## 4 Discussion
A recent study showed that any emotional social content (not only faces, but also scenes) modulate anterior P2 [71]. However, it is important to note that our paradigm generated only salient posterior, but not anterior, P2. During 501-1100 ms, we registered a negative wave (during the perception of dynamic emotional expressions), which we considered to be the early posterior negativity. It reflects a natural selective attention to emotional stimuli [25, 72]. [PERSON] and [PERSON] [73] found similar EPN in response to emotional faces, scenes and words. [PERSON] et al. [74] demonstrated an enhanced early negativity at lateral temporal and occipital electrodes for emotions relative to neutral faces. Finally, our paradigm permitted us to observe the LPP 1100 ms after the gradual appearance of a basic facial emotional expression. LPP amplitude could be interpreted as the allocation of attentional resources towards emotional stimuli of different characteristics [75]. Thus, [PERSON] et al. [76] showed a larger amplitude to emotional expressions relative to neutral ones. [PERSON] et al. [77] found larger amplitudes of the late ERP waves, P3 and LPP, during perception of the beloved of participants in comparison with unknown or famous faces. In a study by [PERSON] et al. [78], high empathy people exhibited a larger amplitude of LPP in response to facial expressions. Similar to [PERSON] et al. [46], who presented dynamic facial expressions, we identified the EPN and LPP, although in our case their latencies were greater. While those researchers found them at 200-250 and 400-600 ms, the latencies in our study were 501-1100 and 1101-1500 ms. We believe that this discrepancy can be explained by the duration of the gradual change to a basic facial emotional expression. In their study, dynamic morphs consisted of three frames, a third of which (the final one, 100% of emotion) was presented at 100 ms. Whereas in our study, we consecutively presented 11 frames for 42 ms, so the final frame appeared only at 450 ms.  
We also compared event-related potentials between participants with different performances of facial emotion recognition. Thus, participants with low accuracy exhibited larger amplitudes of posterior P2 (387-428 ms). This can reflect the attentional resources required for recognition. Of note, our EEG analysis embraced only trials with correct responses. Hence, participants with lower accuracy allocated their attention in such a way as to recognise emotional expressions correctly. This is in accordance with attentional allocation framework which relates a successful behavioural performance with an increased amplitude in attentional ERP waves [79, 80]. For participants with high accuracy, such a task, supposedly, is easier and more habitual. In addition, participants with the faster correct responses displayed a larger amplitude of P2 and LPP after the beginning of the gradual appearance of a facial emotional expression. Therefore, faster, but correct recognition of emotional expressions required a greater allocation of processing resources. In other words, dynamic facial stimuli provoked a greater allocation of attention which helped to recognise emotions faster. Further research could be directed at constructing different designs to attempt to capture subtle physiological differences between all basic facial emotional expressions. We admit, as a limitation, the fact that our design could not allow examination of ERP between facial emotional expressions (only 144 trials for six emotional expressions). For this purpose, one could present much more stimuli (30-50 per emotion, that would make the study much longer) or limit to, for instance, three basic facial emotional expressions. In our study, basic emotions differed much in the accuracy of their recognition. This does reflect typical differences in facial emotion recognition, but this entailed different numbers of trials with correct responses for different basic emotions.