R.A. Maraguei - Research Scientist, Institute of Higher Nervous Activity and Neurophysiology, Russian Academy of Sciences, Moscow
L.A. Potulova - Ph.D.(Biol.), Senior Research Scientist, Institute of Higher Nervous Activity and Neurophysiology, Russian Academy of Sciences, Moscow
G.B. Milovanova - Ph.D.(Biol.), Senior Research Scientist, Institute of Higher Nervous Activity and Neurophysiology, Russian Academy of Sciences, Moscow

The infraslow oscillations of cortical potentials as well as EEG have been proposed to be useful to examine a functional state of man-operator in long-term experiments. To acquire a bioelectrical activity (EEG, EOG, ECG, EMG) we used multichannel DC-amplifier with sintered Ag/AgCl electrodes. The requirements to amplifier and electrodes intended for measure infraslow oscillations are well known. They touch a dynamic range and zero drift of amplifier input stage as well as electrical characteristics of electrodes. The latter are mainly defined by characteristics of electrolyte bulk adjacent to the electrode surface. It is feasible to consider electrode-electrolyte interface as a circuit of electrolyte bulk resistance, electrode potential, electrode resistance and capacity constituted by double-layered charge of electrolyte bulk. In common case the resistance and capacity of electrolyte bulk are nonlinear and may form a high-pass filter disturbing studied processes at low frequencies. The aim of present work was to trend the polarization potentials of electrodes to be sure of stability of their characteristics. Another task was to define the distortion of low-frequency signal made by means of amplifier and electrodes. Monitoring the polarization potentials during 50 hours has revealed the polarization potentials were complied with requirements for bioelectric electrodes and fitted to dynamic range of DC-amplifier used. To examine possible distortions interfered by amplifier and electrodes with low-frequency signals we used a set of test sine waves applied by generator of waveforms to amplifier inputs directly and through electrodes immersed in electroconducting gel. The oscillation frequencies of test sinusoids were matched the following subcircadian ranges: EEG (2 Hz), decasecond (30 mHz), minute (20 mHz), decaminute (500 µHz), hour (250 µHz), several hours (100 µHz). It was shown that neither DC-amplifier, nor electrodes distort low-frequency signals significantly. The visual inspection of output curves has been approved by spectral analysis and 2 measures: total harmonic distortion (THD) and Pearson's correlation coefficient (r) between input and output signals averaged overall cases. In the case of sinusoids applied directly these measures were 0.03 and 0.99 respectively. In the case of electrodes the measures were 0.05 and 0.97 respectively. We conclude that digital DC-amplifier of biopotentials with sintered Ag/AgCl electrodes allows to record bioelectric activity overall frequency ranges from 0 Hz until EEG simultaneously.