Quality Metrics Reference

Comprehensive guide to unit quality assessment using SpikeInterface metrics and Allen/IBL standards.

Overview

Quality metrics assess three aspects of sorted units:

Computing Quality Metrics

# Create analyzer with computed waveforms
analyzer = si.create_sorting_analyzer(sorting, recording, sparse=True)
analyzer.compute('random_spikes', max_spikes_per_unit=500)
analyzer.compute('waveforms', ms_before=1.5, ms_after=2.0)
analyzer.compute('templates')
analyzer.compute('noise_levels')
analyzer.compute('spike_amplitudes')
analyzer.compute('principal_components', n_components=5)

# Compute all quality metrics
analyzer.compute('quality_metrics')

# Or compute specific metrics
analyzer.compute('quality_metrics', metric_names=[
    'firing_rate', 'snr', 'isi_violations_ratio',
    'presence_ratio', 'amplitude_cutoff'
])

# Get results
qm = analyzer.get_extension('quality_metrics').get_data()
print(qm.columns.tolist())  # Available metrics

Metric Definitions & Thresholds

Contamination Metrics

ISI Violations Ratio

Fraction of spikes violating refractory period. All neurons have a ~1.5ms refractory period.

# Compute with custom refractory period
analyzer.compute('quality_metrics',
                 metric_names=['isi_violations_ratio'],
                 isi_threshold_ms=1.5,
                 min_isi_ms=0.0)

Reference: Hill et al. (2011) J Neurosci 31:8699-8705

Signal-to-Noise Ratio (SNR)

Ratio of peak waveform amplitude to background noise.

analyzer.compute('quality_metrics', metric_names=['snr'])

Isolation Distance

Mahalanobis distance to nearest cluster in PCA space.

analyzer.compute('quality_metrics',
                 metric_names=['isolation_distance'],
                 n_neighbors=4)

L-ratio

Contamination measure based on Mahalanobis distances.

D-prime

Discriminability between unit and nearest neighbor.

Completeness Metrics

Amplitude Cutoff

Estimates fraction of spikes below detection threshold.

analyzer.compute('quality_metrics',
                 metric_names=['amplitude_cutoff'],
                 peak_sign='neg')  # 'neg', 'pos', or 'both'

For precise timing analyses: Use < 0.01

Presence Ratio

Fraction of recording time with detected spikes.

analyzer.compute('quality_metrics',
                 metric_names=['presence_ratio'],
                 bin_duration_s=60)  # 1-minute bins

Stability Metrics

Drift Metrics

Measure unit movement over time.

analyzer.compute('quality_metrics',
                 metric_names=['drift_ptp', 'drift_std', 'drift_mad'])

Amplitude CV

Coefficient of variation of spike amplitudes.

Cluster Quality Metrics

Silhouette Score

Cluster cohesion vs separation (-1 to 1).

Nearest-Neighbor Metrics

analyzer.compute('quality_metrics',
                 metric_names=['nn_hit_rate', 'nn_miss_rate'],
                 n_neighbors=4)

Standard Filtering Criteria

Allen Institute Defaults

# Allen Visual Coding / Behavior defaults
allen_query = """
    presence_ratio > 0.95 and
    isi_violations_ratio < 0.5 and
    amplitude_cutoff < 0.1
"""
good_units = qm.query(allen_query).index.tolist()

IBL Standards

# IBL reproducible ephys criteria
ibl_query = """
    presence_ratio > 0.9 and
    isi_violations_ratio < 0.1 and
    amplitude_cutoff < 0.1 and
    firing_rate > 0.1
"""
good_units = qm.query(ibl_query).index.tolist()

Strict Single-Unit Criteria

# For precise timing / spike-timing analyses
strict_query = """
    snr > 5 and
    presence_ratio > 0.99 and
    isi_violations_ratio < 0.01 and
    amplitude_cutoff < 0.01 and
    isolation_distance > 20 and
    drift_ptp < 40
"""
single_units = qm.query(strict_query).index.tolist()

Multi-Unit Activity (MUA)

# Include multi-unit activity
mua_query = """
    snr > 2 and
    presence_ratio > 0.5 and
    isi_violations_ratio < 1.0
"""
all_units = qm.query(mua_query).index.tolist()

Visualization

Quality Metric Summary

# Plot all metrics
si.plot_quality_metrics(analyzer)

Individual Metric Distributions

fig, axes = plt.subplots(2, 3, figsize=(15, 10))

metrics = ['snr', 'isi_violations_ratio', 'presence_ratio',
           'amplitude_cutoff', 'firing_rate', 'drift_ptp']

for ax, metric in zip(axes.flat, metrics):
    ax.hist(qm[metric].dropna(), bins=50, edgecolor='black')
    ax.set_xlabel(metric)
    ax.set_ylabel('Count')
    # Add threshold line
    if metric == 'snr':
        ax.axvline(5, color='r', linestyle='--', label='threshold')
    elif metric == 'isi_violations_ratio':
        ax.axvline(0.01, color='r', linestyle='--')
    elif metric == 'presence_ratio':
        ax.axvline(0.9, color='r', linestyle='--')

plt.tight_layout()

Unit Quality Summary

# Comprehensive unit summary plot
si.plot_unit_summary(analyzer, unit_id=0)

Quality vs Firing Rate

fig, ax = plt.subplots()
scatter = ax.scatter(qm['firing_rate'], qm['snr'],
                     c=qm['isi_violations_ratio'],
                     cmap='RdYlGn_r', alpha=0.6)
ax.set_xlabel('Firing Rate (Hz)')
ax.set_ylabel('SNR')
plt.colorbar(scatter, label='ISI Violations')
ax.set_xscale('log')

Compute All Metrics at Once

# Full quality metrics computation
all_metric_names = [
    # Firing properties
    'firing_rate', 'presence_ratio',
    # Waveform
    'snr', 'amplitude_cutoff', 'amplitude_cv_median', 'amplitude_cv_range',
    # ISI
    'isi_violations_ratio', 'isi_violations_count',
    # Drift
    'drift_ptp', 'drift_std', 'drift_mad',
    # Isolation (require PCA)
    'isolation_distance', 'l_ratio', 'd_prime',
    # Nearest neighbor (require PCA)
    'nn_hit_rate', 'nn_miss_rate',
    # Cluster quality
    'silhouette_score',
    # Synchrony
    'sync_spike_2', 'sync_spike_4', 'sync_spike_8',
]

# Compute PCA first (required for some metrics)
analyzer.compute('principal_components', n_components=5)

# Compute metrics
analyzer.compute('quality_metrics', metric_names=all_metric_names)
qm = analyzer.get_extension('quality_metrics').get_data()

# Save to CSV
qm.to_csv('quality_metrics.csv')

Custom Metrics

from spikeinterface.qualitymetrics import compute_firing_rates, compute_snrs

# Compute individual metrics
firing_rates = compute_firing_rates(sorting)
snrs = compute_snrs(analyzer)

# Add custom metric to DataFrame
qm['custom_score'] = qm['snr'] * qm['presence_ratio'] / (qm['isi_violations_ratio'] + 0.001)

References

• SpikeInterface Quality Metrics
• Allen Institute ecephys_quality_metrics
• Hill et al. (2011) "Quality metrics to accompany spike sorting of extracellular signals"
• Siegle et al. (2021) "Survey of spiking in the mouse visual system reveals functional hierarchy"