# Review of Studies on User Research Based on EEG and Eye Tracking
## 3 Indicators of User Research
### Classification and Characteristics of EEG Signals
#### 3.1.2 Classic Brain EEG ERP Components and Their Meanings  
The ERP, as a current focus of extensive research, is an electroencephalogram (EEG) caused by psychological activity. It reflects the physiological changes in the brain's nerves during the cognitive process and can non-invasively, with high temporal resolution, reflect the process of brain cognition. It is an important means of observing and interpreting people's inner thoughts and behavioral expressions. An ERP generally only has 2 to 10 microvolts, so it is necessary to filter, segment, superimpose, and extract EEG experimental data to obtain corresponding components for further research. Currently, the narrow definition of an ERP refers to classic ERP components, including P1, P2, P3 (P300), N1, and N2; meanwhile, the broad definition of an ERP includes not only narrow content, but also components such as N400 and accompanying negative reactions [26]. As shown in Figure 5, P and N represent the amplitude of positive and negative trends, respectively, and the numbers represent the time position where the amplitude appears. Among them, P1, N1, and P2 are also called the exogenous components or physiological components of an ERP. They are the most primitive components produced by the human brain when stimuli occur, and are mainly affected by the intensity and frequency of physical stimuli; N2 and P3 are the endogenous components or psychological components of an ERP. They are not affected by physical stimuli, but depend on people's memory, perception, intelligence, and other activities. They are some of the stimulus-induced activities produced by the human brain during internal activities [27].  
ERP components not only reflect physiological changes in the brain but, more importantly, can reflect psychological changes and cognitive processes at certain levels, which is of great significance for explaining user behaviors and their psychological states [28]. For example, P300 is related to the last stage of information processing and the measurement of \"cognitive efficiency\", which can reflect the subject's mental load and attention level. The amplitude of the wave generated is positively correlated with the mental state used. It also has a latency period. The more difficult the task during the latency period, the longer the waveform will be. It can be used as a marker for attention and working memory; N1 and P1 are considered to be closely related to early visual cognition, including visual attention and spatial location information capture. They can be used as objective indicators to measure the degree of visual stimulation. Negative emotional stimuli will cause larger wave amplitudes [29]; P2 and N2 reflect attention allocation to novel and potentially significant stimuli. They can be used as objective indicators to measure the intensity of emotional valence. The intensity of negative emotions will make changes in the brain more sensitive; N400 is related to semantic understanding, and can reflect the time process of semantic construction [30]. These findings have an important reference value for exploring users' real thoughts, emotions, and evaluations in cognitive aspects.  
Figure 5: ERP component schematic diagram.