This study compared the body perception of SSD patients with that of healthy controls by using the mental tracking paradigm to measure IA. There was no statistically significant difference in IA between the SSD patients and healthy controls regardless of the absence or presence of emotional interference. These results are consistent with previous studies that showed similar IA between SSD patients and healthy controls [10]. However, regarding the extent to which facial interference affected IA, the difference in IA in the absence of interference and in the presence of neutral facial interference was more pronounced in SSD patients than in healthy controls. Previous studies have reported that neutral face photographs may be perceived as containing emotions in the experimental environment [32]. Therefore, our current findings are partially consistent with our hypothesis that patients with SSD have difficulty recognizing bodily signals when emotional processing is involved.
In this study, only the effect of neutral facial interference on IA was significant; the effects of happy or angry facial interference on IA were insignificant. Previous research has found that neutral faces were interpreted as ambiguous stimuli containing emotion rather than as stimuli without emotion [32]. Therefore, it is more affected by the emotional process of the subject who recognizes it [33]. Patients with somatoform disorder have also been found unable to accurately distinguish emotions from facial expressions [14]. This helps to explain why, in this study, the emotional processing difficulty of SSD patients was more pronounced when they were given emotionally ambiguous, i.e., neutral, facial interference. We speculate that the patients with SSD in this study experienced altered perception of their bodily signals due to the emotional interference caused by the neutral facial photographs. These speculations are consistent with the psychoanalytic explanations of somatization that individuals with somatization are unable to adequately deal with emotional information [12].
Additionally, the results of this study can be interpreted from the perspective of trustworthiness. We automatically form trustworthiness of other individuals through bottom-up emotion-attention interactions from facial features [34]. Previous studies reported that identifying trustworthiness from neutral facial stimuli is related to the function of the autonomic nervous system [35, 36]. SSD patients have been found to show alterations of the autonomic nervous system and low levels of trustworthiness [15]. Taken together, the distinct changes in IA for neutral face interference in this study may have been influenced by the differences in the propensity of trustworthiness readings from facial stimuli of SSD. This interpretation could be tested through further studies with psychological experimental designs involving trustworthiness.
SSD patients in this study showed lower HRV than the controls, suggesting that they experienced autonomic nervous system alterations. The HRV of SSD patients showed a significant correlation with IA. These results are consistent with recent studies that have reported the relationship between HRV and IA [37]. According to the neurovisceral integration model, HRV is known to reflect not only autonomic nervous system activity but also connectivity with the prefrontal cortex, such as the ventromedial prefrontal cortex or the orbitofrontal cortex [38]. The prefrontal cortical network, centered on the ventromedial prefrontal cortex, plays a major role in the regulation of interoceptive signals [39]. Taken together, the results of this study suggest that SSD patients’ prefrontal cortical network alterations may be related to their IA. In addition, previous studies have reported on the close associations between IA, HRV, empathy and emotion recognition [40–43]. Therefore, the correlation between IA and HRV specifically found in SSD patients supports that the pathophysiology of SSD is associated with IA and emotional processing. Future research, including investigation of emotional processing such as empathy and emotion recognition, will help to further expand the results of this study.
There are several limitations to consider when interpreting our findings. First, this study had few participants, making it difficult to derive significant, generalizable results. However, the SSD patients were relatively homogenous with no other major psychiatric illnesses, and the patient and control groups were demographically well matched. Second, this study measured IA using only one paradigm. Although the tasks used in this study proved useful in several previous studies, many complex factors could affect one’s ability to recognize one’s bodily sensations. Using a variety of tools, including the heartbeat discrimination paradigm, will help reinforce our results. Third, because the same paradigm was repeatedly performed, the results may have been impacted by the learning effect rather than the task condition alone.
In conclusion, patients with SSD showed more pronounced IA changes compared with healthy controls when neutral facial interference was given. The results of this study suggest that the perception of bodily signals in patients with SSD is not simply increased or decreased as a whole but may be influenced by various psychological factors, including emotionally ambiguous interference. Our current findings imply that disturbed emotional processing may contribute to the development and maintenance of somatic symptoms in SSD.