Spike Sorting for Extracellular Recordings

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Spike Sorting for Extracellular Recordings

• Kenneth D. Harris
• Rutgers University

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Aims

• We would like to
• Monitor the activity of large numbers of neurons
  simultaneously
• Know which neuron fired when
• Know which neuron is of which type
• Estimate our errors

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Extracellular Recording Hardware

• You can buy two types of hardware, allowing
• Wide-band continuous recordings
• Filtered, spike-triggered recordings

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The Tetrode

• Four microwires twisted into a bundle
• Different neurons will have different amplitudes
  on the four wires

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Raw Data
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High Pass Filtering

• Local field potential is primarily at low
  frequencies.
• Spikes are at higher frequencies.
• So use a high pass filter. 800hz cutoff is good.

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Filtered Data
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Spike Detection

• Locate spikes at times of maximum extracellular
  negativity
• Exact alignment is important is it on peak of
  largest channel or summed channels?

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Data Reduction

• We now have a waveform for each spike, for each
  channel.
• Still too much information!
• Before assigning individual spikes to cells, we
  must reduce further.

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Principal Component Analysis

• Create feature vector for each spike.
• Typically takes first 3 PCs for each channel.
• Do you use canonical principal components, or new
  ones for each file?

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Feature Space
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Cluster Cutting

• Which spikes belong to which neuron?
• Assume a single cluster of spikes in feature
  space corresponds to a single cell
• Automatic or manual clustering?

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Cluster Cutting Methods

• Purely manual time consuming, leads to high
  error rates.
• Purely automatic untrustworthy.
• Hybrid less time consuming, lowest error rates.

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Semi-automatic Clustering
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Cluster Quality Measures

• Would like to automatically detect which cells
  are well isolated.
• Will define two measures.

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Isolation Distance
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L_ratio
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False Positives and Negatives
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Room for Improvement?

• Improved alignment methods, leading to nicer
  clusters.
• Faster automatic sorting.
• Better human-machine interaction.
• Fully automatic sorting.