A tetrode is a group of wire bundles used in electrophysiological

Electrophysiology

Electrophysiology is the study of the electrical properties of biological cells and tissues. It involves measurements of voltage change or electric current on a wide variety of scales from single ion channel proteins to whole organs like the heart...

 studies in the neuroscience

Neuroscience

Neuroscience is the scientific study of the nervous system. Traditionally, neuroscience has been seen as a branch of biology. However, it is currently an interdisciplinary science that collaborates with other fields such as chemistry, computer science, engineering, linguistics, mathematics,...

s to record extracellular field potential

Extracellular field potential

The extracellular field potential is the electrical potential produced by cells, e.g. nerve or muscle cells, outside of the cell. Electrophysiological studies investigate these potentials using extracellular microelectrodes...

s from nervous tissue, e.g. the brain

Brain

The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

. They consist of bundles of 4 thin (e.g. 30 µm diameter) wires glued together. The idea is that the wires are spaced close enough to each other to detect overlapping populations of neurons, but wide enough so that the exact waveform of the individual neurons are different on each of the wires. These differences can then be used to cluster individual neurons from the population of spikes.

Clustering

Clustering techniques involve both examining the waveform of individual spikes, and examining the spike height across all four channels.