gms | German Medical Science

Electroencephalogram (EEG) electrodes are used in various application areas without considering the frequency-specific impedance. For instance, routine EEG examinations explore frequency bands from 0.5 Hz to 50 Hz, while Auditory Brainstem Response (ABR) measurements cover a range of 100 Hz to 3,000 Hz. The impedance of electrodes, inherently dependent on frequency, leads to impedance differences dependent on the frequency range of the application.

To address the impedance characteristic of the electrode, Electrochemical Impedance Spectroscopy (EIS) measurements were conducted using the Gamry Interface 1010T. Impedance measurements ranged from 1 Hz to 20 kHz, with eight measurements per decade. The experimental setup included a counter electrode (CE), a stainless steel A4 cylinder with a diameter of 12 mm, and a reference electrode (RE) (RE-1B by ALS Co., Ltd.). Both CE and RE were submerged in 0.9% sodium chloride solution. Two types of measurement electrodes (WE) were compared, (1) dry flat electrodes and (2) standard silver cup electrodes. For ABR and Alpha Power measurements (8–13 Hz), electrodes were positioned at FPZ (Ground), AFz (positive electrode), and each Mastoid (negative electrodes). Signal Recording was done with Biopac _R© MP36 system (California, United States) with a sampling rate of 100 kHz, 24-bit A/D converter. Alpha power spectral analyses were performed during eyes closed and eyes open condition. For the ABR measurements a 100 μs click stimulus served as the acoustic stimuli at 80 dB SPL, delivered through over-ear headphones (HD580 by Sennheiser). All measurements were conducted on the same subject and same electrode position.

The EIS measurements revealed a larger impedance decrease of about 2 kΩ to 15 Ω for the cup electrodes in comparison to the flat electrodes with an impedance decrease of 250 Ω to 70 Ω see figure 1 left side. This impedance variation leads to different expected electrode impedances in the two application frequency ranges: Alpha band: 200 to 400 Ω, and ABR measurement 20 to 100 Ω. The power spectrum density measurement resulted in a ratio of eyes closed to eyes open of 1.67 (dry flat electrodes) and 1.69 (cup electrodes). The Wave V amplitude in ABR measurements exhibited values of 512 nV (dry flat electrodes) and 559 nV (cup electrodes).

These results suggests that the impedance of the electrode influences the measurement due to its frequency ranges. The alpha band impedance was close and overlaped, which was reflected in the similar ratio results. The ABR results diverged amplitude of wave V which needs a further review of different dry electrode types applied to a larger number of EEG type measurements to quantify this effect.

Figure 1 [Fig. 1]