Abstract
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 manufacturers of amplifiers geared toward the neurofeedback market do not provide on-board impedance monitoring, but provide DC offset measurements. To discover 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 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 is not a reliable measure of electrode connection quality.
Highlights
Many factors are inherent in the quality of an electroencephalograph (EEG) recording, among which are artifact reduction, electrode stability, and the highest possible signal-to-noise ratio
Two analyses were conducted for the direct current (DC) offset and impedance measurements: (1) plots and correlation, and (2) confusion matrices and receiver operating characteristic curve
Statistical analysis of common DC offset and impedance measurements suggests that DC offset may not be a reliable measure of electrode connection quality as compared to impedance
Summary
Many factors are inherent in the quality of an electroencephalograph (EEG) recording, among which are artifact reduction, electrode stability, and the highest possible signal-to-noise ratio. Other sources of EMI, such as radio, may be present in the recording environment. Since the typical neurofeedback provider may not be operating in a shielded room, these sources of EMI are often present, as indicated by artifacts in the traces and characteristic peaks in the spectra. The quality of the electrode connection has been seen as instrumental to ameliorating these factors as achieved by attaining low impedance between the leads. The importance of low impedance for achieving electrode connection quality is well established and continues to be recommended in EEG textbooks (Tatum, 2014; Tyner, Knott, & Mayer Jr., 1983). Impedance measurement below 5,000 ohms remains the adopted standard for EEG recordings (American Association of Sleep Technologists, 2012; American Clinical Neurophysiology Society, 2008)
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