Wearable electroencephalography (EEG) systems are increasingly employed for real-world neurophysiological monitoring, enabling continuous brain signal acquisition outside controlled laboratory or clinical environments [1]. Such systems have potential applications in cognitive neuroscience, brain–computer interfaces, and neurorehabilitation [2–4]. Among wearable EEG devices, eyewear-integrated systems offer a particularly advantageous form factor by embedding electrodes into everyday spectacles, enabling unobtrusive and user-friendly recording while preserving comfort and social acceptability [5].

Despite these advantages, reliable signal acquisition in constrained form factors remains challenging due to limited electrode coverage, variable electrode–skin contact, and inter-subject anatomical variability [6]. Robust detection of electrophysiological markers under these constraints is essential to validate the practical utility of wearable EEG for research and clinical applications [7]. Alpha rhythm modulation during eyes-open/eyes-closed (EOEC) protocols [8–10] and P300 event-related potentials (ERPs) [11–13] elicited by oddball paradigms constitute widely used benchmarks for system sensitivity, signal fidelity, and temporal resolution. Moreover, both paradigms elicit robust cognitive responses that are largely subject-independent, as they rely on fundamental neurophysiological mechanisms (alpha suppression with visual engagement and attentional resource allocation to rare stimuli) rather than requiring extensive training or learned behaviors [14, 15]. Prior wearable EEG validations have largely been restricted to small cohorts or single-subject analyses, limiting generalizability and precluding systematic evaluation of anatomical influences on electrode performance [16].

In this study, we present a comprehensive assessment of a custom-built eyewear EEG frame incorporating two dry electrode types: gold-plated retractile pins (GPR) and soft conductive elastomer (CoE). The evaluation encompasses three critical dimensions: (i) electrode–skin impedance characteristics, (ii) alpha rhythm modulation during EOEC tasks, and (iii) P300 responses elicited by auditory oddball stimuli. Systematic analysis across a cohort stratified by head breadth, inter–tragion length, and head length provides novel insights into electrode design, signal reliability, and the feasibility of real-world EEG acquisition. Specifically, the objectives of this study are to:

To contextualize our work within the evolution of wearable EEG technologies, we divided existing solutions into two broad categories: (1) headset-based EEG systems and (2) eyewear-integrated EEG devices. Headset or headband EEG devices have been the primary focus of commercial and research efforts aiming to simplify scalp-based recording outside the laboratory (table 1). These designs often adopt relatively standardized electrode placement geometries derived from the international 10–20 system. However, in wearable solutions, it is not always feasible to maintain these precise scalp coordinates, leading to electrode locations that may deviate substantially from standard neurophysiological landmarks, which can affect signal fidelity, spatial specificity, and interpretability.

Table 1. Recent contributions in headset EEG systems with the description of the electrode setup (number of electrodes and their locations) and the intended purposes. The status is summarized as research platform (R), prototype (P), or commercial device (C).

DeviceElectrode setupPurposeStatusNeuroDot VR Kit [17]16-temporalVEPF analysisPOpenBCI Mark IV [18]8-scalpResearchRCGX Quick-20r [19]23-scalpResearchCMuse 2 [20]4-headbandMeditation, FocusCBrainBit [21]4-headbandSleep analysisCEmotiv Insight [22]5-headsetEmotion analysisCNeuroSteer [3]3-foreheadCognitive assessmentRNeurosity Crown [23]8-scalpFocus assessmentCBitbrain Diadem [24]12-scalpBCIC

NeuroDotVR (NeuroFieldz Inc. USA) is a headset that integrates both virtual reality and EEG recording systems. The system is designed for neuro-ophthalmic diagnosis and allows the presentation of visual stimuli and simultaneous EEG recording [17]. The EEG setup is based on eight custom-made hydrogel electrodes covering the O1, Oz, and O2 regions, with a reference placed on the earlobe. Given its size and purpose, this device was not designed for wearable use. The OpenBCI Ultracortex Mark IV (OpenBCI, USA) is a dry-electrode headset designed for research applications and compatible with different biosensing devices. Owing to its low cost and versatility, this setup has been used in diverse studies across BCI and emotion recognition [18]. The CGX Quick-20r (CGX Systems, USA) is a research-grade EEG headset equipped with 20 channels and dry active electrodes, with reference and bias on the earlobes, embedded electronics, and continuous impedance checking. These features, make it suitable for basic and clinical research [19]. Muse 2 (InteraXon Inc. Canada) is a consumer-grade EEG headband featuring four flat, gold-plated electrodes on hairless areas, paired with a PPG sensor for heart rate detection. It has been widely used as a portable EEG tool for ERP detection and cognitive fatigue assessment [20, 25, 26]. BrainBit (BrainBit Inc. USA) offers spring-loaded electrodes targeting temporal and occipital regions, designed for meditation, focus, and sleep analysis, and has been applied to eye movement assessment and pediatric epilepsy detection [4, 21]. Neurosteer (Neurosteer Inc. USA) uses a minimal three-electrode adhesive strip for forehead EEG acquisition, targeting neurodegenerative disease detection [3, 27]. Emotiv Insight (Emotiv, USA) and Neurosity Crown (Neurosity Inc. USA) are consumer-grade multi-channel headsets combining dry and semi-dry electrodes for emotion, focus, and performance monitoring [22, 23, 28]. The Bitbrain Diadem (Bitbrain, Spain) provides 12 scalp channels for portable research applications, often used in BCI and HRI studies [24, 29].

