*2.7. Experimental Conditions*

Since the observer's state, as a rule, is not at a constant level, but fluctuates at different time scales, in the short term, this causes a difference in the subject's behavior when presenting stimuli, even if their ambiguity does not change. Meanwhile, in the long term, cognitive fatigue occurs, and the training effect takes place. In this work, we have eliminated short-term fluctuations and focused on the long-term changes in the person's condition over a 40-min task. To exclude the influence of short-scale fluctuations, we divided the entire experiment into four consecutive intervals of 10-min duration each (see Figure 1B). For each interval, we averaged ERSP and source power (SP) over all trials. To assess the behavioral characteristics, we used the median DT and the error rate (ER), reflecting the percentage of erroneous responses at each interval.
