## 4 Discussion  
The primary aim of this study was to investigate the impact of a multimodal IVR intervention, which integrated cognitive training, physical exercise, and mindfulness practices, on HRV parameters in adults with PCC. Our main hypothesis was that participants undergoing the multimodal IVR intervention would exhibit significant improvements in both time- and frequency-domain HRV measures throughout the intervention period. More specifically, we anticipated a progressive increase in HRV from the baseline, mid-term, and end of the intervention, demonstrating a trend toward recovery and enhanced ANS function. We also sought to determine whether an extended intervention duration (24 sessions) would provide greater improvements in HRV than a shorter one (16 sessions). Contrary to our primary hypothesis, the study revealed an unexpectedly significant reduction in the time-domain HRV parameters from the baseline to the mid-term intervention. Several factors may account for this unexpected result. Firstly, the observed reduction in HRV could have reflected an overload response during the early phases of the intervention: a phenomenon that has been observed in other studies [51, 52]. For instance, reductions in HRV have been previously observed during the initial weeks of training and/or in the early stages of a rehabilitation program, followed by recovery when the training load was reduced. The cognitive demands imposed by the intervention probably also influenced this outcome. Cognitive training tasks, and particularly those requiring executive control, have been shown to significantly reduce HRV in comparison with tasks that do not require such high levels of executive performance [53, 54]. In our study, the cognitive training component was designed to engage participants in certain executive function tasks that demanded considerable mental effort and concentration. This may have led to a temporary reduction in HRV, particularly in the early sessions. Participants also engaged in physical exercise and mindfulness practices, both of which placed additional demands on cognitive and physical resources. Finally, mental fatigue, which is often associated with prolonged cognitive overload, is another factor that could have influenced HRV in the early stages of the intervention. The relationship between mental fatigue and HRV is complex and not entirely understood: reported study findings have been mixed. Some studies have suggested that mental fatigue may produce an increase in HRV [55, 56], while others found that prolonged dedication to cognitive tasks resulted in reductions in HRV [57, 58]. This inconsistency reflects the complex nature of mental fatigue and its impact on autonomic function. In addition, participants' lack of familiarity with the IVR technology used in the intervention could have triggered certain stress responses in the early sessions. The novelty of the IVR system, combined with the immersive nature of the intervention, may have caused some participants to experience heightened stress, which is known to be associated with reduced HRV [59]. According to the generalized unsafety theory [60], when an individual encounters a threat or stressor--such as an unfamiliar environment or task--the body initiates a default stress response. This induces physiological changes, such as an increase in heart rate and/or a reduction in HRV. From the technological side, exposure to the novel, multisensory stimuli inherent in IVR systems typically elevates attentional demands, promoting increased vigilance and autonomic arousal, which can transiently suppress HRV. Over time, as participants became more familiar with the IVR system and their perceptions of threat diminished, it is likely that their HRV levels began to recover, reflecting a reduction in their stress response. A secondary aim of this study was to determine whether increasing the number of multimodal IVR sessions (from 16 to 24 sessions) would produce greater improvements in HRV.