Filtering (Band-Pass and Notch)

Filtering separates a wide-band channel into the signals it contains: a band-pass filter keeps one range of frequencies — about 300 Hz to 6 kHz for spikes, 0.1 to 300 Hz for field potentials — and a notch filter removes one narrow band, usually the 50 or 60 Hz mains line and its harmonics. Both are applied digitally, live in the acquisition system or afterwards in analysis; filter type and order set how sharply the band edges cut and how much the waveform's phase is distorted, which matters for spike shape.

Filtering cannot add what the electrode did not record. Spikes and field potentials are the same voltage on the same site, so the band-pass is a division of one signal, and a site chosen for one band still records the other; what area and impedance change is the noise in each band, not its presence. A notch filter removes the mains line but rings near it, and it is a repair for a grounding or reference ↗ problem that is better fixed at the rig. Common average referencing removes what filtering cannot: noise shared across channels at every frequency. NeuroNexus headstages record wide-band, with the pass-band set in software from below 1 Hz to several kilohertz, so both bands are in the file and the split can be changed afterwards; a filter applied at acquisition in Allego is a view of the data, not a change to the file.