# The Influence of Electrode Impedance on the Parameters of Visual Evoked Potentials
## 3 Results  
The analysis of the impedance characteristics for each of the cup electrodes after their repeated use is presented in Figure 3. In order to indicate the significant differences occurring between new and used electrodes, the graphs additionally show the waveforms of characteristics obtained for new electrodes. The average impedance values were calculated based on measurements of twenty electrodes of each type, i.e., gold, silver, silver/silver chloride. Figure 3: Impedance characteristics of the (**a**) gold (Au), (**b**) silver (Ag), and (**c**) silver/silver chloride (Ag/AgCl) reusable cup electrodes after a subsequent VEP test. The dotted line represents the characteristics of the new electrodes. The obtained results confirm the generally known fact that the value of the impedance decreases as the frequency value increases [17; 18]. Moreover, it can be confirmed that there is a dependence of the impedance value of reusable electrodes on the number of performed VEP examinations. For two types of electrodes (i.e., Ag and Ag/AgCl), the impedance value increases with the number of examinations. In the case of gold electrodes, the impedance decreased significantly after the 11 th, 12 th, 13 th, 14 th, and 15 th use due to non-uniform degradation of the electrode coating. In fact, gold electrodes are only silver with a gold coating. However, after 16 uses, the impedance increased. Considering the course of all the presented characteristics, the choice of the new Ag/AgCl electrode for recording brain potentials seems to be the most favorable. However, a linear course of characteristics above the frequency of 20 Hz was achieved by Au and Ag electrodes. On the other hand, in the frequency range of up to 10 Hz, the smallest impedance fluctuations were observed for Ag electrodes. The Ag/AgCl electrodes show significant fluctuations, despite achieving the lowest impedance values compared with the other electrodes. Also, for Au electrodes, significant variation in impedance values can be seen. Abrasion of the electrode coating is responsible for the impedance fluctuations. It is worth adding that the characteristics of new electrodes do not show such irregularities. Taking into account the analyses shown in Figure 3, Ag electrodes should be used for recording brain potentials only in the slow-wave range (i.e., delta and theta activity). The influence of new cup electrodes on the VEP parameters, i.e., amplitudes and latencies, and the statistical analysis are presented in Figure 4 and Table 1 and Figure 5 and Table 2 respectively. Figure 4: Variation analysis of the VEP amplitudes for twenty new electrodes of each type: (**a**) gold (Au); (**b**) silver (Ag); (**c**) silver/silver chloride (Ag/AgCl), where N1-P1, N1-P2, and N2-P2 are the VEP components. Within the box, a vertical red line means the median value of the data set, the bottom and top blue lines of the box indicate the 25 th and 75 th percentiles, and the whiskers present the min and max. Based on the results obtained from the analysis of VEP signals recorded with the new electrodes, variability and significant fluctuations in the values of both the amplitudes and latencies were observed. In the case of new Ag and Ag/AgCl electrodes, the variability in the parameters is comparable but significantly smaller than with Au electrodes. The results suggest that gold electrodes should not be used for recording VEP potentials. The effect of the number of examinations performed with reusable cup electrodes on the VEP parameters is presented in Figures 6 and 7. Based on the results of twenty VEP examinations, obtained for twenty electrodes of each type, i.e., Au, Ag, and Ag/AgCl, the mean values of all the parameters were calculated. Of these figures, the mean values of the VEP after the first, fifth, tenth, fifteenth, and twentieth examinations are shown.