# The Influence of Electrode Impedance on the Parameters of Visual Evoked Potentials
## 1 Introduction  
In clinical practice, despite the definition of general principles for conducting clinical tests of brain potentials, the methods of their analysis, and the way they are described, a problem still exists with both the correctness of their interpretation and the accuracy of signal recordings [1, 2, 3, 4, 5, 6]. In general, three main parameters are analyzed: frequency, time, and amplitude. In order to determine the accuracy of brain potentials (i.e., evoked or spontaneous), it is required to prepare a description of the characteristics recorded during the study. This description should be based on the results of the value analysis and the amplitude distribution in individual brain areas and their frequency range. The description of the examination also includes information on the presence or absence of the required normal brain waves [4]. Based on the analysis of the amplitude and frequency values, the periodicity of the potentials, along with their number, shape, degree of symmetry, and reactivity (i.e., in response to eye opening and closing or in routine activation trials) are determined. In the case of abnormal changes (i.e., seizure, spike-wave complex, slow or fast activities), additional analyses of individual potentials are performed, and an interpretation of the results is placed in the general description [4, 7]. For the correct interpretation of the test results, additional information on the patient's age, case history, and current neurological status is necessary. It is worth mentioning that regardless of the patient's age group and health status, personal constancy, and the nature of the recording (i.e., resting,activation), all brain examinations (i.e., EEG, EP) are determined by genetic characteristics and homeostasis, as well as by gender [4; 8; 9]. The evoked potential examination, on the other hand, focuses mainly on the analysis of latency values while also taking into account the amplitude distribution of characteristic refractions [5; 6]. In this case, in addition to the patient's age and gender, the patient's medical history and current neurological status also should be taken into account in the final evaluation. In the general diagnostic evaluation of brain potentials to extract and quantify parameters of brain activity (i.e., frequency, amplitude, phase of waveforms, degree of symmetry), it is acceptable to use algorithms based on regression methods, wavelet transform, or artificial neural networks [8; 10; 11; 12]. However, their accuracy depends on, among other things, the electrodes used to record brain signals. Published papers [8; 9; 13; 14; 15; 16; 17] discuss a number of factors affecting the accuracy of brain signal analysis, with a focus on changes in frequency ranges. One factor that significantly affects the frequency values of recorded signals is the scalp-electrode impedance, which, in the case of brain potentials, should not exceed 5 k\(\Omega\)[1; 1; 1; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26]. On the other hand, in one study [15], no significant effect was found for changes in electrode impedance on the amplitude values of recorded brain signals. Currently, many types of electrodes have been proposed, e.g., dry-contact and non-contact electrodes, dry foam-based electrodes, polymer and flexible silicone-based dry electrodes, 3D printable dry electrodes, passive and active dry electrodes, textile electrodes, or even polymer tattoo electrodes [18; 19; 20; 21; 22; 23; 24; 25]. However, reusable electrodes, i.e., gold (Au), silver (Ag), silver/silver chloride (Ag/AgCl), and sintered bridge electrodes (SBEs) are still used in clinical practice [1; 1; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69]. The cup electrodes are used in both EEG and EP studies, whereas SBEs are only used in EEG examinations.