Comparing DC offset and impedance readings in the assessment of electrode connection quality

Abstract

Electroencephalograph (EEG) electrode impedance measurements of 5,000 ohms or less are required by common standards of practice to minimize artifacts due to electro-magnetic interference (EMI). Some amplifiers geared toward the neurofeedback market do not include on-board impedance monitoring, but provide direct current (DC) offset measurements. To examine if DC offset is a reliable measure of connection quality, measurements of DC offset and impedance, each independently taken by students in a university graduate level course in neurofeedback over a one-year period were analyzed retrospectively. DC offset was not found to have predictive value of a standard impedance level. Additionally, 19 channel EEGs collected within manufacturer recommended parameters of DC offset using a high-impedance amplifier were analyzed to assess the level of EMI pollution of quantitative EEG (QEEG) data. Visible peaks of EMI in the spectra in at least one channel in each of these recordings were identified. A sample of EMI pollution of QEEG results is presented. Together, these findings suggest that DC offset may not be a reliable measure of electrode connection quality.