Spike sorting is a class of techniques used in the analysis of

electrophysiological Electrophysiology (from Greek , ''ēlektron'', "amber" etymology of "electron"">Electron#Etymology">etymology of "electron" , ''physis'', "nature, origin"; and , ''-logia'') is the branch of physiology that studies the electrical properties of bi ...

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, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...

s from background electrical noise. Neurons produce

action potentials An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, c ...

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 A microelectrode is an electrode used in electrophysiology either for recording neural signals or for the electrical stimulation of nervous tissue (they were first developed by Ida Hyde in 1921). Pulled glass pipettes with tip diameters of 0. ...

(exception: 'spikes' in

EEG Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocortex ...

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 component of the field due to the synaptic mechanism is referred to as the

local field potential Local field potentials (LFP) are transient electrical signals generated in nervous and other tissues by the summed and synchronous electrical activity of the individual cells (e.g. neurons) in that tissue. LFP are "extracellular" signals, meaning ...

(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 Principal component analysis (PCA) is a popular technique for analyzing large datasets containing a high number of dimensions/features per observation, increasing the interpretability of data while preserving the maximum amount of information, and ...

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 num ...

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 (called ''anode'' in British English). Th ...

), though more electrodes may be used. Recording electrodes can be metal wires or fine print on a

PCB PCB may refer to: Science and technology * Polychlorinated biphenyl, an organic chlorine compound, now recognized as an environmental toxin and classified as a persistent organic pollutant * Printed circuit board, a board used in electronics * ...

with gold or platinum plated at their exposed tips to ensure good contact and prevent change in resistance while the experiment is going on. image:spike clusters.png Principal Component weights of spikes from two different neurons image:spike cutouts sorted.png Spike shapes colored according to their assignment to different neurons. The blue trace could not be assigned unequivocally.

See also