Extracellular Recording
Extracellular recording is the measurement of neural activity as the voltage that neurons' own currents produce in the fluid around them, taken with an electrode placed in the tissue rather than inside a cell.
It is the technique that thin-film silicon probes and grids exist for, and it is what electrophysiology means in practice for most laboratories working in the living brain (in vivo) rather than in slices or cultures.
The electrode sets what the recording can contain. A site sees only the neurons within its detection radius, about a hundred micrometers, so which cells appear in a recording is decided by where the sites are and how far apart they sit. The same site returns two signals by frequency — spikes from individual neurons above about 300 Hz and the local field potential below it — and site area weighs the two: a small site favors isolated spikes, a large one a cleaner field potential. Every channel is a difference from a reference, so the reference position is part of the measurement, not a wiring detail.
Intracellular methods capture one cell completely; extracellular recording captures many cells partially, for as long as the electrode remains in place — hours in an acute preparation, months in a chronic one. This is why the probe or grid is chosen against the measurement. The primer How an Electrode Records Neural Activity works through it from the electrode's side.
Typical values
| Spike band | above about 300 Hz |
|---|---|
| Field-potential band | below about 300 Hz |
| Detection radius of a site, spikes | about 100 µm |