# The Influence of Electrode Impedance on the Parameters of Visual Evoked Potentials
## 2 Materials and Methods  
Confirmation of the dependency between the number of tests performed with reusable cup electrodes and the amplitudes or the latencies necessitated a study in which the response of brain signals to stimuli with fixed parameters could be recorded. The visual evoked potential (VEP) examination was prepared for this purpose. All the VEP tests were performed on the author of this paper. To determine the absence of contraindications before starting the VEP examinations, this author was tested for demyelinating diseases, diabetes, basic retinal disease, and glaucoma. All these tests were repeated after a year. Twenty reusable cup electrodes of each type were examined, i.e., gold (Au, DEGM152600 GVB-geliMED GmbH), silver (Ag, DESM152600 GVB-geliMED GmbH), and silver/silver chloride (Ag/AgCl, DAGM152600 GVB-geliMED GmbH). Additional electrodes (i.e., ground) required for recording VEP potentials were not analyzed. For every examination, new ground electrodes were used. In accordance with recommendations [1; 2; 3; 4; 5; 6], all the electrodes used in one study were of the same type. For each of the ten electrode pairs, twenty VEP tests were made. Only one pair of electrodes was tested in one day. Moreover, taking into account the principles of recording brain potentials [5; 6], the interval between VEP examinations was 24 h, i.e., the recordings were performed at one time of day. After cleansing the scalp with a NUPREP gel and ECOLAB Skinsept Pur, electrodes were bonded to the skin using the TEN20 conductive EEG paste according to the 10-20 international system in the following places: occipital lobe (active/positive electrode Oz), forehead (reference/negative electrode Fz) and right earlobe (ground electrode A2). Each VEP examination was carried out with the flash method using the EEG Neuron-Spectrum-5 equipment and LED goggles emitting red light. Stimulation parameters were selected according to recommendations [5; 6], i.e., stimulation frequency: 1 Hz; stimulation duration: 5 ms; frequency band of the recorded signal: 2-100 Hz; sampling frequency: 10 kHz. All the VEP examinations were performed in a single-channel Oz-Fz electrode array [5; 6], i.e., MO-MF, while the stimulation was performed independently in the right eye, in the left eye, and then binocularly, with eighty recordings averaged each time. Each VEP test was prepared twice, and the final averaged recording from the two examinations was analyzed. The following amplitudes--N1-P1, N1-P2, and N2-P2--and latencies--N1, P1, N2, and P2--of the waves were included in the calculations. The VEP reports were generated automatically by the Neuron-Spectrum.NET software 1.0.82.0. An example of a VEP test report is shown in Figure 1.  
Then, to check the value of the scalp-electrode impedance for each of the electrodes, the EEG Neuron-Spectrum-5 amplifier system was used. After the duration of the VEP examination, all the electrodes were cleaned according to the recommendations presented in [1; 2; 3; 4] and prepared for further tests. In the next step, the electrochemical impedance spectroscopy (EIS) method was used to check the existence of a relationship between the impedance values of reusable cup electrodes and the number of VEP examinations. To prevent any noises, all measurements were performed within a Faraday cage. After each VEP examination, the electrodes were tested at a frequency range of 0.1-100 Hz and a temperature of 37 \({}^{\circ}\)C using the Autolab PGSTAT302N platform, the frequency response analyzer FRA32M, and an electrochemical cell (Metrohm, no. 6.1418.250) with a reference silver/silver chloride electrode (Metrohm 6.0733.100) and a counter platinum electrode (Metrohm 3.109.0790). The TEN20 conductive EEG paste was used in the electrochemical cell.