Tri-manual interaction in hybrid BCI-VR systems: integrating gaze, EEG control for enhanced 3D object manipulation

Abstract

Brain-computer interface (BCI) integration with virtual reality (VR) has progressed from single-limb control to multi-limb coordination, yet achieving intuitive tri-manual operation remains challenging. This study presents a consumer-grade hybrid BCI-VR framework enabling simultaneous control of two biological hands and a virtual third limb through integration of Tobii eye-tracking, NeuroSky single-channel EEG, and non-haptic controllers. The system employs e-Sense attention thresholds (>80% for 300 ms) to trigger virtual hand activation combined with gaze-driven targeting within 45° visual cones. A soft maximum weighted arbitration algorithm resolves spatiotemporal conflicts between manual and virtual inputs with 92.4% success rate. Experimental validation with eight participants across 160 trials demonstrated 87.5% virtual hand success rate and 41% spatial error reduction (σ = 0.23 mm vs. 0.39 mm) compared to traditional dual-hand control. The framework achieved 320 ms activation latency and 22% NASA-TLX workload reduction through adaptive cognitive load management. Time-frequency analysis revealed characteristic beta-band (15-20 Hz) energy modulations during successful virtual limb control, providing neurophysiological evidence for attention-mediated supernumerary limb embodiment. These findings demonstrate that sophisticated algorithmic approaches can compensate for consumer-grade hardware limitations, enabling laboratory-grade precision in accessible tri-manual VR applications for rehabilitation, training, and assistive technologies.

1 Introduction

The urgent need for intuitive multi-limb control in rehabilitation and assistive technologies drives the development of advanced brain-computer interfaces (BCIs) in virtual reality (VR). Current limitations in consumer accessibility and practical deployment of BCI-VR systems create a critical gap between laboratory demonstrations and real-world applications. This study addresses this gap by developing a cost-effective tri-manual control system that enables simultaneous operation of two biological hands and a virtual third limb, offering transformative potential for individuals with motor impairments and enhancing human capabilities in complex manipulation tasks.

The evolution of brain-computer interfaces (BCIs) in virtual reality (VR) has transitioned from single-limb control paradigms to sophisticated multi-limb coordination systems, yet achieving intuitive tri-manual operation—simultaneous control of two biological hands and a virtual third limb—remains a significant technological and neurophysiological challenge. This capability represents a fundamental advancement beyond conventional dual-hand interactions, offering transformative potential for rehabilitation medicine, assistive technologies, and human augmentation applications where additional manipulative capacity could enhance functional independence and task performance.

Early foundational work in BCI-VR integration established motor imagery paradigms for 3D modeling interfaces, with Shankar and Rai (2014) and Verma and Rai (2013) demonstrating classification accuracies comparable to traditional input modalities in CAD environments. While these studies laid crucial groundwork for neural-driven interaction, they remained constrained to single-actuator systems with limited applicability to complex multi-limb scenarios. Subsequent integration of eye-tracking by Zander et al. (2010) demonstrated hybrid selection capabilities through P300-gaze models, yet their approach struggled with multi-object environments, inducing cognitive overload that limited practical deployment (Škola and Liarokapis, 2018).

The democratization of consumer-grade BCI technologies, particularly single-channel systems like NeuroSky’s MindWave, has introduced new accessibility opportunities while creating novel trade-offs between signal fidelity and usability—challenges magnified in tri-manual tasks requiring concurrent attention division across multiple control modalities (Dhole et al., 2019; Perales and Amengual, 2013; Ramakuri et al., 2017; Värbu et al., 2022). Contemporary efforts to synchronize multimodal inputs have prioritized specialized hardware solutions, such as Kim and Kim (2019) FPGA-synchronized EEG systems (Kundu et al., 2024), but their incompatibility with consumer VR ecosystems limits widespread adoption. Similarly, while Chin et al. (2010)‘s PID-controlled hybrid systems achieved high precision in 2D tasks, they failed to address the unique cognitive demands of tri-limb coordination in three-dimensional space.

Recent advances in hybrid BCI systems have demonstrated significant performance improvements through multimodal physiological signal integration. Tan et al. (2022) developed autonomous hybrid BCI systems combining EEG and eye-tracking in virtual environments, introducing particle swarm optimization fusion methods that automatically determine optimal weighting coefficients for multimodal data integration. Their sliding window analysis approach for eye-gaze variance detection provides effective autonomous control strategies, achieving superior accuracy and information transfer rates compared to single-modality systems. However, their variance-based detection methods may lack the stability required for continuous tri-manual control scenarios.

The integration of synchronized EEG and eye-tracking in fully immersive VR environments has emerged as a critical methodological advancement. Larsen et al. (2024) developed comprehensive frameworks for multimodal data acquisition, demonstrating average hardware offsets of 36 ms between data streams with 5.76 ms jitter tolerance. This synchronization methodology proves essential for applications requiring precise temporal coordination between neural signals and gaze behavior, particularly in complex manipulation tasks where timing accuracy directly impacts user experience and task success.

Contemporary research has identified attention-aware adaptation as a key requirement for practical BCI-VR deployment. Long et al. (2024) explored multimodal attention detection using combined EEG and eye-tracking features, addressing the challenge of extended training periods that typically hinder widespread adoption of attention-aware BCI systems. Their work demonstrated that multimodal approaches can significantly improve classification accuracy while reducing calibration requirements, making BCI-VR systems more accessible for everyday applications.

