Reference Electrode
The reference electrode is the second contact every recording channel is measured against: an extracellular channel reports the voltage at its site minus the voltage at the reference, so where the reference sits is half of every measurement.
Ground is distinct: it is the potential the recording system is built on, tied to the animal so the amplifier and the tissue share a common potential, and the two should not be confused even when they are wired together.
Placement decides what is subtracted. A reference near the sites, in quiet tissue or on the probe itself, cancels the most shared noise and leaves the local signal — the choice for spikes. A reference on the skull or in a distant structure cancels less but preserves slow, shared activity such as the field potential across a region — the choice for LFP and surface recording. A reference inside the structure being recorded subtracts the very signal of interest. Between adjacent sites, the difference of two channels is its own reference and is the basis of current-source-density analysis on a laminar probe.
On a NeuroNexus probe the reference and ground are separate wires on the package; on a thin-film grid the reference is a decision about the surface, worked through in the grid referencing guide. Many recordings that appear to have a noisy electrode have a reference problem; the surgical guides cover placement, and common average referencing corrects afterwards for what the reference did not cancel.
Typical values
| Channel value | site voltage minus reference voltage |
|---|---|
| Near reference | cancels shared noise; keeps local spikes |
| Distant reference | keeps slow shared activity; LFP and surface work |
| On a NeuroNexus package | reference and ground on separate wires |