Spike sorting

Spike sorting refers to a technique used in analyses of the electrical activity (electrophysiology) of nerve cells (neurons) to distinguish the activity neighboring neurons based on the shape of the 'spikes' they produce.

Neurons produce action potentials 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). Spike sorting refers to the process of assigning spikes with different time courses to different neurons. The background to this is that the exact time course depends on the size and shape of the neuron, the position of the recording electrode relative to the neuron, etc. These electrode, 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 it can be used to distinguish spikes produced by different neurons, i.e. to separate the activity produced by each. This process is termed spike sorting.

Technically this is often achieved based on different sizes of the spikes (simple but inaccurate version) or more sophisticated analyses such as principle component or wavelet transformations the spike shapes with appropriate software. The results may be improve using special micro electrodes called Tetrodes.

principle component weights of spikes from two different neurons

Spike shapes colored according to their assignment to different neurons. The blue trace could not be assigned unequivocally.