## 3 Results
### RQ1: Which Electrophysiological Signals May Index Emotional Processing? To monitor human body responses related to emotion processing, several electrophysiological recording techniques have been used. EEG was the preferred method, reported in 27 studies [25; 26; 28; 32; 34; 37; 38; 39; 40; 41; 42; 43; 44; 45; 47; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 63; 65; 66]. ECG was used in 11 studies [27; 29; 30; 33; 36; 44; 46; 48; 50; 61; 62]. EDA was reported in 10 studies [25; 27; 29; 31; 32; 43; 46; 54; 61; 64], and only one study used EOG [36]. From these studies, four of them fused ECG with EDA [27; 29; 46; 61], two of them fused EEG with EDA [25; 32], one of them fused EEG and ECG [50], and the remaining one fused ECG with EOG [36]. Table 3 summarizes electrophysiological signals, methods and features analyzed for correlation with emotional states. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Electrophysiological** & **Analysis Method** & **Analyzed Feature** & **Emotional Correlation** \\ \hline \hline \multirow{8}{*}{Spontaneous activity in frequency domain} & Delta & High activity in schizophrenia patients \\ \cline{2-4}  & Alpha & High band power and high recovery \\  & & from discomfort \\ \cline{2-4}  & & & High activity in fathers of autistic \\  & & Beta & children \\ \cline{2-4}  & & & Inhibitory control \\ \cline{2-4}  & & & High activity with fearful pictures \\ \cline{2-4}  & & Theta & High activity in SAD participants \\ \cline{2-4}  & & & Higher activity in female than male \\ \hline \hline \multirow{8}{*}{EEG} & P1 & Larger amplitude with positive stimuli \\  & & than neutral in children with ADHD \\ \cline{2-4}  & & P2 N2 & Higher amplitudes for high arousal and \\ \cline{2-4}  & & negative valence stimuli in healthy \\ \cline{2-4}  & & & participants \\ \cline{2-4}  & & N4 & Absence in Asperger participants \\ \cline{2-4}  & & & Less N400 in Neurotic participants \\ \cline{2-4}  & & EPN & The less prominent the EPN, the better \\ \cline{2-4}  & & & the sad and afraid states were recognized \\ \cline{2-4}  & & & Amplitude reduction in reappraisal \\ \cline{2-4}  & & & conditions \\ \cline{2-4}  & & & Amplitude reduction in suppression \\ \cline{2-4}  & & & strategy (marker depression and \\ \cline{2-4}  & & & East-Asian descendants) \\ \cline{2-4}  & & & Increase with acceptance regulation \\ \cline{2-4}  & & & Increments marker for SAD and \\ \cline{2-4}  & & & schizophrenic participants, and low \\ \cline{2-4}  & & & empathy levels \\ \hline \hline \multirow{8}{*}{ECG} & \multirow{2}{*}{Time Domain} & HR & Decrease in fear (anterior cruciate \\  & & & ligament reconstruction) \\ \cline{2-4}  & & & Decrease while watching tender scenes \\ \cline{2-4}  & & RSA & No effect \\ \cline{2-4}  & & & Lower high frequency component in \\ \cline{2-4}  & & & positive, negative, and neutral stimuli \\ \cline{2-4}  & & & (BPD+PTSD). Decrease after negative \\ \cline{2-4}  & & & stimuli (anorexia nervosa) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Electrophysiological signals, methods and features analyzed for correlation with emotional states. In general, the experimental setups of the studies were based on the technical configurations summarized in Table 4.