The shift toward EEG-enabled eyeglasses has led to the emergence of concepts, prototypes, research and commercial products for physiological monitoring [34]. Unlike headsets designed for full-scalp coverage, eyeglass-based EEG concentrates electrodes around the temples, ears, and nose bridge. While this improves wearability and aesthetics, it raises fundamental questions about whether these non-standard locations can reliably capture neural markers. Validating these placements is therefore essential for advancing wearable EEG hardware design. Furthermore, the geometric constraints of eyewear frames amplify the influence of anthropometric differences (e.g. temple curvature, skull shape, hair coverage), which remain underexplored in most published studies. Table 2 summarizes the existing examples of eyewear-based EEG devices reported in the literature and available on the market, specifying the number of electrodes, intended purpose, and details of eyewear setup validation, including the number of subjects involved.

Table 2. Recent contributions in eyewear EEG systems. The status is summarized as research platform (R), prototype (P), or commercial device (C). The type of validation is related to the tests performed to validate the setup and hardware architecture: EEG inspection (EEG), alpha wave modulation in eyes-open eyes-closed (EOEC), steady-state visually evoked potentials (SSVEP), motor movement (MM); the number in parentheses is the number of subjects involved in the validation tasks. * indicates discontinued devices. ** e-Glass studies do not report validation EEG measurements performed with the proposed platform; proposed algorithms were tested on the wearable platform.

DeviceElectrode numberPurposeValidation (# subjects)StatusAttentivU [30]3Cognitive performanceEEG (1)RAAVAA glasses4BCI—CSmith lowdown focus5Focus training—C*e-Glass [31]4Seizure detection, cognitive workload**RZero8Seizure detection—CGAPses [32]8ResearchEOEC (1), SSVEP (5), MM (5)REEGlasses [33]3ResearchEOEC (1), MM (1)PE-glasses [5]2ResearchEOEC (10), SSVEP (10)P

AttentivU is a research prototype of smart glasses enabling EEG and EOG measurements in an eyewear form factor, with electrodes at TP9/TP10 and on the nose pads for EOG. Its electronics are housed in the temple arms [30]. AttentivU has been applied to cognitive workload classification and wearable BCI studies [35, 36]. AAVAA glasses (AAVAA Inc. Canada) integrate four EEG channels with gesture and head movement sensors for hands-free interaction. The Smith Lowdown Focus is a now-discontinued consumer-grade eyewear solution that emphasized aesthetics and social acceptability, integrating EEG, EOG, and EMG sensors. The e-Glass is a research platform with four EEG channels (around F7, F8, T7, and T8), embedded electronics, and algorithms optimized for the detection of epileptic seizures and cognitive workload monitoring [31, 37]. Zero glasses (SK Biopharmaceuticals, South Korea) target epilepsy monitoring, embedding eight electrodes connected to temple tips for seizure detection. GAPses is a research prototype featuring pronged electrodes for improved contact in hairy regions, validated in steady-state visual evoked potential and oculomotor detection protocols [32]. EEGlasses is an early research prototype featuring three EEG channels (two electrodes on the temple tips and one above the nose bridge, with reference and bias on the nose pads). Signal recording was performed on an external biosensing board. The whole setup was tested on one subject in a single motor-action session [33]. The challenge of maintaining a stable contact with the head was also addressed in the E-glasses. Soft polymeric electrodes were mounted on a spring-loaded mechanism integrated in the eyeglasses’ temples to ensure constant contact pressure on a wide range of head’s dimensions [5]. Despite these innovations, none of these studies systematically correlate performance with anthropometric diversity, nor rigorously benchmark non-standard electrode placements against conventional neurophysiological markers. This motivates our focus on impedance and ERP validation at temple and over-ear sites.