Feature Normalization for BCI Applications

Feature normalization is CRITICAL for cross-session BCI performance!EEG amplitude varies 50-200% across sessions due to electrode impedance, skin conductance, and user state. Without proper normalization, accuracy can drop by 15-30%.

Why Normalization Matters

EEG and BCI feature amplitudes vary significantly across sessions due to:

Physiological Factors

Electrode impedance: Changes with skin preparation, gel application, contact quality
Skin conductance: Varies with hydration, temperature, anxiety
Skull thickness: Subject-specific (affects multi-subject transfer)
Cortical activity: User state (fatigue, attention, arousal) affects signal amplitude

Technical Factors

Amplifier gain: May differ between recording sessions
Reference electrode: Position and quality affects all channels
Environmental noise: EMG, EOG, line noise levels vary
Hardware: Different recording systems have different amplitude scales

Impact Without Normalization

Scenario	Accuracy Drop	Notes
Same session	~0%	Minimal impact
Cross-session (same day)	10-15%	Electrode re-application
Cross-session (next day)	15-25%	Day-to-day variability
Cross-session (week later)	25-30%	Maximum degradation
Multi-subject transfer	15-20%	Individual differences

When to Normalize

✅ Always Normalize For

Cross-session BCI applications (session 1 → session 2)
Multi-subject studies (subject A → subject B)
Transfer learning scenarios
Combining data from different recording systems
Online BCI where electrode impedance changes during use

⚠️ Consider Normalizing For

Single-session studies (helps with model convergence)
When features have very different scales (e.g., mixing CSP + bandpower)
When training data has high variance in recording quality

❌ Don’t Normalize

Raw EEG before feature extraction (preprocessing handles this)
If all data is from same session with stable conditions

Normalization Methods

Z-score Normalization (Recommended) ⭐

Formula: z = (x - μ) / σ Result: Mean = 0, Standard deviation = 1 When to use: Default choice for most BCI applications

using NimbusSDK

# Estimate params from training data
norm_params = estimate_normalization_params(train_features; method=:zscore)

# Apply to train and test
train_norm = apply_normalization(train_features, norm_params)
test_norm = apply_normalization(test_features, norm_params)

Pros:

Standard statistical normalization
Preserves relative distances between features
Works well with Gaussian-like distributions
Most common in BCI literature

Cons:

Sensitive to outliers (use robust method if many artifacts)

Min-Max Normalization

Formula: x_norm = (x - min) / (max - min) Result: All values in [0, 1] When to use:

When you need bounded features
For algorithms sensitive to feature range

norm_params = estimate_normalization_params(train_features; method=:minmax)
normalized = apply_normalization(test_features, norm_params)

Pros:

Bounded output range [0, 1]
Easy to interpret

Cons:

Very sensitive to outliers
Training min/max may not represent test data range

Robust Normalization

Formula: x_norm = (x - median) / MAD
where MAD = median(|x - median(x)|) × 1.4826 Result: Median = 0, Robust standard deviation ≈ 1 When to use:

Data with many artifacts/outliers
Poor artifact rejection in preprocessing
Online BCI with variable data quality

norm_params = estimate_normalization_params(train_features; method=:robust)
normalized = apply_normalization(test_features, norm_params)

Pros:

Resistant to outliers
More stable with noisy data
Better for real-world conditions

Cons:

Less standard in literature
Slightly different interpretation than z-score

API Reference

Core Functions

`estimate_normalization_params`

estimate_normalization_params(features::Array{Float64, 3}; 
                              method::Symbol=:zscore) -> NormalizationParams

Compute normalization parameters from training data. Arguments:

features: Training features (n_features × n_samples × n_trials)
method: :zscore, :minmax, :robust, or :none

Returns: NormalizationParams object with computed statistics Example:

train_features = randn(16, 250, 80)
params = estimate_normalization_params(train_features; method=:zscore)

`apply_normalization`

apply_normalization(features::Array{Float64, 3}, 
                   params::NormalizationParams) -> Array{Float64, 3}

Apply pre-computed normalization to features. Arguments:

features: Features to normalize (n_features × n_samples × n_trials)
params: Normalization parameters from estimate_normalization_params

Returns: Normalized features with same shape Example:

test_features = randn(16, 250, 20)
test_norm = apply_normalization(test_features, params)

`check_normalization_status`

check_normalization_status(features::Array{Float64, 3}; 
                          tolerance::Float64=0.1)

Check if features appear normalized and get recommendations. Returns: Named tuple with:

appears_normalized::Bool
mean_abs_mean::Float64
mean_std::Float64
recommendations::Vector{String}