Crucially, prior work has overlooked the unique requirements of virtual third limbs—systems requiring seamless arbitration between gaze-driven targeting and BCI-triggered actuation while preserving manual dexterity for concurrent tasks. Hou and Chen (2021) demonstrated that eye-triggered interaction in virtual reality achieves comparable accuracy to controller-based input in 3D target selection tasks, with performance gaps narrowing in three-dimensional environments. However, their study highlighted critical limitations in calibration stability and device precision, indicating that eye-based interaction alone remains insufficient for dynamic multi-limb coordination scenarios requiring sustained precision and reliability.

The concept of embodiment in virtual supernumerary limb control has gained particular attention following recent neurophysiological investigations. Alsuradi et al. (2024) conducted comprehensive studies of neural signatures associated with motor imagery of supernumerary thumbs in VR environments, achieving 78% classification accuracy for distinguishing motor imagery from baseline conditions. Their findings revealed distinct beta-band energy modulations during successful virtual limb control, providing crucial neurophysiological evidence for the feasibility of controlling additional virtual body parts through mental imagery. Similarly, Arai et al. (2022) explored embodiment of supernumerary robotic limbs in virtual reality, demonstrating that users can develop ownership sensations over additional virtual limbs through visuotactile feedback mechanisms.

This study introduces a novel consumer-grade framework for tri-manual VR interaction that addresses three historical barriers to practical deployment: (1) replacing laboratory-grade EEG arrays with accessible single-channel systems without sacrificing spatial precision, (2) mitigating cognitive overload through real-time adaptive task modulation, and (3) resolving temporal conflicts between manual and virtual actuation mechanisms. By integrating Tobii eye-tracking (120 Hz), non-haptic controllers, and NeuroSky’s e-Sense attention metrics, our system enables users to manipulate two physical objects while a virtual limb retrieves a third target—a paradigm advancing beyond conventional dual-hand or BCI-only control approaches.

2 Related work

The convergence of brain-computer interface technologies with virtual reality systems has emerged as a transformative area of research, offering novel approaches to human-computer interaction and neurorehabilitation. Recent advances in consumer-grade hardware have made multimodal BCI-VR systems increasingly accessible, enabling new paradigms for real-time neural signal processing and immersive interface design.

Early foundational work in BCI-VR integration focused primarily on rehabilitation applications, with researchers demonstrating the potential for motor imagery-based control in virtual environments (Vourvopoulos and Bermúdez i Badia, 2016). These studies established that virtual reality could serve as an effective training medium for BCI systems, providing rich visual feedback and contextual cues that enhance user engagement and learning outcomes. Building upon this foundation, recent research has expanded to explore more sophisticated multimodal approaches that combine EEG with other physiological signals.

Recent advances in hybrid BCI systems have demonstrated significant improvements in performance through the integration of multiple physiological signals. Tan et al. (2022) developed an autonomous hybrid BCI system that combines EEG and eye-tracking in virtual environments, introducing a particle swarm optimization (PSO) fusion method to automatically determine optimal weighting coefficients for multimodal data integration. Their sliding window analysis approach for eye-gaze variance detection provides an effective autonomous control strategy that eliminates the need for manual triggering, achieving superior accuracy and information transfer rates compared to single-modality systems. This work established important precedents for autonomous decision-making in hybrid BCI applications, demonstrating that intelligent fusion algorithms can adapt to individual differences in signal quality and user performance.

The integration of eye tracking with EEG in virtual reality environments represents a significant methodological advancement in the field. Larsen et al. (2024) developed a comprehensive framework for synchronized EEG and eye tracking data acquisition in fully immersive VR, demonstrating an average hardware offset of 36 ms between data streams with 5.76 ms jitter. Their work validated the feasibility of combining commercial EEG and VR technologies for neuroscientific research, providing essential timing accuracy measurements for multimodal BCI applications. This synchronization methodology has proven crucial for applications requiring precise temporal coordination between neural signals and gaze behavior.

Recent innovations in attention-aware VR systems have demonstrated the potential for real-time adaptation based on multimodal physiological signals. Long et al. (2024) explored the detection of external and internal attention states using combined EEG and eye tracking features, addressing the challenge of long training periods that typically hinder the widespread adoption of attention-aware BCI systems. Their work showed that multimodal approaches can significantly improve classification accuracy while reducing calibration requirements, making BCI-VR systems more practical for everyday applications.

The emergence of novel interaction paradigms has further expanded the possibilities for BCI-VR integration. Reddy et al. (2024) introduced an innovative eye-brain-computer interface that combines gaze tracking with stimulus-preceding negativity (SPN) for target selections in extended reality environments. Their approach addresses the “Midas touch” problem in gaze-based interfaces by using anticipatory neural responses as confirmation signals, eliminating the need for deliberate physical actions while maintaining selection accuracy. This work demonstrates how passive neural signatures can serve as implicit control mechanisms in immersive environments.

The concept of embodiment in virtual reality has gained particular attention in the context of supernumerary limb control. Alsuradi et al. (2024) conducted a comprehensive investigation of neural signatures associated with motor imagery of a supernumerary thumb in VR environments, achieving 78% classification accuracy for distinguishing motor imagery from baseline conditions. Their findings revealed distinct beta-band energy modulations during successful virtual limb control, providing neurophysiological evidence for the feasibility of controlling additional virtual body parts through mental imagery. Similarly, Arai et al. (2022) explored the embodiment of supernumerary robotic limbs in virtual reality, demonstrating that users can develop a sense of ownership over additional virtual limbs through visuotactile feedback mechanisms.

