Spring 2012, Volume 3, Number 3 67 Basic and Clinical A Brief Look into Spike Sorting Methods Mojtaba Kermani 1,2 , Seyed Mohammad Noorbakhsh 2,* , Abbas Haghparast 1 1. Neuroscience Research Center, Shahid Beheshti University of Medical Sciences, P.O. Box 19615-1178, Tehran, Iran 2. ScienceBeam Institute, Tehran, Iran * Corresponding Author: Seyed Mohammad Noorbakhsh, M.D., Science Beam Institute, Tehran, Iran Tel.: (+98912) 670-1773/ Fax: (+9821) 221-76125 E-mail: Noorbakhsh@sciencebeam.com Spike sorting is a class of techniques used in the analysis of electrophysiological data. Studying the dynamics of neural activity via electrical recording relies on the ability to detect and sort neural spikes recorded from a number of neurons by the same electrode. This article reviews methods for detecting and classifying action potentials, a problem commonly referred to as spike sorting. A B S T R A C T Article info: Received: 18 February 2012 First Revision: 19 April 2012 Accepted: 09 May 2012 Key Words: Spike, Data Acquisition, Filtering, Feature Extraction, Clustering. 1. Introduction ne of the major questions contemporary neuroscience seeks to elucidate concerns the mechanisms used by dedicated parts of brains (our brain, vertebrate brains, in- sect brains) to perform speciic tasks. Our brains are buzzing with electrical activity created by so- dium and potassium ions moving in and out of neurons through specialized pores. Classical methods for explor- ing the mechanisms of brain function involve recording the electrical activity of single nerve cells. Neurophysi- ologists often record the currents lowing across cell membranes using an insulated wire probe whose ‘listen’ to a few neurons close-by the electrode tip that ire action potentials or ‘spikes’ (Fig.1).Any such probe will record currents passing across all sorts of physiological mem- branes close to its tip, but we can approximate this as a small number of close, high-amplitude sources imposed on a background of synaptic, somatic and axonal cur- rents from more distant sources. Each neuron has spikes O of a characteristic shape, which is mainly determined by the morphology of their dendritic trees and the distance and orientation relative to the recording electrode (Le- wicki, 1998).When a recording electrode measures ac- tion potentials from multiple cells, these contributions must be disentangled from the background noise and from each other before the activity of individual neu- rons can be analyzed. This procedure of estimating one or more single cell point processes from a noisy time se- ries is known as spike sorting. Spike sorting, then, is the process of identifying the waveforms associated with action potentials in the foreground cell(s) and thereby detecting iring events. As a matter of fact, Spike sort- ing is the grouping of spikes into clusters based on the similarity of their shapes. Given that, in principle, each neuron tends to ire spikes of a particular shape, the re- sulting clusters correspond to the activity of different putative neurons. The end result of spike sorting is the determination of which spike corresponds to which of these neurons.