Example:

status = check_normalization_status(features)
if !status.appears_normalized
    println("Recommendations:")
    for rec in status.recommendations
        println("  • ", rec)
    end
end

Best Practices

✅ Correct Workflow

# 1. Preprocessing (outside NimbusSDK)
# Raw EEG → Filter → Artifact removal → Epochs → CSP extraction

# 2. Estimate normalization from TRAINING data only
train_features = csp_features_train  # (16 × 250 × 80)
norm_params = estimate_normalization_params(train_features; method=:zscore)

# 3. Apply to BOTH training and test data
train_norm = apply_normalization(train_features, norm_params)
test_norm = apply_normalization(test_features, norm_params)

# 4. Save normalization params with your model
using JLD2
@save "model_with_norm.jld2" model norm_params

# 5. Training
metadata = BCIMetadata(250.0, :motor_imagery, :csp, 16, 4, nothing)
train_data = BCIData(train_norm, metadata, train_labels)
model = train_model(RxLDAModel, train_data)

# 6. Deployment - load and apply same params
@load "model_with_norm.jld2" model norm_params
new_data_norm = apply_normalization(new_data, norm_params)
results = predict_batch(model, BCIData(new_data_norm, metadata))

Common Pitfalls

❌ Pitfall 1: Normalizing Train and Test Separately

Wrong:

# WRONG - computes different params for each!
train_norm = normalize_features(train_features)
test_norm = normalize_features(test_features)  # ❌ Different scale!

Correct:

# Compute params from training only
params = estimate_normalization_params(train_features)
train_norm = apply_normalization(train_features, params)
test_norm = apply_normalization(test_features, params)  # ✅ Same scale

Impact: Test accuracy drops by 10-30%

❌ Pitfall 2: Normalizing Before Feature Extraction

Wrong:

# WRONG - normalize raw EEG
raw_eeg_norm = normalize_features(raw_eeg)
csp_features = extract_csp(raw_eeg_norm)  # ❌ Breaks CSP assumptions

Correct:

# Extract features first
csp_features = extract_csp(raw_eeg)
csp_norm = normalize_features(csp_features)  # ✅ Normalize features

Why: CSP assumes specific covariance structure in raw data. Normalizing first destroys this.

❌ Pitfall 3: Forgetting to Save Parameters

Wrong:

# Training session
train_norm = normalize_features(train_features)
model = train_model(RxLDAModel, train_data)
save_model(model, "model.jld2")

# Later, deployment session
test_norm = normalize_features(test_features)  # ❌ Different normalization!

Correct:

# Training session
params = estimate_normalization_params(train_features)
train_norm = apply_normalization(train_features, params)
model = train_model(RxLDAModel, train_data)
@save "model.jld2" model params  # ✅ Save params

# Later, deployment session
@load "model.jld2" model params
test_norm = apply_normalization(test_features, params)  # ✅ Same params

Examples

Example 1: Cross-Session Motor Imagery

using NimbusSDK

# Session 1: Training
train_features = randn(16, 250, 80)  # CSP features
train_labels = repeat(1:4, inner=20)

# Estimate normalization
norm_params = estimate_normalization_params(train_features; method=:zscore)

# Normalize training data
train_norm = apply_normalization(train_features, norm_params)

# Train model
metadata = BCIMetadata(250.0, :motor_imagery, :csp, 16, 4, nothing)
train_data = BCIData(train_norm, metadata, train_labels)
model = train_model(RxLDAModel, train_data; iterations=50)

# Save model AND normalization params
@save "motor_imagery_model.jld2" model norm_params

# Session 2: Testing (new day, re-applied electrodes)
@load "motor_imagery_model.jld2" model norm_params

test_features = randn(16, 250, 20)  # Different amplitude due to new impedances
test_labels = repeat(1:4, inner=5)

# Apply SAME normalization
test_norm = apply_normalization(test_features, norm_params)
test_data = BCIData(test_norm, metadata, test_labels)

# Inference
results = predict_batch(model, test_data)
accuracy = sum(results.predictions .== test_labels) / length(test_labels)
println("Cross-session accuracy: $(round(accuracy * 100, digits=1))%")

Example 2: Checking Data Quality

# Load your features
features = load("my_features.jld2")["features"]

# Check normalization status
status = check_normalization_status(features)

println("Appears normalized: ", status.appears_normalized)
println("Mean |μ|: ", status.mean_abs_mean)
println("Mean σ: ", status.mean_std)

if !status.appears_normalized
    println("\nRecommendations:")
    for rec in status.recommendations
        println("  • ", rec)
    end
    
    # Apply normalization
    normalized = normalize_features(features; method=:zscore)
    println("\nAfter normalization:")
    status2 = check_normalization_status(normalized)
    println("Appears normalized: ", status2.appears_normalized)
end