The extension of BCI-VR technologies into augmented reality environments represents an emerging frontier with significant potential for practical applications. Xu et al. (2024) demonstrated the feasibility of combining AR-based frequency-related potential (FRP) BCI with eye-tracking for hands-free control of multi-robot systems, achieving a recognition success rate of 90.67% in online experiments. Their Microsoft HoloLens2-based implementation showcases how AR environments can provide more natural and intuitive interfaces for complex control tasks, particularly in scenarios where traditional manual control methods are insufficient. This work bridges the gap between laboratory-based BCI research and real-world robotic control applications, highlighting the potential for immersive interface technologies to enhance human-machine collaboration.

The methodological landscape has been enriched by advances in signal processing and machine learning approaches specifically designed for VR environments. Prapas et al. (2024) provided a systematic review of BCI-AR systems, highlighting the need for standardized evaluation protocols and benchmarking frameworks. Their analysis revealed that current BCI-VR implementations often lack consistent performance metrics, making it difficult to compare results across different studies and technologies.

Technical challenges in multimodal BCI-VR systems have been addressed through various innovative approaches. The integration of multiple physiological signals, including EEG, eye tracking, and potentially other modalities, requires sophisticated synchronization and processing algorithms (Guo et al., 2023). Recent work has demonstrated that hybrid approaches combining different signal types can overcome individual limitations, such as the temporal resolution constraints of eye tracking or the spatial resolution limitations of single-channel EEG.

Consumer hardware limitations continue to influence system design choices, with researchers developing creative solutions to maximize performance within cost constraints. The availability of affordable VR headsets with integrated eye tracking, such as those used by Larsen et al. (2024), has democratized access to multimodal research platforms. However, these systems still require careful calibration and synchronization to achieve the temporal precision necessary for real-time BCI applications.

Current research directions indicate a growing emphasis on practical applications and real-world deployment considerations. Unlike laboratory-based studies that rely on expensive research-grade equipment, recent work has focused on developing systems using commercially available hardware that can be deployed in clinical or home settings. This shift toward practical implementation has highlighted new challenges related to system robustness, user training requirements, and long-term reliability.

3 Methodology

3.1 Experimental platform and hardware configuration

This study developed a comprehensive experimental framework to investigate tri-manual control in virtual reality through the integration of consumer-grade brain-computer interface technology, eye-tracking systems, and traditional manual controllers. The experimental design aimed to validate whether sophisticated algorithmic approaches could compensate for hardware limitations while enabling intuitive control of two biological hands and one virtual limb simultaneously.

The experimental platform was constructed around an HTC Vive Pro virtual reality headset operating at 90 Hz refresh rate with 2,880 × 1700 combined resolution, augmented with Tobii Pro Nano eye-tracking modules integrated into the head-mounted display. Eye-tracking data was captured at 120 Hz with manufacturer-specified accuracy of 0.5° visual angle under optimal conditions. Neural signals were acquired using a NeuroSky MindWave Mobile 2 headset positioned at the Fp1 location according to the international 10–20 system, sampling at 512 Hz with 12-bit resolution. The single dry electrode configuration provided access to both raw EEG data and proprietary eSense metrics including attention and meditation scores updated at 1 Hz intervals. Manual input was captured through two standard HTC Vive controllers with haptic feedback disabled to prevent interference with force measurements.

All hardware components were synchronized using FPGA-generated triggers from a National Instruments PCIe-6341 data acquisition card operating at 200 MHz, implementing IEEE 1588 Precision Time Protocol to maintain temporal alignment across data streams with measured jitter below 1.1 ms. This approach built upon synchronization methodologies established by Kim et al. (2019) for EEG systems, though adapted for consumer-grade hardware integration. The experimental setup incorporated an OptiTrack motion capture system with eight Prime 13 cameras arranged in an octagonal configuration, providing sub-millimeter tracking accuracy for ground truth validation of both controller positions and head movements at 240 Hz sampling rate.

Figure 1 illustrates the complete tri-manual VR hybrid control framework integrating multimodal sensory inputs with real-time processing and feedback mechanisms. The system architecture begins with gaze tracking data captured by the Tobii Pro Nano eye tracker, which undergoes Kalman filtering to reduce noise and improve target prediction accuracy within the 45° visual cone. The filtered gaze coordinates combine with eSense attention metrics from the NeuroSky EEG headset to enable target locking when attention thresholds exceed 80% for 300 ms continuous duration. The central dynamic priority arbitration module resolves conflicts between manual controller inputs and virtual hand commands using the softmax-weighted mechanism described in Section 3.7, ensuring seamless transitions between control modalities. The framework supports simultaneous manipulation of three colored spheres (red, blue, gray) through operational interaction paradigms where two spheres are controlled directly via handheld controllers while the third sphere responds to the hybrid gaze-BCI control. Real-time performance visualization displays pupil diameter variations, gaze angles, and position guidance metrics, providing immediate feedback for system optimization (Pester et al., 2022). The bottom panel demonstrates the physical experimental setup with a participant wearing the integrated HTC Vive Pro headset equipped with both EEG sensors and eye-tracking modules, alongside the Unity-based monitoring interface showing real-time data streams for pupil diameter, gaze angles, and position guidance. This comprehensive framework achieves the 89 ms end-to-end latency necessary for natural tri-manual coordination while maintaining the 92.4% conflict resolution success rate reported in our results.

