Signal-to-Noise Ratio (SNR)

Signal-to-noise ratio (SNR) is the amplitude of the neural signal divided by the amplitude of the background noise; for spikes it is the peak spike amplitude over the RMS noise on the same channel. An SNR of 3 to 4 is the working floor for a sortable unit and 5 to 6 is a good recording. Below 3 a spike is a threshold crossing that could be noise, and sorting fails first.

Both halves of the ratio are set at the electrode. The signal is set by distance — a spike is tens to a few hundred microvolts within a few tens of micrometers of the cell body and gone at about a hundred — so a site's SNR for a given neuron is mostly where the site is. The noise floor is set by the site's impedance ↗ (thermal noise rises with the square root of resistance), by the amplifier, and by what the reference ↗ fails to cancel: mains, muscle, the reference's own signal. A small site trades a higher noise floor for a more local signal; a coating lowers the floor without giving up the site size. The practical levers are in order: a clean, low-impedance site; a reference in quiet tissue; the amplifier at the head; shielding and grounding of the rig — the sequence in Noise Reduction and Troubleshooting ↗. SNR should be known during the session, not after it: a site that reads low while the animal is on the rig can be moved or excluded, and Allego reports per-channel signal quality live for that reason.