Example 3: Robust Normalization for Noisy Data

# Data with artifacts
features = randn(16, 250, 100)
# Add some extreme outliers (artifacts)
features[5, 100, 20] = 1000.0  # Muscle artifact
features[8, 50, 45] = -500.0   # Eye blink

# Standard z-score would be affected by outliers
zscore_params = estimate_normalization_params(features; method=:zscore)
println("Z-score std range: ", extrema(zscore_params.stds))

# Robust normalization handles outliers better
robust_params = estimate_normalization_params(features; method=:robust)
println("Robust MAD range: ", extrema(robust_params.mads))

# Apply robust normalization
normalized = apply_normalization(features, robust_params)

# Outliers are less influential
println("Normalized range: ", extrema(normalized))

Performance Impact

Expected Improvements with Proper Normalization

Scenario	Without Normalization	With Normalization	Improvement
Same session	85%	86%	+1%
Cross-session (same day)	65%	80%	+15%
Cross-session (next day)	55%	78%	+23%
Cross-session (week later)	45%	75%	+30%
Multi-subject transfer	35%	55%	+20%

Values are typical for motor imagery BCI. Your results may vary.

Summary

Key Takeaways

✅ Always normalize for cross-session BCI
✅ Estimate params from training data only
✅ Apply same params to test/deployment data
✅ Save params with your model
✅ Use z-score as default (or robust for noisy data)
✅ Normalize after feature extraction, before training ❌ Never normalize train and test separately
❌ Never normalize raw EEG (do it after features)
❌ Never forget to save normalization params

Expected Impact

🎯 10-30% accuracy improvement for cross-session BCI
🎯 Enables transfer learning between subjects
🎯 Reduces calibration requirements for new users
🎯 More robust to hardware/setup variations

Next Steps

Preprocessing Requirements

Complete preprocessing guide

Julia SDK

Full SDK API reference

Code Examples

Working code examples

Batch Processing

Processing multiple trials

References

Academic Papers

Ledoit & Wolf (2004): “Honey, I shrunk the sample covariance matrix” - Optimal shrinkage estimation
Blankertz et al. (2011): “Single-trial analysis and classification of ERP components” - Discusses normalization in BCI
Lotte et al. (2018): “A review of classification algorithms for EEG-based BCI” - Section on preprocessing

Getting started

API documentation

Core concepts

Model specification

Inference configuration

BCI Models

Examples & tutorials

Development

​Feature Normalization for BCI Applications

​Why Normalization Matters

​Physiological Factors

​Technical Factors

​Impact Without Normalization

​When to Normalize

​✅ Always Normalize For

​⚠️ Consider Normalizing For

​❌ Don’t Normalize

​Normalization Methods

​Z-score Normalization (Recommended) ⭐

​Min-Max Normalization

​Robust Normalization

​API Reference

​Core Functions

​estimate_normalization_params

​apply_normalization

​check_normalization_status

​Best Practices

​✅ Correct Workflow

​Common Pitfalls

​❌ Pitfall 1: Normalizing Train and Test Separately

​❌ Pitfall 2: Normalizing Before Feature Extraction

​❌ Pitfall 3: Forgetting to Save Parameters

​Examples

​Example 1: Cross-Session Motor Imagery

​Example 2: Checking Data Quality

​Example 3: Robust Normalization for Noisy Data

​Performance Impact

​Expected Improvements with Proper Normalization

​Summary

​Key Takeaways

​Expected Impact

​Next Steps

Preprocessing Requirements

Julia SDK

Code Examples

Batch Processing

​References

​Academic Papers

Feature Normalization for BCI Applications

Why Normalization Matters

Physiological Factors

Technical Factors

Impact Without Normalization

When to Normalize

✅ Always Normalize For

⚠️ Consider Normalizing For

❌ Don’t Normalize

Normalization Methods

Z-score Normalization (Recommended) ⭐

Min-Max Normalization

Robust Normalization

API Reference

Core Functions

`estimate_normalization_params`

`apply_normalization`

`check_normalization_status`

Best Practices

✅ Correct Workflow

Common Pitfalls

❌ Pitfall 1: Normalizing Train and Test Separately

❌ Pitfall 2: Normalizing Before Feature Extraction

❌ Pitfall 3: Forgetting to Save Parameters

Examples

Example 1: Cross-Session Motor Imagery

Example 2: Checking Data Quality

Example 3: Robust Normalization for Noisy Data

Performance Impact

Expected Improvements with Proper Normalization

Summary

Key Takeaways

Expected Impact

Next Steps

References

Academic Papers