## 7 Discussion  
The presented review reported the approaches adopted in scientific research regarding the evaluation of embodiment in MR. As described in the Introduction section, MR is a technology of enormous potential in many kinds of applications, especially in the era of the metaverse, in which it will be possible, and more convenient from many points of view, to perform in the virtual world many activities (e.g., training) that until now were performed exclusively in physical environments. In this regard, the great versatility and effectiveness to generate virtual scenarios with critical situations with a very high realism could, in fact, revolutionize current training and learning practices of users. Therefore, the present review aimed at identifying the scientific works proposing approaches for analyzing the effects of MR on cognition, and assessing how the MR is perceived as close to the real world by the users (i.e., embodiment). The presented work was conducted with a particular focus on the neurophysiological evaluation of embodiment in MR. In fact, with respect to subjective and behavioral ones, the neurophysiological-based measurements offer deeper insights into the underlying neural mechanisms, detect subtle changes in embodiment perception, and potentially reveal unconscious processes. Among the wide range of emerged methods, it appears to be clear that the measurements derived from the cerebral activity are the most promising for obtaining an objective index for evaluating embodiment variations within virtual environments. In this regard, several EEG spectral indexes were demonstrated to be significantly sensitive to embodiment variations, both in VR and AR environments. Recent research showed how the increase of the EEG PSD computed in alpha band is frequently related to the high-embodiment experience in MR. Similarly, the increase of the EEG PSD computed in theta and alpha bands within the central region is significantly correlated with high-embodiment perception while interacting with virtual body entities. In fact, previous research demonstrated that the central region is strictly representative of the motor cortex activity. Therefore, the cerebral activity increase within such a region can be consistently related to high-embodiment perception, since it represents the fact that the user is perceiving the virtual entity (e.g., a virtual arm and/or hand) as part of his own physical body, as confirmed by the high and significant correlation between the neurophysiological and behavioral features [33; 60]. In this context, it was also deeply investigated the impact of the emotional interaction on the sense of presence in MR. More specifically, different research demonstrated how the combination of EEG spectral features computed in theta and alpha bands are significantly sensitive to embodiment variations while interacting with emotive avatars in MR [31; 47]. Additionally, a further consistent contribution that emerged from the scientific literature showed how the time-locked EEG features are capable of providing a significant contribution to the embodiment assessment \begin{table}
\begin{tabular}{c c c} \hline \hline
**Type** & **Feature** & **Results** \\ \hline \multirow{4}{*}{**Cardiac**} & Normalized HR & High correlation with the increase of spatial presence in VR and AR. \\ \cline{2-3}  & Normalized HF & High correlation with the sense of presence in VR. \\ \cline{2-3}  & Normalized LF & Negative and significant correlation with the increase of embodiment in VR. \\ \hline \multirow{2}{*}{**Electrodermal activity**} & Normalized SCL (i.e., phasic EDA) & Positive and significant correlation with the embodiment increase in VR and AR. \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Overview of the autonomic (i.e., cardiac and electrodermal) correlates of the embodiment variations in MR that emerged from the considered scientific literature.