A nerve activity monitoring method comprises: receiving an input signal indicative of nerve activity of a subject, which may be via electrode 108； receiving physiological data indicative of physiological activity of said subject, which may be via sensor 111； establishing a relationship between the physiological data and the input signal； identifying a plurality of periodic portions in the input signal based on said relationship； and outputting the periodic portions. The electrode and sensor system 116 may be implantable. A further embodiment discloses a nerve activity monitoring method comprising: calculating a frequency spectrum of at least a portion of an input signal indicative of activity in a nerve of a subject； identifying a demodulation frequency within the frequency spectrum； demodulating at least two periodic portions of the input signal based on the demodulation frequency, to generate at least two demodulated signal portions； and averaging the at least two demodulated signal portions to generate an averaged signal.
