tetrodes

A tetrode is a four-wire electrode used for extracellular recording of neural activity. Each of the four closely spaced microwires serves as a separate recording site, and the signals are analyzed jointly to improve the isolation of individual neurons compared with single-wire electrodes.

Tetrodes are typically composed of tungsten, stainless steel, or platinum-iridium microwires with diameters of roughly 12–25 micrometers. The four wires are grouped so that their tips are near each other (tip distances on the order of tens of micrometers). They are mounted on a microdrive or probe holder to enable controlled advancement into brain tissue, and often include an insulating coating except at the tips.

During recording, extracellular action potentials are captured on the four channels. Spike sorting uses waveform shape and amplitude across channels to assign spikes to individual neurons. Because the relative timing and amplitude of a neuron's spikes differ across the tetrode's tips, neurons in the surrounding tissue can be discriminated more reliably than with a single electrode.

Advantages include improved single-unit discrimination, higher yield in densely packed tissue, and the ability to track units by moving the electrode with depth. Limitations include tissue damage from insertion, impedance changes over time, and the need for complex data processing. Wiring and mechanical stability also affect signal quality.

Tetrodes are widely used in neuroscience research, including studies in rodents and primates, to investigate neural coding, sensory processing, and behavior. The tetrode concept originated in mid-20th-century electrophysiology and has evolved with advances in materials, microfabrication, and chronic implant techniques.