NeuroSkill Metric Definitions & Interpretation Guide

⚠️ Research Use Only: All metrics are experimental and derived from consumer-grade hardware (Muse 2/S). They are not FDA/CE-cleared and must not be used for medical diagnosis or treatment.

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Hardware & Signal Acquisition

NeuroSkill is validated for Muse 2 and Muse S headbands (with OpenBCI support in the desktop app), streaming at 256 Hz (EEG) and 64 Hz (PPG).

Electrode Positions (International 10-20 System)

Preprocessing Pipeline

1. Filtering: High-pass (0.5 Hz), Low-pass (50/60 Hz), Notch filter
2. Spectral Analysis: Hann-windowed FFT (512-sample window), Welch periodogram
3. GPU acceleration: ~125ms latency via gpu_fft

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EEG Frequency Bands

Relative power values (sum ≈ 1.0 across all bands):

JSON Field Names

"bands": {
  "rel_delta": 0.28, "rel_theta": 0.18, "rel_alpha": 0.32,
  "rel_beta": 0.17, "rel_gamma": 0.05
}

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Core Composite Scores (0–1 Scale)

Focus

• Formula: σ(β / (α + θ)) — beta dominance over slow waves, sigmoid-mapped
• > 0.70: Deep concentration, flow state, task absorption
• 0.40–0.69: Moderate attention, some mind-wandering
• < 0.40: Distracted, fatigued, difficulty concentrating

Relaxation

• Formula: σ(α / (β + θ)) — alpha dominance, sigmoid-mapped
• > 0.70: Calm, stress-free, parasympathetic dominant
• 0.40–0.69: Mild tension present
• < 0.30: Stressed, anxious, sympathetic dominant

Engagement

• 0–1 scale: Active mental investment and motivation
• > 0.70: Mentally invested, motivated, active processing
• 0.40–0.69: Passive participation
• < 0.30: Bored, disengaged, autopilot mode

Meditation

• Composite: Combines alpha elevation, physical stillness (IMU), and HRV coherence
• > 0.70: Deep meditative state
• < 0.30: Active, non-meditative

Mood

• Composite: Derived from FAA, TAR, and BAR
• > 0.60: Positive affect, approach motivation
• < 0.40: Low mood, withdrawal tendency

Cognitive Load

• Formula: (P_θ_frontal / P_α_temporal) · f(FAA, TBR) — working memory usage
• > 0.70: Working memory near capacity, complex processing
• 0.40–0.69: Moderate mental effort
• < 0.40: Task is easy or automatic
• Interpretation: High load + high focus = productive struggle. High load + low focus = overwhelmed.

Drowsiness

• Composite: Weighted TAR + TBR + falling Spectral Centroid
• > 0.60: Sleep pressure building, micro-sleep risk
• 0.30–0.59: Mild fatigue
• < 0.30: Alert

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EEG Ratios & Spectral Indices

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Complexity & Nonlinear Metrics

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Consciousness Metrics

Derived from the nonlinear metrics above:

Status thresholds: ≥ 50 Green, 25–50 Yellow, < 25 Red.

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Cardiac & Autonomic Metrics (from PPG)

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Motion & Artifact Detection

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Signal Quality (Per Electrode)

If any electrode is below 0.7, recommend the user adjust the headband fit or moisten the electrode contacts.

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Sleep Staging

Based on 5-second epochs using relative band-power ratios and AASM heuristics:

Healthy Adult Targets (~8h Sleep)

• N3 (Deep): 15–25% of total sleep
• REM: 20–25%
• Sleep Efficiency: > 85%
• Sleep Onset Latency: < 20 min

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Composite State Patterns

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ZUNA Embeddings

NeuroSkill uses the ZUNA Neural Encoder to convert 5-second EEG epochs into 128-dimensional vectors stored in an HNSW index:

• Search: Sub-millisecond approximate nearest-neighbor queries
• UMAP: GPU-accelerated 3D projection for visual comparison
• Storage: Local SQLite + HNSW index in ~/.skill/YYYYMMDD/