,25. Such an approach offers even lower visibility than the c-grid but leads to recording signals with lower amplitudes due to the small distances between the electrodes²⁶.

Most critical steps
EEG in general, and especially mobile ear-centered EEG, remains a challenging technology. Therefore, the careful preparation of the participant and placement of the grids is essential to ensure good data quality over time. The preparation starts with the hair and the skin of the participants. The hair and skin around the ear should be washed and dried. In addition to that, the experimenter needs to carefully clean the area around the ear with abrasive gel and alcohol and ensure that the grids are firmly attached with the adhesive stickers. These steps are important and should be performed carefully to ensure good electrode-skin adhesion and a low impedance for longer periods. The skin cleaning especially can make the difference between a successful and an unsuccessful recording.

Even with proper care, however, the impedance for individual electrodes may still be poor directly after the placement of the electrodes. In general, the electrode-skin interface stabilizes over time, and we often observe that the impedance decreases within 5 min to 15 min. If the signal quality remains poor, it is recommended to completely remove the grids, wipe off any residual gel around the participant's ear, and fit a new one. It is faster to fit a new one as opposed to cleaning and preparing the previously removed grid. It is not recommended to add electrode gel to individual electrodes once the grid is fitted as this can compromise the adhesion strength of the stickers and can even lead to the bridging of the neighboring electrodes.

After the grid has been placed and when the impedance of the electrodes is low, data recording can begin. For longer recordings (>1 h), a brief data quality check should be conducted at the beginning. For instance, a 3 min auditory oddball task is exemplified in this study, which can be conducted and analyzed quickly to ensure a good signal quality.

In some cases, recording with the c-grid might not be possible at all, such as when the grid is too small for the ear (even after cutting) or when the hairline is too close to the ear, meaning that the grid does not stick to the skin. If the grid "hovers" over some hair, researchers cannot expect high-quality data.

Troubleshooting
Bad impedance and/or signal
To avoid these problems, it is imperative that the skin is carefully cleaned before fitting. Additionally, one should make sure to test the functionality of each electrode before the fitting. For example, one should check that the grid is plugged in correctly into the connector and that each electrode has firm contact with the skin and then wait for a few minutes until the impedance and the signal improve. To further check the functionality after fitting, the individual electrodes should be pressed, and the resulting signal should be checked. If the corresponding signal of each electrode shows a response, the electrode is functional in principle. If all of the above steps do not help, one should remove the grid, wipe off the residual gel around the participant's ear, and fit a new one.

Situations with no signal
Firstly, one should make sure the grid is connected properly to the amplifier, as well as ensuring that the grid connector is not upside down. There will be a signal only if the ground and reference electrodes are connected; whether the ground and reference will be on the left, the right, or on both sides depends on the connector.

Signal getting worse during the recording
There may be several reasons for this issue that need to be addressed. First, some of the electrodes might have become detached from the skin. This can happen when the adhesive is compromised by the remains of the electrode gel, by hair under the electrodes, or due to interference from the participant (e.g., scratching around the ear or adjusting glasses). Second, there might be problems with the connection between the grid and the amplifier (i.e., the grid might have been pulled out of the amplifier, or its position might have shifted). Finally, the grid might have sustained damage during usage. This can happen if the tail of the c-grid is bent too strongly.

Channels showing identical signals
In this case, the electrodes are bridged. One should remove the grid, wipe off the residual gel around the participant's ear, and fit a new one. One should also make sure to only use lentil-sized drops of electrode gel on each electrode to avoid bridging.

Participants reporting that the placement is uncomfortable
The most common reason for decreased comfort is that the grid is placed too close to the backside of the ear. One should make sure to leave 1 mm to 2 mm between the c-grid and the back side of the ear. A small piece of tape attached behind the ear helps to increase comfort.

Modifications of the method
The c-grid comes in one size. However, it allows for some flexibility regarding its size. By cutting the plastic of the inner side, the size may be reduced to fit larger ears. One should pay special attention in order to not cut into the electrodes or the conductive paths.

Depending on the amplifier used and the recording scenario, there are different ways to place the amplifier on the body. The fixed length of the tail of the grid and the fact that it points horizontally away from the ear limits the possible locations for placing the connector of the amplifier. Different manufacturers provide adapter cables that connect the grid to a specific amplifier (either mobile or lab-based). Different solutions have been proposed for placing the amplifier; some researchers use a headband³, whereas others integrate it into a basecap²⁷. For shorter experiments, a headband is suitable. For longer experiments, the amplifier can be taped to the clothes⁶ or body², stored in custom-made straps, taped to headphones worn around the neck¹, or taped to a neck protector commonly used for mountain biking. We have developed a prototype that combines a neck speaker (for presenting auditory stimuli) with a mobile EEG amplifier and connectors to the c-grid (building instructions can be found here: https://github.com/mgbleichner/nEEGlace). We have used this approach successfully in a recent study (in preparation) in which we recorded ear-EEG for 4 h while participants worked in an office.

Future applications
The c-grid is a promising tool for long-term recordings in everyday life. For instance, one can use it to investigate sound processing in everyday life¹. With long-term recordings, circadian variations in cognition and auditory function can also be investigated^28,29. For diagnostic purposes, the grid can be used for the long-term monitoring of epileptic seizures², sleep staging⁶, or for measuring attention for hearing devices^7,11.

Conclusion
This protocol comprehensively equips researchers for experimenting with these c-grids in and beyond the lab. If researchers follow this protocol and carefully perform the steps, including the most important ones, such as skin cleaning and fitting the c-grid, they can expect high-quality data for their ear-EEG experiments.