Figure 1

3.2 Virtual environment development

The virtual environment was developed in Unity 2020.3 LTS, featuring a minimalist workspace containing three colored spheres (red, blue, green) with 10 cm diameter positioned at vertices of an equilateral triangle with 40 cm sides. Dynamic physics simulation operated at 1,000 Hz using NVIDIA PhysX 4.1, with collision detection resolution set to 0.1 mm. Visual rendering employed temporal anti-aliasing and motion blur reduction algorithms to minimize latency perception. The experimental setup incorporated design principles from Škola and Liarokapis (2018) regarding embodied VR environments for motor imagery training, while extending their framework to support tri-manual interaction paradigms.

3.3 Participant recruitment and demographics

Eight right-handed adults (22–26 years, 4 M/4F, mean age 24.1 ± 1.5 years) with normal or corrected-to-normal vision (20/20 Snellen equivalent) participated in the study. All participants were neurologically healthy with no history of motor or attention disorders, confirmed via pre-screening questionnaire.

Each participant underwent a structured 30-min training protocol:

• - Phase 1 (10 min): VR environment familiarization and control mapping.

• - Phase 2 (10 min): Two-hand baseline performance assessment.

• - Phase 3 (10 min): Personalized calibration of e-Sense attention thresholds (>80% sustained for 300 ms) and gaze stabilization parameters (0.65–0.85 N·s/m damping coefficients).

Following training, participants completed 20 experimental trials using the tri-manual system in a counter-balanced design. Each trial lasted 60 s, during which participants performed the designated manipulation tasks. Trial order was randomized using a Latin square design to control for learning and fatigue effects. Each 60-s trial was segmented into 3.75-s epochs (50% overlap, Hamming window), yielding approximately 32 epochs per trial and 640 total training samples across all trials for analysis. Mandatory 5-min rest periods were enforced after every 5 trials to prevent fatigue accumulation.

3.4 Calibration procedures

The calibration procedure began with standard nine-point eye-tracking calibration achieving mean accuracy below 1° visual angle for all participants. Personalized eSense attention thresholds were established through a staircase procedure where participants performed simple focusing tasks while monitoring real-time attention metrics. The threshold for virtual hand activation was set at the 80th percentile of sustained attention levels maintained for 300 ms, with individual values ranging from 75 to 85 across participants. Gaze stabilization parameters were tuned using proportional-derivative control with damping coefficients between 0.65 and 0.85 N·s/m based on individual saccadic characteristics measured during smooth pursuit tasks.

3.5 Tri-manual control architecture

The tri-manual control architecture implemented a hierarchical decision system integrating multiple input modalities. Manual control operated through direct position mapping with 1:1 scaling between physical controller movement and virtual hand displacement. Eye-tracking data underwent Kalman filtering to reduce noise and predict gaze targets within a 45° visual cone projected from the cyclopean eye position. This approach extended the hybrid selection capabilities demonstrated by Zander et al. (2010) while addressing their identified limitations in multi-object environments. The state-space formulation employed transition and observation models:

3.6 Neural signal processing

Neural signal processing implemented a comprehensive pipeline to extract reliable features from the inherently noisy single-channel EEG signal. The NeuroSky MindWave Mobile 2 acquired data from the Fp1 position at 512 Hz sampling rate with 12-bit resolution, incorporating hardware-level preprocessing through its ThinkGear ASIC chip including 50/60 Hz notch filtering and 3-100 Hz band-pass filtering. Our software pipeline processed the signal in 2-s sliding windows with 50% overlap, applying artifact detection through multiple criteria: amplitude thresholds of ±100 μV for eye blink detection, gradient thresholds exceeding 50 μV/sample for muscle artifact identification, and variance thresholds where σ > 35 μV indicated poor electrode contact. Signals exceeding these thresholds triggered a 500 ms blanking period where virtual hand activation was disabled, reducing false positive rates from 18.3 to 2.3% in pilot testing.

The filtering stage employed a 4th-order Butterworth band-pass filter (4-40 Hz) followed by moving average detrending with a 500 ms window to remove slow drifts while preserving neural dynamics. Feature extraction utilized Welch’s method with 256-point FFT and 50% overlapping Hamming windows to compute power spectral density, from which we derived band powers for theta (4–8 Hz), alpha (8–12 Hz), and beta (12–30 Hz) frequencies. The primary attention metric was calculated as the ratio β/(α + θ), normalized by a 30-s rolling baseline to account for individual differences and temporal variations. Additional features included signal quality indices (0–200 scale), temporal derivatives of band powers, and zero-crossing rates, creating a 10-dimensional feature vector updated at 8 Hz.

False trigger mitigation employed a three-tier validation system specifically designed for single-channel limitations. First, temporal consistency checks required sustained attention levels exceeding personalized thresholds for 300 ms continuous duration, with any signal quality drop below 150 resetting the timer. Second, confidence-weighted gating scaled activation thresholds inversely with signal quality: thresholds increased by 20% when quality dropped below 100, and virtual hand control was completely disabled below quality index 50. Third, contextual validation compared current attention patterns against a 60-s historical buffer, rejecting activations that deviated more than 2.5 standard deviations from recent patterns. This multi-layered approach achieved 87.5% true positive rate while maintaining false positive rates below 5% across all participants, despite the single-channel constraint.

3.7 Multimodal arbitration system

Multimodal arbitration resolved conflicts between manual and virtual control through a softmax-weighted mechanism that dynamically adjusted authority based on confidence metrics and temporal stability:

where ϵ_BCI ∈[0,1] (BCI confidence), t_gaze ∈[0,2 s] (gaze stabilization time). This achieved 210 ± 35 ms handover latency with 92.4% conflict resolution success, prioritizing manual control during spatial overlaps.

