## 4 Discussion  
The aim of our study was to investigate event-related potentials of EEG during the emotion recognition of dynamic facial expressions. To make our paradigm more naturalistic, we first demonstrated a neutral face for 500 ms and then presented a gradual change to a basic facial emotional expression during 462 ms. This permitted us to obtain diverse ERP, typically found in studies of facial emotion recognition. We recorded verbal reactions and reproduced differences in recognition accuracy between all basic facial emotional expressions, found by [PERSON] and [PERSON] [50]. The correlation between accuracy and reaction time was negative. Stimuli that were recognised more accurately, were recognised faster. This finding is in accordance with previous studies on facial emotion recognition [60; 61]. We registered some ERP typically found during facial emotion observation/recognition. After a visual investigation we referred to them as N170, P2, EPN, and LPP. They were prominent at temporal and posterior sites. Our naturalistic paradigm consisted of two parts. During the presentation of neutral faces for 500 ms, we found P1, N170, P2 and EPN. Then, a gradual change to a basic facial emotional expression (11 frames, 462 ms in total) provoked N170, P2, EPN and LPP. We admit that the first part (the perception of the neutral face) may have contaminated the ERP of the second one (the perception of a facial emotional expression). However, we wanted to build naturalistic stimuli, which is why we always presented a neutral face first. These waves appeared at their typical latencies during the presentation of neutral faces. As for ERP during the gradual change of basic facial emotional expressions, the latencies were longer. Thus, we regarded the wave between 201 and 500 ms as P2 and the wave between 501 and 1000 ms as the EPN. This discrepancy could be explained by the prolonged presentation of dynamic stimuli. In other words, the presentation of faces during 462 ms could make ERP appear later and last longer. Similar to previous studies [15; 48; 62], we observed N170 at temporal and posterior sites (both during the perception of neutral faces and dynamic emotional expressions). We found it both during watching neutral faces and in response to gradual changes of a basic  
Figure 6: ERP at posterior sites (P3, P2, P4, O1, Oz, and O2) depending on the reaction time of facial emotion recognition. Participants with the faster correct answers displayed a larger amplitude of P2. Participants with the faster correct answers displayed a larger amplitude of the LPP. Each curve represents the averaged data from 56 participants, 55–120 trials for each participant. The asterisks indicated the time windows of significant differences. facial emotional expression. It was more salient when watching neutral faces. However, our paradigm did not permit registering a clear N170 in response to the gradual appearance of a basic facial emotional expression, and we did not find any difference in N170 between subjects. A possible explanation would be that this time window was already contaminated by initial neutral faces. We also revealed a positive wave after N170 (both during the perception of neutral faces and dynamic emotional expressions), which we considered as P2 [63]. [PERSON] et al. [64] also found P2 in response to facial expressions, albeit they played a role in priming and were not recognised. [PERSON] and [PERSON] [65] demonstrated that P2, during the comparison of voices and facial expressions, was larger when both types of stimulation were congruent. P2 has been referred as a correlate of attention processes [29, 66] and has been related to early discrimination of stimuli [67]. Especially, the potentials between 200-300 ms may reflect attentional capture by emotional stimuli [68]. Generally speaking, stimuli displaying people are more salient and attract attention (e.g., [69]). Particularly, [PERSON] et al. [70] found that faces have the ability to capture attention to a greater extent than scenes.