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. 2022 Feb 17;22(4):1568.
doi: 10.3390/s22041568.

Advancing towards Ubiquitous EEG, Correlation of In-Ear EEG with Forehead EEG

Swati Mandekar  1   2 , Abigail Holland  2 , Moritz Thielen  2 , Mehdi Behbahani  1 , Mark Melnykowycz  2
Affiliations

Advancing towards Ubiquitous EEG, Correlation of In-Ear EEG with Forehead EEG

Swati Mandekar et al. Sensors (Basel). .

Abstract

Wearable EEG has gained popularity in recent years driven by promising uses outside of clinics and research. The ubiquitous application of continuous EEG requires unobtrusive form-factors that are easily acceptable by the end-users. In this progression, wearable EEG systems have been moving from full scalp to forehead and recently to the ear. The aim of this study is to demonstrate that emerging ear-EEG provides similar impedance and signal properties as established forehead EEG. EEG data using eyes-open and closed alpha paradigm were acquired from ten healthy subjects using generic earpieces fitted with three custom-made electrodes and a forehead electrode (at Fpx) after impedance analysis. Inter-subject variability in in-ear electrode impedance ranged from 20 kΩ to 25 kΩ at 10 Hz. Signal quality was comparable with an SNR of 6 for in-ear and 8 for forehead electrodes. Alpha attenuation was significant during the eyes-open condition in all in-ear electrodes, and it followed the structure of power spectral density plots of forehead electrodes, with the Pearson correlation coefficient of 0.92 between in-ear locations ELE (Left Ear Superior) and ERE (Right Ear Superior) and forehead locations, Fp1 and Fp2, respectively. The results indicate that in-ear EEG is an unobtrusive alternative in terms of impedance, signal properties and information content to established forehead EEG.

Keywords: BCI; biopotential electrodes; correlation; forehead EEG; impedance spectroscopy; in-ear EEG.

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Conflict of interest statement

The research work was carried out as a Master’s thesis project at IDUN Technologies AG, where Swati Mandekar, Abigail Holland, Moritz Thielen and Mark Melnykowycz were employed during the research study.

Figures

Figure 1
Figure 1
Foam earplug earpieces with three electrodes located at 120° apart of sizes 8 × 5 mm (area: 40 mm2).
Figure 2
Figure 2
(a) Electrode placement on the head for EEG measurement according to 10–20 International electrode placement, Fp1 and Fp2 refer to forehead locations, M1 and M2 are mastoids and Iz is inion; (b) labeling scheme for the in-ear electrodes, Exy where x ∈ {L, R} refers to the left or right ear and y is the position of the electrodes.
Figure 3
Figure 3
Impedance measurement setup on a subject. Electrodes on earpieces are working electrodes (WE), counter electrode (CE) is at inion, and reference electrode (RE) is between WE and CE, equidistant from both.
Figure 4
Figure 4
EEG recording setup on a subject. The ground was located at the inion, the reference at the contra-lateral mastoid.
Figure 5
Figure 5
Experimental procedure, the impedance measurements were carried out prior to the EEG recording. Left ear measurements were performed first with electrodes ELE, ELJ, ELH, and Fp1; followed by right ear measurements with electrodes ERE, ERJ, ERH, and Fp2. Each participant was subjected to three trials (designated as EEG Data Acquisition 1, 2 and 3).
Figure 6
Figure 6
EEG data pre-processing pipeline. EEG data consisted of 2 min data of eyes-open and eyes-closed condition. The raw data were restructured with labels and then bandpass filtered, and split into epochs. Epochs were cleaned according to epoch acceptance criteria to have labeled data which were ready for further computations.
Figure 7
Figure 7
Electrical equivalent circuit of skin electrode interface [35].
Figure 8
Figure 8
Mean impedance magnitude (a) and phase angle (b) with standard deviation of electrodes at in-ear locations E, J and H. n = 8.
Figure 9
Figure 9
Power spectral density of electrodes for left ear measurements of eyes-open (a) and eyes-closed (b) conditions.
Figure 10
Figure 10
Power spectral density of electrodes for right ear measurements of eyes-open (a) and eyes-closed (b) conditions.
Figure 11
Figure 11
For eyes-open (EO) and the eyes-closed (EC) conditions, the epochs of the ear electrodes are shown. Two distinct epochs are indicated by two distinct colors, (a) good epochs for eyes-open, (b) bad epochs for eyes-open, (c) good epochs for eyes-closed, (d) bad epochs for eyes-closed.
Figure 12
Figure 12
For eyes-open (EO) and the eyes-closed (EC) conditions, the epochs of the forehead electrodes are shown. Two distinct epochs are indicated by two distinct colors, (a) good epochs for eyes-open, (b) bad epochs for eyes-open, (c) good epochs for eyes-closed, (d) bad epochs for eyes-closed.
Figure 13
Figure 13
Signal-to-noise ratio (SNR) of electrodes on the left and right sides of the head.
Figure 14
Figure 14
Pearson correlation coefficient matrix of in-ear and forehead locations for left (a) and right ear (b).
See this image and copyright information in PMC

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