## 4 Discussion
### Intensity-Dependent Affective and Autonomic Modulation  
Analysis of the psychophysiological differences associated with perceived pain intensity indicated that individuals experiencing more intense pain reported greater negative valence and higher arousal (Table 6), suggesting that higher pain intensity corresponds to deeper negative emotional experiences. A study of chronic musculoskeletal pain showed a significant link between pain and negative affect, where increased pain predicted elevated negative emotions (sadness) [42]. However, this study does not match our current work perfectly. Furthermore, individuals who frequently experience pain exhibit attentional biases toward negative emotional stimuli [43; 44]. According to the four-stage pain processing model, increasing pain intensity elicits unpleasantness, encompassing negative emotions such as sadness or anger [45]. This underscores that pain is not solely a physical sensation, but also an inherently emotional experience. For instance, while some individuals can tolerate pain of a certain intensity, others find the same level overwhelmingly distressing. These differences are likely due to emotional reactivity. Consequently, an individual's mental state or capacity for emotion regulation can directly influence their pain experience. These observations suggest that psychological interventions, such as emotion regulation training or stress reduction programs, may be effective in pain management. They also highlight the importance of integrating physiological and emotional regulation strategies in pain interventions.  
From a physiological perspective, SCR and SCL were significantly elevated at higher pain intensities, indicating intensified SNS activation among individuals who perceived pain more acutely. SCR emerged as the most sensitive index of pain intensity changes, underscoring its potential as a key biomarker for emotional pain detection. These observations are consistent with previous findings on automatic physical pain detection [8], reinforcing the notion that emotional pain elicits robust autonomic responses, particularly within the SNS. Moreover, people reporting higher psychological pain tend to exhibit stronger emotional reactivity and perceive emotions more intensely. In practice, emotionally sensitive individuals may respond more vigorously to anger-inducing stimuli (such as unfair treatment and humiliating situations), leading to elevated SNS activity and stronger SC responses [46]. Similarly, those experiencing greater social rejection or psychological distress show higher SC activity [47]. These results reveal that individuals who perceived pain more intensely from the same stimulus exhibited heightened SNS activation, as evidenced by increased SCR and SCL. Thus, these results demonstrate that emotional pain elicits varying levels of physiological reactivity, depending on an individual's psychological state or emotion regulation capacity.  
Meanwhile, SDNN and PTT exhibited significant trends that may be linked to inhibition or delayed regulation of the PNS. Notably, the group reporting lower pain intensity tended to show higher PTT values, suggesting that reductions in peripheral blood flow may correlate with stimulus intensity. This finding also implies the potential influence of individual differences and psychological coping strategies. Furthermore, no distinctions were observed in HRV-related indices (RMSSD, HF, and pNN50) across varying pain intensities, indicating that the impact of emotional pain on HRV may be relatively minor or that large inter-individual variability hinders the attainment of statistical significance. The limited range of pain intensities in the experimental design and the possibility that subjective self-ratings and physiological responses may not perfectly align should be considered.