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:

Category	Question	Key Metrics
Contamination (Type I)	Are spikes from multiple neurons?	ISI violations, SNR
Completeness (Type II)	Are we missing spikes?	Amplitude cutoff, presence ratio
Stability	Is the unit stable over time?	Drift metrics, amplitude CV

Computing Quality Metrics

python

import spikeinterface.full as si

# 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.

python

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

Value	Interpretation
< 0.01	Excellent (well-isolated single unit)
0.01 - 0.1	Good (minor contamination)
0.1 - 0.5	Moderate (multi-unit activity likely)
> 0.5	Poor (likely multi-unit)

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

Signal-to-Noise Ratio (SNR)

Ratio of peak waveform amplitude to background noise.

python

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

Value	Interpretation
> 10	Excellent
5 - 10	Good
2 - 5	Acceptable
< 2	Poor (may be noise)

Isolation Distance

Mahalanobis distance to nearest cluster in PCA space.

python

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

Value	Interpretation
> 50	Well-isolated
20 - 50	Moderately isolated
< 20	Poorly isolated

L-ratio

Contamination measure based on Mahalanobis distances.

Value	Interpretation
< 0.05	Well-isolated
0.05 - 0.1	Acceptable
> 0.1	Contaminated

D-prime

Discriminability between unit and nearest neighbor.

Value	Interpretation
> 8	Excellent separation
5 - 8	Good separation
< 5	Poor separation

Completeness Metrics

Amplitude Cutoff

Estimates fraction of spikes below detection threshold.

python

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

Value	Interpretation
< 0.01	Excellent (nearly complete)
0.01 - 0.1	Good
0.1 - 0.2	Moderate (some missed spikes)
> 0.2	Poor (many missed spikes)

For precise timing analyses: Use < 0.01

Presence Ratio

Fraction of recording time with detected spikes.

python

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

Value	Interpretation
> 0.99	Excellent
0.9 - 0.99	Good
0.8 - 0.9	Acceptable
< 0.8	Unit may have drifted out

Stability Metrics

Drift Metrics

Measure unit movement over time.

python

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

Metric	Description	Good Value
`drift_ptp`	Peak-to-peak drift (μm)	< 40
`drift_std`	Standard deviation of drift	< 10
`drift_mad`	Median absolute deviation	< 10

Amplitude CV

Coefficient of variation of spike amplitudes.

Value	Interpretation
< 0.25	Very stable
0.25 - 0.5	Acceptable
> 0.5	Unstable (drift or contamination)

Cluster Quality Metrics

Silhouette Score

Cluster cohesion vs separation (-1 to 1).

Value	Interpretation
> 0.5	Well-defined cluster
0.25 - 0.5	Moderate
< 0.25	Overlapping clusters

Nearest-Neighbor Metrics

python

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

Metric	Description	Good Value
`nn_hit_rate`	Fraction of spikes with same-unit neighbors	> 0.9
`nn_miss_rate`	Fraction of spikes with other-unit neighbors	< 0.1

Standard Filtering Criteria

Allen Institute Defaults

python

# 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

python

# 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

python

# 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)

python

# 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

python

# Plot all metrics
si.plot_quality_metrics(analyzer)

Individual Metric Distributions

python

import matplotlib.pyplot as plt

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

python

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

Quality vs Firing Rate

python

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

python

# 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

python

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"