  Intracellular recordings of neuronal membrane potential are a central tool in
neurophysiology. In many situations, especially in vivo, the traditional
limitation of such recordings is the high electrode resistance, which may cause
significant measurement errors. We introduce a computer-aided technique, Active
Electrode Compensation (AEC), based on a digital model of the electrode
interfaced in real time with the electrophysiological setup. The
characteristics of this model are first estimated using white noise current
injection. The electrode and membrane contribution are digitally separated, and
the recording is then made by online subtraction of the electrode contribution.
Tests comparing AEC to other techniques demonstrate that it yields recordings
with improved accuracy. It enables high-frequency recordings in demanding
conditions, such as injection of conductance noise in dynamic-clamp mode, not
feasible with a single high resistance electrode until now. AEC should be
particularly useful to characterize fast phenomena in neurons, in vivo and in
vitro.
