REAPPRAISAL OF THE EFFECT OF ELECTRODE PROPERTY ON RECORDING SLOW POTENTIALS

Subdural electrodes made of stainless steel, which were believed to be unsuitable for recording slow potentials, can still record Bereitscha ftspotential (BP) (Neshige, R., Luders, H. and Shibasaki, H. Recording of movement-related potentials from scalp and cortex in man. Brain, 1 988, 11: 719-736) and ictal DC shifts (Ikeda, A., Terada, K., Mikuni, N., Burgess, R.C., Comair, Y., Taki, W., Hamano, T., Kimura, J., Luder s, H.O. and Shibasaki, H. Subdural recording of ictal DC shifts in neo cortical seizures in humans. Epilepsia, 1996b, 37: 662-674) sufficient ly. In this study, therefore, the effects of different kinds of metals on slow potential recordings were reevaluated. First, slow electro-oc ulograms (EOGs) were recorded with 3 different levels of input impedan ce (200 M Omega, 470 k Omega and 10 k Omega) of a DC amplifier by usin g surface electrodes made of silver (Ag), silver/silver chloride (Ag/A gCl) and stainless steel. Secondly, BP was recorded by using the above electrodes with a long time constant of 3 s and with a fixed input im pedance of 100 M Omega. As a result: (1) slow EOGs were equally record ed with the input impedance of 200 MO and 470 k Omega regardless of th e kind of metals used, although stainless steel electrodes caused base line fluctuation, (2) low input impedance of 10 k Omega allowed only t he Ag/AgCl electrode to record slow EOGs without any decay, and (3) el ectrodes made of stainless steel could record BP as efficiently as the other two types of electrode with high input impedance. In conclusion , electrodes with a large surface area contact such as cup electrodes and an amplifier with a large input impedance, electrodes made of Ag, and even of stainless steel, can record slow potentials reasonably wel l. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.