## 3 Results and Discussion
### Classification of Emotions
This makes emotion recognition in real-life circumstances a challenging area of research. Table 4 shows the AUROC values for each participant, for each of the emotions as a positive class. AUROCs larger than 0.50, marked in bold, indicate that the emotion can better be differentiated from the other emotions than a random classifier would do. Over all participants, the following emotions are well discriminable with the current approach: 'happiness' (discriminable for 77.8% of the considered participants),'stress' (100.0%) and 'calm' (100.0%). The emotions'surprise' (0.0%), 'guilt' (0.0%), 'address' (16.7%), 'under time pressure' (0.0%),'relaxed' (0.0%) and 'worried' (0.0%) were overall seemingly hard to discriminate. This research was the first to use mental heart rate as a derived physiological signal to monitor emotions in real-life circumstances. Other physiological signals already have been used for emotion recognition in lab circumstances. In the study of [PERSON], for example, six basic emotions were classified with an accuracy of 26% and eight non-basic emotions with an accuracy of 15%, based on fMRI measurements [18]. Based on HR measured with PPG in lab conditions, five emotions were recognized with an accuracy of 87.4%. However, these results can poorly be compared with the current result, because ambulatory assessment is more challenging than lab conditions and the number of measured emotions differs. In ambulatory assessment, positive and negative emotions could already be discriminated from each other with an accuracy of 82%, based on voice recordings in a call centre. However, this result can hardly be compared with the 49.7% achieved in the current study, because in the latter a much larger number of emotions were being classified. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline  & **P1** & **P2** & **P5** & **P6** & **P7** & **P8** & **P10** & **P11** & **P13** & **P25** & **P26** \\ \hline Happiness & 0.50 & **0.52** & **0.61** & **0.56** & 0.42 & 0.37 & & **0.57** & **0.60** & **0.67** & \\ Anger & & 0.13 & **0.62** & 0.27 & & 0.41 & & **0.51** & **0.62** & & \\ Surprise & & 0.23 & 0.44 & 0.32 & & & 0.12 & 0.43 & & & \\ Fear & 0.50 & 0.38 & & 0.38 & & & **0.60** & & & \\ Guilt & & 0.21 & & & & & & & & & \\ Sadness & & **0.57** & & 0.41 & & 0.48 & 0.12 & 0.33 & 0.32 & & \\ Love & & **0.58** & & 0.25 & & & & 0.50 & 0.32 & & \\ Longing & & & & & & & 0.38 & & **0.53** & & \\ Pride & & 0.20 & **0.62** & & & & & **0.52** & 0.32 & & \\ Shame & & & & & & & & **0.61** & 0.19 & & \\ _Stress_ & & & **0.72** & & & **0.69** & & & & & \\ _Under time_ & & & & & & 0.42 & & & & & 0.44 \\ _pressure_ & & & & & & & & & & **0.83** & \\ _Calm_ & & & & & & & & & & 0.32 & & \\ _Relaxed_ & & & & & & & & & & & 0.50 & **0.55** \\ _Energetic_ & & & & & & & & & & & 0.31 \\ _Worried_ & & & & & & & & & & & \\ \hline \end{tabular}
\end{table} 
Table 4: AUROC evaluation of the emotion classification. For each participant (P1–P26), every labelled emotion is considered as positive class. AUROCs larger than 0.50, marked in bold, indicate that the emotion can better be differentiated from the other emotions than a random classifier would do. The emotions in italics are the emotions that were not asked to be labelled but were added on initiative of the participants themselves.