## 8 Experimental Results and Discussion  
Electroencephalographs (EEGs), which measure electrical signals generated by the brain (brain waves), are used to study a person's brain activity. EEG signals are obtained as a result of the work of neurons in the human brain and can be intercepted using electrodes attached to the surface of the scalp [22; 23; 24]. A series of experiments were conducted to analyze the possible effects of the electromagnetic fields generated by smartphones on the activity of the human brain. Thirty volunteers (16 men (53.3%) and 14 women (46.7%) with an average age of 45.2 years) participated in the studies; however, we plan to increase these numbers in future studies among adults and children. The participants in the study stated that they were physically and mentally healthy, that they had not taken any medication before the tests, and that they were voluntarily undergoing these tests. The experiments were conducted in two stages. The first stage involved studying the EEG signals of the subjects without using a mobile phone. The second stage of the experiment was performed while the subjects used a mobile phone (Figure 6). The EEG recordings from the two experiments were processed in the MATLAB environment in the time and frequency domain of the signal. The aim was to make a comparative analysis of the signal spectra from the two experiments. Measures to reduce any other brain activity have been taken to assess the effect of cell phone electromagnetic radiation on a person's brain activity. The experiments were conducted in a quiet and cozy room, with the test subjects placed in comfortable armchairs with their eyes closed to reduce side stimuli. During the experiments, participants held the phone at a distance of about 1 cm away from the head, listening to a quiet countdown from one to one hundred, which was started by a researcher in another room. The aim of the experiment was to be as close as possible to a real conversation as shown in Figure 6.  
A mobile phone with a SAR of 0.36 W/kg was used during the experiment. The experiment lasted about an hour with the first 30 min without a phone and the second 30 min with a phone. The obtained signals were filtered and divided into frequency ranges, respectively: delta \(\delta\) (1-4 Hz), theta \(\theta\) (4-8 Hz), alpha \(\alpha\) (8-13 Hz), and beta \(\beta\) (13-32 Hz). With the help of the Pwelch function of the MATLAB program, the spectra of the signals before and after a call with a mobile device were obtained. The spectra of the two experiments for all electrodes were compared, and differences in the spectra were found at several measurement points (Figure 7). Figure 5: (**a**) Visualization of thermal effects in a human head before use of a GSM device; (**b**) visualization of thermal effects in a human head during use of a GSM device; and (**c**) visualization of thermal effects in a human head after use of a GSM device. At the points with the numbers T3, T5, and F7, which are the closest to the mobile phone, a significant change in the spectral activity of the brain was found. The largest change in the spectrum was found in T3, where the changes were in the theta, alpha, and beta frequency ranges. The changes in the spectra at points T5 and F7 were only in the theta and alpha ranges. Interestingly, this dependence was found in all participants in the experiments but to varying degrees. To compare the average spectral exposure with and without GSM for the ranges of delta \(\delta\) (1-4 Hz), theta \(\theta\) (4-8 Hz), alpha \(\alpha\) (8-13 Hz), and beta \(\beta\) (13-32 Hz), Student's \(t\)-test for related samples was used ([PERSON], 2008). Statistically significant differences, where \(p<0.05\), are visualized in Figure 7.  
The change in brain activity in a person's head on the side of a mobile phone has a short-term effect that is shown to be due to the operation of a mobile phone.