# The Influence of Electrode Impedance on the Parameters of Visual Evoked Potentials
## 4 Discussion
According to the recommendations presented in [1, 2, 3, 4, 5, 6], all types of reusable cup electrodes can be used for recording VEPs, i.e., gold, silver, and silver/silver chloride, but for adult subjects, pure silver electrodes are preferred. The purpose of these studies was to test all cup electrodes; therefore, the selection of the electrode type according to the age of the subject was meaningless. On the other hand, the usage of a particular electrode in the examination is determined by the value of the scalp-electrode impedance, which should not exceed 5 k\(\Omega\), and its general technical condition, i.e., no visible changes in the coating color or evidence of mechanical damage [1, 2, 3, 4, 5, 6, 8, 9, 13]. It should be noticed that during these studies, the scalp-electrode impedance measured with the VEP equipment for all electrodes (i.e., new and used electrodes) was below 5 k\(\Omega\). For each of the gold and silver electrodes tested, partial coating-color changes appeared on average after the fifteenth VEP examination. In the case of the Ag/AgCl electrode, tamishing of the coating occurred after the twentieth test. It is well known that degradation of the electrode coating, especially that occurring with silver and gold electrodes, occurs due to the application of conductive pastes and the cleaning fluids applied after each examination of brain potentials. The frequency properties of each electrode, after another VEP examination, were tested using the electrochemical impedance spectroscopy (EIS) method, which confirmed the results reported in other studies [13,15]. When new electrodes were tested, a decrease in impedance values was observed with increasing frequency values. On the other hand, the gradual degradation of the electrode coating following successive VEP recordings has contributed to not only an increase in impedance values after a subsequent examination using the same pair of reusable cup electrodes but also significant fluctuations in the frequency range up to 10 Hz. Thanks to the VEP registration, in addition to analyzing the course of latency changes, it was possible to observe values of the wave amplitude. As a result of the study, an increase in the amplitude of individual waveforms by an average of 26.42% and a prolongation of latencies by an average of 8.16% were observed by comparing the values of VEP parameters after the first and twentieth recording using reusable cup electrodes. An increase in electrode impedance undoubtedly causes an extension of latency, but the issue of increasing amplitude is debatable. From a technical point of view, the observed increase in electrode impedance, causing an increase in amplitude and an extension of latency, is not correct. In clinical practice, an increased amplitude often occurs regardless of changes in electrode impedance. According to [28], a larger VEP amplitude indicates stronger cortical activation in response to the visual input. The variability in VEP parameters, and especially in the amplitude, after subsequent stimuli is the result of a change in the sensitivity of the nervous system. A commonly observed phenomenon is either a decrease in reactivity to repeated stimuli (i.e., habituation), or an increase in reactivity to the stimulus (i.e., sensitization). In this paper, the sensitization phenomenon was observed. The sensitization phenomenon can be eliminated by using various types of visual stimuli, i.e., flash and checkerboards, during the one VEP test. However, the results of VEP parameter measurement using different types of stimuli cannot be compared with each other. The analysis of the VEP amplitude is not reliable and should not be taken into account in the quality assessment of cup electrodes. The results of EIS examinations suggest that reusable electrodes, regardless of the material they are made of (i.e., gold, silver, silver/silver chloride), in the study of evoked brain potentials should be used only once, because the correctness of the examination depends on the latency of the individual VEP components (e.g., P1, N1).