Systematic Investigation of Optimal Electrode Positions and Re-Referencing Strategies on Ear Biosignals

This study aims to determine the optimal electrode positions and re-referencing strategies for ear biosignals by systematically comparing ear and conventional biosignals. We analysed four physiological signals: electroencephalography (EEG), electromyography (EMG), electrooculography (EOG), and electrocardiography (ECG), with five re-referencing strategies. The signals were recorded from conventional locations and around the ears. Quantitative measures, such as the signal-to-noise ratio, F1 score, and correlation, were employed to identify the optimal electrode positions and re-referencing strategies for each physiological signal type. The optimal ear electrode positions were selected based on their proximity to conventional configurations: the upper part/behind the ear for EEG, the lower part for blink detection and vertical EOG, and the front for clench EMG and horizontal EOG. While no single re-referencing strategy consistently performed best for all physiological signals, we found that (mean-)ipsilateral re-referencing strategies were more suitable for EEG, EMG, blink detection, and vertical EOG, whereas (mean-)contralateral re-referencing strategies were more effective for horizontal EOG and ECG compared to other strategies. The optimal ear electrode positions and re-referencing methods varied slightly across subjects and demonstrated comparable performance to the conventional configurations in terms of quantitative measures. Ear biosignals have the potential for use in developing human-computer interfaces with comparable performance to conventional electrode configuration while being more practical in terms of attachment and detachment. This study provides in-depth insight into the optimal electrode locations and re-referencing strategy for developing ear-biosignal-based human-computer-interface applications.