Local Field Potential (LFP)

It is recorded on the same site and in the same channel as spikes and is separated from them by filtering, not by a different electrode; where spikes report individual neurons, the LFP reports the population's rhythms and state. Between about 100 Hz and the spike filter lies the high-frequency field, whose power rises when many neurons fire together; the house field-potential channel is recorded at 0.1 to 300 Hz so that it keeps that band.

Because it sums many sources, the LFP is larger than a spike — a few hundred microvolts to a millivolt — and it does not fade within a hundred micrometers; it is present on every site of a probe and on the surface of the brain, which is what a thin-film grid records as electrocorticography. Site area matters less for its amplitude than for its noise: a larger, lower-impedance site averages more tissue and gives a cleaner field potential, which is why surface grids use sites from tens of micrometers to a millimeter across.

The reference matters more for the LFP than for spikes. Slow potentials travel through tissue, so a distant reference adds signal from everywhere between it and the site; a reference close to the sites, or a difference between neighboring sites, keeps the local part. Along a shank whose sites span the cortical layers, the differences between adjacent sites locate the current sources that generate the LFP, which is the basis of current-source-density analysis. Bands, spectrograms and coherence are under spectral analysis; the laminar method is current source density.