Gaussian Process Regression (Matérn 3/2 kernel) predicted NASA-TLX scores (R² = 0.79)

• Normalized task duration

• EEG variance (15–22 Hz β-band)

• Gaze entropy Hg = −∑ p(x) logp(x)

When predicted load exceeded 68/100, the system:

• Increased BCI confirmation threshold by 20%

• Reduced virtual hand speed to 80% of maximum

• Applied stabilization forces (F = 2.5 N/mm).

3.8 Cognitive load adaptation

Cognitive load adaptation employed Gaussian Process Regression with Matérn 3/2 kernel to predict NASA-TLX scores from real-time physiological markers. Input features included normalized task completion time, EEG beta-band (15–22 Hz) variance computed over 2-s sliding windows, and gaze entropy H_g = −Σp(x)log p(x) calculated from fixation probability distributions. When predicted cognitive load exceeded 68/100 based on training data correlations (R² = 0.79), the system implemented three adaptive mechanisms:

• Increasing BCI confirmation threshold by 20% to reduce false activations.

• Reducing maximum virtual hand velocity to 80% of baseline 0.5 m/s.

• Applying stabilizing forces proportional to position error at 2.5 N/mm to assist precise positioning.

This adaptive approach addressed concerns raised by Dhole et al. (2019) and Perales and Amengual (2013) regarding cognitive overload in consumer-grade BCI systems and serious game applications.

3.9 Task complexity quantification

Task complexity was quantified through a weighted linear model validated against pilot performance data:

where Δx = target displacement (0–1.8 m), Hg = gaze entropy, Nd = distractor count. Coefficients (λ1 = 0.5, λ2 = 0.3, λ3 = 0.2) derived from PCA explained 82% variance in trial performance.

Data streams (gaze, EEG, controller poses) synchronized at 200 Hz via hardware timestamps. The framework achieved 89 ms end-to-end latency using optimized sensor fusion, validating real-time tri-manual coordination.

3.10 Experimental protocol

Experimental trials followed a structured protocol where participants manipulated three colored spheres to match randomized target configurations displayed as wireframe outlines. Each trial began with spheres in standardized starting positions, followed by an auditory cue initiating the 30-s manipulation period. Participants controlled two spheres directly using handheld controllers while the third sphere required coordinated gaze targeting and BCI activation. Success was defined as achieving all three spheres within 5 mm of target positions simultaneously for 1 s. Real-time visual feedback included color-coded proximity indicators and haptic-like visual pulsing for the virtual hand to compensate for absent tactile sensation. This protocol design built upon tri-manual coordination principles identified by Hou and Chen (2021) in their comparative study of eye-based and controller-based VR selection, though extending their framework to incorporate BCI control.

3.11 Data acquisition and synchronization

Data acquisition occurred through a custom C++ application implementing lock-free circular buffers for each sensor stream, with hardware timestamps enabling post-hoc synchronization to microsecond precision. This synchronization approach followed methodologies established by Kundu et al. (2024) for FPGA-accelerated BCI signal processing. The complete data pipeline processed approximately 18 MB/s across all channels, with real-time compression achieving 3:1 ratios for storage efficiency. All experimental data followed Brain Imaging Data Structure (BIDS) formatting standards, including comprehensive metadata documentation of hardware configurations, software versions, and participant demographics. Signal quality metrics were computed online, with automatic trial rejection for epochs containing excessive artifacts defined as EEG amplitudes exceeding ±100 μV or eye-tracking data loss above 20%.

3.12 Performance evaluation metrics

Performance evaluation encompassed both objective and subjective measures collected immediately after each trial. Objective metrics included task completion time with millisecond resolution, spatial error vectors computed as Euclidean distance between achieved and target positions for each sphere, and trajectory efficiency calculated as the ratio of actual to optimal path length. Movement smoothness was quantified through jerk minimization scores, while coordination between manual and virtual control was assessed through cross-correlation of velocity profiles.

Subjective assessment employed modified NASA-TLX questionnaires presented in VR, capturing workload across six dimensions: mental demand, physical demand, temporal demand, performance satisfaction, effort, and frustration levels on 100-point scales (Hart, 2006; Hart and Staveland, 1988).

3.13 Statistical analysis

Statistical analysis protocols were predetermined to avoid multiple comparison issues, with primary outcomes defined as virtual hand success rate and spatial positioning accuracy. The experimental design provided 80% power to detect a 15% difference in success rates based on pilot data variability (G*Power 3.1.9.4, effect size f = 0.35). All continuous measures underwent Shapiro–Wilk normality testing, with appropriate parametric or non-parametric analyses selected accordingly.

For normally distributed data, we employed:

• - Paired t-tests for two-condition comparisons with Cohen’s d effect sizes.

• - Repeated measures ANOVA with Greenhouse–Geisser correction for sphericity violations.

• - Pearson correlations with 95% confidence intervals.

• For non-normal distributions:

• - Wilcoxon signed-rank tests with rank-biserial correlation as effect size.

• - Friedman tests for multi-condition comparisons.

• - Spearman’s rho for correlation analyses.

Mixed-effects models (lme4 package in R 4.3.0) accounted for repeated measures within participants, with random intercepts and slopes for learning effects across trials. Significance thresholds were set at α = 0.05 with Bonferroni correction (adjusted α = 0.05/6 = 0.0083) for planned comparisons across six performance metrics. All analyses were performed using SPSS 28.0 and R 4.3.0.

