Spike sorting is a class of techniques used in the analysis of
data. Spike
sorting algorithm
In computer science, a sorting algorithm is an algorithm that puts elements of a List (computing), list into an Total order, order. The most frequently used orders are numerical order and lexicographical order, and either ascending or descending ...
s use the shape(s) of waveforms collected with one or more electrodes in the brain to distinguish the activity of one or more
neuron
A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...
s from background electrical noise.

Neurons produce
action potentials
An action potential (also known as a nerve impulse or "spike" when in a neuron) is a series of quick changes in voltage across a cell membrane. An action potential occurs when the membrane potential of a specific cell rapidly rises and falls. ...
that are referred to as 'spikes' in laboratory jargon. Frequently this term is used for electrical signals recorded in the vicinity of individual neurons with a
microelectrode (exception: 'spikes' in
EEG recordings). In these recordings action potentials appear as sharp spikes (deviations from the baseline). These extracellular electrodes pick up all the components constituting the field at the point of its contact. This includes the component due to the synaptic currents and the action potentials. The synaptic currents have slower time course and the spikes have faster time course. They are thus easily separated by filtering: highpass for spikes and low pass for the synaptic mechanisms. The synaptic current contributes to the global
local field potential (LFP).
Spike sorting refers to the process of assigning spikes to different neurons. The background to this is that the exact time course of a spike event as recorded by the electrode depends on the size and shape of the neuron, the position of the recording electrode relative to the neuron, etc. These electrodes, positioned outside of the cells in the tissue, however, often 'see' the spikes generated by several neurons in their vicinity. Since the spike shapes are unique and quite reproducible for each neuron they can be used to distinguish spikes produced by different neurons, i.e. to separate the activity produced by each.
Technically this is often achieved based on different sizes of the spikes (simple but inaccurate version) or more sophisticated analyses which make use of the entire waveform of the spikes. The techniques often use tools such as
principal components or
wavelet
A wavelet is a wave-like oscillation with an amplitude that begins at zero, increases or decreases, and then returns to zero one or more times. Wavelets are termed a "brief oscillation". A taxonomy of wavelets has been established, based on the n ...
analysis.
Multiple electrodes record different waveforms for each individual spike elicited by the neurons in the vicinity of the electrodes. The geometric configuration of the electrodes can then be used to define additional dimensions to analyze which spikes originated from which individual cell in the recorded population of cells. Thus the spike sorting using multiple electrodes is better than sorting based simply on waveform shape. The most popular setup involves the use of four micro electrodes, called 'tetrodes' (different from the vacuum tube
Tetrode
A tetrode is a vacuum tube (called ''valve'' in British English) having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids, and a plate electrode, plate (called ''anode'' in Bri ...
), though more electrodes may be used. Recording electrodes can be metal wires or fine print on a
PCB with gold or platinum plated at their exposed tips to ensure good contact and prevent change in resistance while the experiment is going on.
References
{{Reflist
External links
Spike sortingarticle in Scholarpedia
Electrophysiology
Neurophysiology
Articles containing video clips