# Automating Stimulation Frequency Selection for SSVEP-Based Brain-Computer Interfaces
## 5 Experimental Evaluation
### The Experiment Description
#### 5.1.2 Design and Procedure  
For this study, we used a portable Mitsar-EEG-SmartBClx24 amplification unit, which provides continuous recording of EEG signals over the whole experiment. The EEG data were recorded from 21 channels using SmartCAPx24 cup Ag/Ag-Cl electrodes located according to the international \"10-20%\" electrode placement system, as shown in Figure 1. The electrode corresponding to the location of FCz was used as a reference (Ref) electrode. The location of the grounding (Gnd) electrode corresponded to the AFz electrode. The sampling rate of the EEG signal was 250 Hz. As the main operating frequency of our devices' electrical network was 50 Hz, we activated a notch filter in the \"EEGStudio\" software designed by the amplification unit manufacturer to record EEG signals. The cutoff frequency of the notch filter was equal to 50 Hz, and a bandwidth range of \(\pm\)5 Hz around it was used. The use of this filter was necessary to suppress the artifact of power line interference [53]. Any additional tools for automatic artifact detection and removal in EEG signals during the recording stage were disabled to preserve the integrity of the signals and allow for their use in future studies related to adaptive artifact removal in EEG signals.  
In many researches, LCD monitors of personal computers are used as a solution for generating and presenting photostimuli to subjects. This approach has its disadvantages, such as the occurrence of harmonics in the EEG signal spectrum that are multiples of the screen refresh rate. Therefore, we used a special device of our own design, which provides a convenient process of generation and photostimuli presentation with a specified duration and frequency More details about the device and its characteristics will be described in the following section.  
Throughout the experiment, each subject was seated in a comfortable chair in a relaxed state. The room where the recording was performed was shaded to reduce interference from strong light. During the registration process for each subject, we carefully controlled the impedance values, which on average lay in the range of 4 to 7 kOhm. After the electrode placement procedure was completed, each subject was given 5 min to rest, and then the process of EEG data recording began. Before the photostimuli presentation, a 60 s fragment of the EEG signal in an eyes-closed state was recorded for each subject. The corresponding EEG signal fragments were marked with special labels in the EEG recording software. After this stage, the process of presenting photostimuli of given frequencies began.  
In our study, we used a range of photostimulus frequencies from 5 to 25 Hz with steps of 1 Hz. Stimulation at each frequency was performed for 60 s. Each fragment of the EEG signal during the presentation of the photostimulus was marked with appropriate labels. During stimulation, participants were required to focus their attention on the flashes. They were also allowed to blink as needed. After each photostimuli presentation, participants were given 2-3 min to rest. An obligatory stage during each rest period involved verbal questioning of the participants about their well-being and discomfort experienced during the stimulation. Since none of the participants experienced discomfort, the survey data were not used in further analysis. Sufficient resting time was provided between stimulations to prevent visual fatigue. Thus, for each participant, a _.xdf_ file was  
Figure 1: Scheme of the EEG cap with Ag/AgCl sintered sensors by MCScap for the 24-channel SmartBCI wireless wearable EEG system.  
recorded, containing EEG signals from 21 channels, along with an additional signal from our photostimuli presentation device. Each file had an average duration of approximately one and a half hours.