4 Results

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4.1 Time-frequency analysis of tri-manual control during object grasping

The experimental framework demonstrates neural dynamics during tri-manual ball grasping tasks using consumer-grade BCI hardware. Figure 2 presents comprehensive time-frequency analysis of single-channel EEG signals across three distinct task phases, revealing how attention-driven neural patterns enable virtual hand control while maintaining manual dexterity for simultaneous object manipulation.

Figure 2

These results were obtained through a comprehensive EEG preprocessing pipeline designed to handle single-channel noise constraints. Raw EEG signals from the NeuroSky MindWave Mobile 2 (Fp1 position, 512 Hz sampling, A1 reference) underwent multi-stage processing: hardware-level 50/60 Hz notch filtering and 3–100 Hz band-pass filtering, followed by software-based artifact rejection using amplitude thresholds (±100 μV for blinks), gradient thresholds (>50 μV/sample for muscle artifacts), and signal quality gating. The preprocessing pipeline employed 4th-order Butterworth filtering (4–40 Hz), Welch’s method for spectral analysis (256-point FFT, 50% overlap), and extracted features including band powers (theta: 4–8 Hz, alpha: 8–12 Hz, beta: 12–30 Hz) and the attention metric β/(α + θ).

False trigger mitigation was critical for reliable virtual hand control. We implemented temporal consistency checks requiring sustained attention >80% for 300 ms, confidence-weighted thresholds that increased by 20% during poor signal quality (index <100), and contextual validation rejecting outliers >2.5σ from 60-s baselines. The CSP-LSTM classifier processed 10-dimensional feature vectors through bidirectional layers (128 units total) with dropout (p = 0.3), achieving the reported 87.5% success rate despite single-channel limitations. This preprocessing reduced false positive rates from 18.3 to 2.3%, enabling the reliable beta ERD/ERS detection shown in the red windows.

Phase 1 (Forming—Palm Contact) shows significant initial beta suppression (15–20 Hz) at object approach (cluster-based permutation test: p = 0.003, corrected), with distinct power increases in the 750–1,000 ms window marking contact detection (mean power increase: 2.8 ± 0.6 μV², t(7) = 4.67, p = 0.002, d = 1.65). Phase 2 (Carving - Palm Contact) demonstrates sustained alpha desynchronization (8–12 Hz) during continuous manipulation (ERD magnitude: −42.3 ± 8.1%, significantly different from baseline, p < 0.001). Phase 3 (Detailing - Palm Contact) exhibits gamma bursts (30–40 Hz) corresponding to precision adjustments (peak gamma power: 1.9 ± 0.4 μV², F(2,14) = 18.3, p < 0.001, η²p = 0.723 for phase comparison).

4.2 Eye-tracking integration for virtual hand targeting

This section examines gaze-based target selection mechanisms that complement BCI control for virtual hand positioning. Figure 3 demonstrates how Tobii Pro eye-tracking (120 Hz) integrates with e-Sense metrics to achieve precise spatial targeting during multi-object manipulation tasks.

Figure 3

4.3 Comparative performance analysis: two-hand vs. tri-manual control

To validate the effectiveness of our tri-manual control paradigm, we conducted comprehensive performance comparisons between traditional two-hand control and our proposed BCI-VR hybrid system. Table 1 summarizes the quantitative improvements across six key performance metrics.

Figure 4 presents a comprehensive multi-dimensional analysis of the performance differences. The normalized comparison (Figure 4A) reveals consistent improvements across all metrics, with the most substantial gains in task completion efficiency. The learning curves (Figure 4B) demonstrate faster skill acquisition with the tri-manual system, reaching performance plateau by trial 10 compared to trial 15 for two-hand control.

Figure 4

The efficiency-precision correlation analysis (Figure 4C) shows a stronger positive relationship in the tri-manual condition (r² = 0.82) compared to two-hand control (r² = 0.64), indicating more coordinated performance improvements. The radar chart (Figure 4D) visualizes the multi-metric performance profile, clearly illustrating the expanded performance envelope achieved through tri-manual control.

Statistical distribution analysis (Figure 4E) confirms lower variance in the tri-manual condition across all primary metrics, suggesting more consistent and reliable performance. The improvement summary (Figure 4F) highlights that all metrics showed statistically significant enhancements, with task completion showing the greatest improvement at 65.6%.

Critically, the cognitive load-performance trade-off analysis (Figure 4G) reveals that the tri-manual system achieves superior task performance while simultaneously reducing cognitive burden, addressing a fundamental limitation of traditional multi-tasking interfaces. The statistical significance matrix (Figure 4H) confirms robust improvements across all metric pairs (p < 0.05), validating the comprehensive superiority of the tri-manual approach.

These findings demonstrate that augmenting human control capabilities through BCI-integrated virtual limbs not only enhances quantitative performance metrics but also fundamentally improves the user experience by reducing cognitive strain while expanding operational capabilities.

To validate the robustness of our findings, we conducted additional statistical analyses. A two-way repeated measures ANOVA examining the interaction between control type (two-hand vs. tri-manual) and trial progression (trials 1–20) revealed significant main effects for both control type [F(1, 7) = 52.3, p < 0.001, η²p = 0.882] and trial number [F(19, 133) = 14.2, p < 0.001, η²p = 0.670], with a significant interaction [F(19, 133) = 4.91, p < 0.001, η²p = 0.412]. Post-hoc polynomial contrasts confirmed a steeper linear learning trend for tri-manual control [F(1, 7) = 18.7, p = 0.003].

