Machine-learning tools were used, as finding the differences with eyes seemed impossible.

The differences between sub-cohorts in such signals were expected to be found.

In the classification problem, the avalanche analysis for feature extractions from the resting state signal and linear discriminant analysis for classification were used. The discussion of such differences will be presented however, an explanation of such differences would require the recruitment of a much larger cohort. The cross-correlation method was used to find differences, and, in fact, such differences manifested themselves in the shape of event-related potentials on the cognitive electrodes. These potentials were plotted for all three patients' sub-cohorts and all three experiments. The event-related potentials were chosen as we expected to find the differences in the patients' responses to three different mental tasks arranged in the experiments commonly known in experimental psychology: face recognition, digit span, and task switching. he dense array electroencephalographic amplifier with 256 electrodes was used for recordings. The aim of this article was to examine whether there are differences in the brain cortical activity of these three sub-cohorts and, if possible differentiate and classify them using the machine-learning tools. The research referred here aimed at carrying out the investigation comprising the electrophysiological analysis of the patients who suffered from COVID-19 and had symptoms of brain fog (sub-cohort A), suffered from COVID-19 and did not have symptoms of brain fog (sub-cohort B), and the control group that had no COVID-19 and no symptoms (sub-cohort C). The maximum magnitude of the third wave of COVID-19 in Poland was in April 2021. 4Department of International Relations, Faculty of Political Science and Journalism, Maria Curie-Sklodowska University in Lublin, Lublin, Polandīrain fog is a kind of mental problem, similar to chronic fatigue syndrome, and appears about 3 months after the infection with COVID-19 and lasts up to 9 months.3Department of Computer Science, Ben-Gurion University of the Negev, Beer-Sheva, Israel.2Department of Cognitive and Brain Sciences, Ben-Gurion University of the Negev, Beer-Sheva, Israel.1Department of Neuroinformatics and Biomedical Engineering, Institute of Computer Science, Maria Curie-Sklodowska University in Lublin, Lublin, Poland.Wojcik 1 *, Oren Shriki 2,3, Lukasz Kwasniewicz 1, Andrzej Kawiak 1, Yarden Ben-Horin 2, Sagi Furman 2, Krzysztof Wróbel 1, Bernadetta Bartosik 1 and Ewelina Panas 4

Digit span test 2.3.1