# Monitoring Brain State and Behavioral Performance during Repetitive Visual Stimulation
## 4 Discussion  
A group of volunteers was tasked to classify Necker cubes of different ambiguity within 40 min. The subjects reported the orientation (left or right) of each presented cube, while the stimulus morphology ranged from low ambiguity (LA) to high ambiguity (HA). By observing behavioral responses, we found that decision time decreased with the time on task for both HA and LA stimuli (Figure 2D). At the same time, the subjects improved the correctness of the interpretation of the HA stimuli, but not the LA stimuli (Figure 2F). Analysis of the EEG spectral power on the sensor level and in the source space revealed an increase in the prestimulus power at 9-11 Hz with the time on the task (Figures 3B and 4B). Finally, we found that the prestimulus EEG power negatively correlated with the decision time to LA stimuli (Figure 5A) and the number of erroneous responses to HA (Figure 5D) stimuli. First, we hypothesized that the prestimulus EEG power reflects changes in a person's condition. The condition, in turn, affected the performance of processing the ongoing visual stimulus [11, 16]. Thus, our results showed that the high pre-stimulus 9-11 Hz EEG power predicted faster decision times and greater accuracy. It is worth noting that we compared EEG power and behavioral estimates between time segments, each of which lasted 10 min. Therefore, we have associated the described effects with slow changes in the observer's state. Taken together, we proposed a possible application of our findings in passive brain-computer interfaces to monitor the human's condition and predict decision speed and errors. Along with possible practical applications, our result can help to reveal specific features of ambiguous perception. To start a discussion on this aspect, we will look at the limitations of our experimental design. It consists of two confounding variables, mental fatigue, and learning; both can be time-dependent. Thus, the revealed EEG changes practically do not reflect one of these factors. At the same time, we suppose that fatigue  
Figure 5: Results of the correlation analysis: Regression plots illustrate the relationship between SP and DT to LA stimuli (**A**); SP and DT to HA stimuli (**B**), SP and ER for LA stimuli (**C**); SP and ER for Ha stimuli (**D**). The colored dots correspond to each participant’s data; the lines have the same slope estimated for these participants via correlation analysis with repeated measures. and learning affect the observer state regardless of the stimulus ambiguity. In contrast, our results show a different time-on-task effect on processing HA and LA stimuli. For instance, we revealed a reduction in error rate for HA stimuli only. In addition, the prestimulus EEG power negatively correlates with the decision time to LA stimuli, not to HA stimuli, and negatively correlates with the error rate for HA, but not for LA stimuli. Another limitation is the small sample size. Thus, there is a risk that the individual characteristics of the participants, e.g., gender and age, will affect their perception of ambiguous stimuli. For our group, we observed no gender and age effects on the decision time and the ERSP, due to the almost uniform distribution of these factors between participants. At the same time, we expect that another group of younger or older subjects may demonstrate different effects on the behavioral and neural activity levels. The decision time (DT) may decrease due to neural adaptation (NA), which occurs when the same visual stimulus is repeatedly presented within a short interval and causes a decrease in neural response to repetitive versus non-repeated stimulus [33]. The NA is thought to arise from at least two types of neural activity. One explanation is that only the part belonging to the neuronal ensemble is sensitive to stimulus recognition. Thus, the neurons that are not critical for stimulus recognition decrease their responses when the stimulus reappears, while on the contrary, neuronal populations carrying essential information continue to give a robust response.