The correlation between gaze entropy and task performance was significantly stronger in the tri-manual condition (Pearson’s r = −0.79, 95% CI [−0.91, −0.58], p < 0.001) compared to two-hand control (r = −0.52, 95% CI [−0.74, −0.21], p = 0.004), with Fisher’s r-to-z transformation confirming this difference (z = 2.31, p = 0.021).

4.4 Machine learning performance and cognitive load assessment

The final experimental validation examines classifier performance, learning dynamics, and comprehensive workload metrics that establish the tri-manual system’s practical viability (Figure 5).

Figure 5

4.5 Comprehensive system validation and neuroergonomic assessment

Figure 6 synthesizes multi-modal performance metrics to establish the tri-manual framework’s superiority across control authority, neural dynamics, cognitive load, and user preference dimensions.

Figure 6

Table 2 presents a comprehensive technical performance comparison across all control modalities evaluated in this study, highlighting the significant advantages of the tri-manual hybrid approach across multiple performance dimensions.

4.6 Statistical validation summary

To ensure the reliability and generalizability of our findings, we performed comprehensive statistical validation:

• 1. **Assumption Testing**: All parametric analyses were preceded by Shapiro–Wilk normality tests. For repeated measures ANOVA, Mauchly’s test of sphericity was applied, with Greenhouse–Geisser corrections where necessary (ε < 0.75).

• 2. **Power Analysis**: Post-hoc power analysis using G*Power confirmed adequate statistical power (1-β > 0.80) for all significant findings, with observed power ranging from 0.82 to 0.99 for main effects.

• 3. **Effect Size Interpretation**: Following Cohen’s guidelines, effect sizes were classified as small (d ≥ 0.2), medium (d ≥ 0.5), or large (d ≥ 0.8). All significant comparisons showed large effect sizes (d > 1.4), indicating robust practical significance.

• 4. **Multiple Comparison Control**: Family-wise error rate was controlled using Bonferroni correction across the six primary performance metrics (adjusted α = 0.0083).

• 5. **Reliability Analysis**: Test–retest reliability across trials showed high consistency (ICC(3, 1) = 0.89, 95% CI [0.84, 0.93], p < 0.001).

5 Discussion

This study successfully demonstrates the feasibility of integrating consumer-grade multimodal technologies to create a sophisticated BCI-VR system capable of three-hand coordination. The achieved 87.5% virtual hand success rate and 41% spatial error reduction validate our hypothesis that carefully designed algorithmic approaches can compensate for hardware limitations, enabling laboratory-grade precision with cost-effective components. These findings contribute significantly to the growing body of evidence supporting the practical deployment of BCI-VR systems in real-world applications.

Our three-hand control paradigm represents a novel contribution to the field of multimodal BCI-VR systems. The integration of e-Sense attention thresholds with dynamic gaze-manual arbitration mechanisms achieved an average activation latency of 320 ms, substantially outperforming the 450–800 ms delays reported in existing literature. This performance improvement aligns with recent advances in autonomous hybrid BCI systems, particularly the sliding window approach demonstrated by Tan et al. (2022), who achieved superior performance through PSO-based fusion optimization. However, our attention-based triggering mechanism offers advantages over variance-based detection methods in continuous control scenarios, as it provides more stable activation signals during extended interaction sessions. When compared to the specialized applications demonstrated by Chen et al. (2022) in driving scenarios, our system’s 87.5% success rate in general manipulation tasks suggests broader applicability across diverse interaction contexts.

The neurophysiological findings from our time-frequency analysis provide compelling evidence for the neural basis of supernumerary limb control in virtual environments. The observed beta-band (15-20 Hz) energy modulations during successful virtual hand activation are consistent with recent work by Alsuradi et al. (2024), who reported similar neural signatures during motor imagery of a supernumerary thumb in VR. The stability of these neural markers across increasing task complexity levels (L1-L5) suggests that users can develop robust neural representations of additional virtual limbs, supporting the embodiment mechanisms described by Arai et al. (2022) in their study of supernumerary robotic limb embodiment.

Our soft maximum weighted arbitration algorithm addresses a critical challenge in multimodal BCI systems—the resolution of conflicts between simultaneous input modalities. The 92.4% conflict resolution rate achieved by our system represents a significant advance over static threshold approaches commonly used in existing research. This dynamic adaptation capability is particularly relevant given the attention-awareness requirements identified by Long et al. (2024) in their work on multimodal attention detection using EEG and eye tracking features in VR environments.

The performance characteristics of our system compare favorably with recent developments in BCI-VR integration. While Reddy et al. (2024) achieved impressive results using stimulus-preceding negativity for target selection in XR environments, their approach requires specific anticipatory neural responses that may not be suitable for continuous control tasks. Our attention-based triggering mechanism provides a more generalizable approach that can be applied across diverse interaction scenarios without requiring task-specific neural training.

The methodological innovations presented in this work address several limitations identified in the systematic review by Prapas et al. (2024), who highlighted the lack of standardized evaluation protocols in BCI-AR systems. Our comprehensive evaluation framework, incorporating both objective performance metrics and subjective user experience measures, provides a template for future comparative studies in the field.

The cost-effectiveness and accessibility demonstrated by our system have significant implications for the practical deployment of BCI-VR technologies. Unlike research-grade systems that typically require extensive technical expertise and substantial financial investment, our consumer-hardware approach enables deployment in clinical, educational, and home settings. This accessibility is particularly important for rehabilitation applications, where long-term training and practice are essential for therapeutic effectiveness.

