Introduction
In conventional electroencephalography (EEG), scalp abrasion and application of a conductive paste are unavoidable for quality recording [1]. These are needed primarily to minimize the electrode-scalp impedance, which, in turn, addresses the finite input impedance of the EEG amplifier. The electrode-scalp impedance is estimated by measuring the interelectrode impedance. This quantity informs the EEG technician whether there has been sufficient abrasion at a particular site. Here, I discuss in detail the theory behind the current methodology and explore alternative means of insuring quality recording.
To measure the interelectrode impedance, a low frequency, limited current source is connected across two electrodes. By recording the voltage across the two electrodes, the impedance between the two points is deduced by Ampere's law. This interelectrode impedance serves as an indirect measurement the electrode-scalp impedance since it includes the contact impedances for two electrode-scalp contacts, two layers of epidermis, and the tissue in-between. Tissue impedance can be ignored, since it is highly conductive and contributes little to the interelectrode impedance.
Experts recommend that (1) the interelectrode impedance be less than the input impedance of the amplifier by at least a factor of 100, (2) the interelectrode impedance be reduced to less than 10 kΩ as measured at the frequencies of interest (e.g., 10 Hz), and (3) when skin potentials are of concern, that is, the frequencies of interest are below 0.1 Hz, the interelectrode impedance be reduced below 2 kΩ by puncturing the skin with a needle or lancet [1]. In this case, the technician is advised to abrade until a drop of blood is seen!
The latter recommendation addresses a secondary concern linked not with the impedance of the electrode-scalp contact but the epidermis itself. Underlying the surface of the skin are ions which distribute themselves in a way that generates a voltage. This voltage can fluctuate when pores open due to heat or arousal, or when varying pressure on the skin causes the ions to redistributed.
Skin Impedance
References
[1] T. W. Picton, S. Bentin, P. Berg, E. Donchin, S. A. Hillyard, R. Johnson, Jr., G. A. Miller, W. Ritter, D. S. Ruchkin, M. D. Rugg, and M. J. Taylor, "Guidelines for using human event-related potentials to study cognition: recording standards and publication criteria," Psychophysiology, vol. 37, no. 2, pp. 127-52., 2000.