The cognitive load management achieved through our Gaussian process regression adaptation mechanism represents a crucial advance for user acceptance and sustained usage. The 22% reduction in NASA-TLX scores demonstrates that adaptive systems can maintain user comfort while providing sophisticated functionality, addressing one of the primary barriers to widespread BCI adoption identified in existing literature.

While our study demonstrates significant advances in multimodal BCI-VR integration, several limitations remain that define important directions for future research. The reliance on single-channel EEG, while cost-effective, limits the system’s ability to decode complex cognitive states beyond basic attention metrics. Future work should explore selective multi-channel configurations that maintain cost efficiency while expanding decoding capabilities, potentially incorporating the channel reduction methodologies suggested by recent neuroplasticity research (Zheng et al., 2024).

The experimental tasks evaluated in this study, while comprehensive within the virtual environment, require validation in more diverse real-world scenarios. The translation from virtual to physical manipulation tasks remains an open question that future longitudinal studies should address. Additionally, the demographic limitations of our participant pool (healthy young adults) necessitate expanded evaluation with clinical populations to fully establish therapeutic efficacy.

Our work contributes to the ongoing effort to establish standardized evaluation protocols for BCI-VR systems, as called for by recent systematic reviews in the field. The comprehensive metrics we employed—including technical performance, user experience, cognitive load, and neurophysiological validation—provide a framework that future studies can adopt and extend. This standardization is essential for enabling meaningful comparisons across different technological approaches and research groups.

The integration challenges we addressed, particularly the synchronization of multiple data streams and the management of temporal accuracy, align with the methodological requirements identified by recent work in multimodal BCI systems. Our solutions to these challenges provide practical guidance for researchers developing similar systems and contribute to the growing knowledge base for multimodal integration techniques.

6 Limitations and future directions

Despite achieving 87.5% virtual hand success rates, this study presents several limitations that define critical pathways for future development. The primary constraint involves single-channel EEG acquisition, which restricts cognitive state decoding beyond basic attention metrics, preventing implementation of sophisticated control paradigms like motor imagery classification demonstrated with high-density arrays (Lotte et al., 2018). Future iterations should explore hybrid approaches maintaining cost efficiency while incorporating selective multi-channel configurations through systematic electrode reduction studies.

Task complexity limitations emerge from focusing on structured grasping tasks within controlled environments, potentially insufficient for real-world applications. As Zhou et al. (2024) highlighted regarding BCI medical applications, laboratory-to-clinical transitions often reveal performance degradation due to environmental complexity and user variability. Future research requires systematic evaluation across progressive scenarios including dynamic tracking, multi-target selection, and realistic rehabilitation protocols similar to VR-robot therapy applications (Said et al., 2022).

Participant demographics restricted to healthy young adults constrains generalizability to target populations including motor-impaired individuals, elderly users, and neurological patients. This limitation is particularly relevant given increasing interest in BCI-VR neurorehabilitation (Vourvopoulos and Bermúdez i Badia, 2016) and cognitive enhancement in aging populations (Perrot and Maillot, 2023). Longitudinal studies should assess learning effects and neural adaptation processes, building upon neuroplasticity insights (Zheng et al., 2024).

Technical limitations of consumer-grade hardware impose performance constraints despite demonstrated sub-millimeter precision. Environmental factors including electromagnetic interference and lighting conditions significantly impact stability in clinical scenarios. Future developments should incorporate machine learning approaches for adaptive noise cancelation and signal enhancement.

The practical deployment implications of our work extend beyond laboratory validation to address real-world implementation challenges. The success demonstrated by Xu et al. (2024) in multi-robot control scenarios using AR-BCI interfaces provides a compelling precedent for the practical utility of attention-based control mechanisms in complex operational environments. Our consumer-grade hardware approach aligns with this trend toward accessible deployment, though our focus on fine manipulation tasks presents different challenges than those encountered in high-level robotic command scenarios. The 320 ms activation latency achieved by our system approaches the real-time requirements necessary for practical applications, though future work should explore optimization strategies similar to those employed in driving assistance systems, where sub-200 ms response times are often critical for safety and user acceptance.

The system lacks integration with emerging AI technologies that could enhance personalization capabilities. Zhang et al. (2024) demonstrated ChatGPT integration potential for mild cognitive impairment treatment, suggesting avenues for incorporating large language models and predictive analytics. Additionally, standardized evaluation protocols are needed for systematic field progress (Prapas et al., 2024), alongside comprehensive safety assessments and ethical guidelines for vulnerable populations.

7 Conclusion

This research demonstrates that consumer-grade hardware, when combined with sophisticated algorithmic approaches, can achieve performance levels previously associated with research-grade equipment. The successful implementation of three-hand coordination in a VR environment, with 87.5% success rate and sub-millimeter precision, validates the potential for practical BCI-VR applications in rehabilitation, training, and assistive technologies.

The neurophysiological evidence we present supports the feasibility of supernumerary limb control through attention-based mechanisms, contributing to our understanding of neural plasticity and embodiment in virtual environments. The methodological framework we developed addresses key challenges in multimodal integration and provides a foundation for future research in practical BCI-VR systems.

Most importantly, this work demonstrates that the future of BCI technology lies not necessarily in more expensive or complex hardware, but in intelligent system design that leverages multiple complementary modalities to create robust, accessible, and effective human-computer interfaces. As the field moves toward practical deployment, this philosophy of cost-effective innovation will be essential for realizing the transformative potential of BCI-VR technologies in improving human capabilities and quality of life.

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