EEG
The absence of relevant group differences in the autonomic state allows to exclude that the HEP amplitude was influenced in highs and lows differentially, although the modulation of heart rate and heart rate variability is not necessarily associated with HEP modulation (Park & Blanke, 2019b).
No significant group difference in the HEP amplitude was observed all over the scalp except for the right parietal site (P4). Earlier findings in the general population (Baranauskas et al., 2021; Luft & Bhattacharya, 2015) indicate that the right hemisphere is more involved than the left one in the heartbeat detection task (Katkin & Reed, 1988). The highs’ lower HEP amplitude at P4, with respect to lows, accords with their lower performance at the heartbeat detection test (Rosati et al., 2021). In this respect, a limitation of the study is the absence of medium hypnotizable participants, which prevents to associate the high or low hypnotizability with the performance of the general population (Jensen et al., 2017). The significant difference between highs’ and lows’ HEP observed in correspondence of sensori-motor areas (P4) is produced by negative and positive potentials in highs and lows, respectively. A possible interpretation of this finding, based on the bilateral connections of the posterior insula with the parietal cortex (Dionisio et al., 2019) is that the activation of the highs’ right parietal cortex by insular volleys may be reduced by a diminished contribution of the left insula. The grey matter volume (GMV) of the left posterior (Huber et al., 2014) or entire insula (Picerni et al., 2019), in fact, is negatively correlated with hypnotizability.
The increase in the HEP amplitude occurring at P4 in highs during hypnosis supports the view of hypnosis as a state of consciousness associated with enhanced attention to internal signals (Demertzi et al., 2015). In the general population, indeed, the HEP is higher during interoceptive than exteroceptive attention (Petzschner et al., 2019). The same mechanism could be involved in the maintenance of HEP amplitudes during REM sleep in the general population (Baranauskas et al., 2021). On the other hand, disengagement from environmental information during hypnosis (Kramer et al., 2014) could be responsible for larger interoceptive representation (Luft & Bhattacharya, 2015). The lows, in contrast, might decrease their HEP amplitude during the session owing to distraction due to their low absorption abilities.
The absence of significant difference in the HEP amplitude between the second part of the hypnotic induction (IND2) and neutral hypnosis can be due to the highs’ spontaneously entering the hypnotic state even independently from induction procedures. Unfortunately, an objective, discriminant index of hypnosis is not available yet.
The HEP amplitude at midline Cz (Condition effect) was lower during neutral hypnosis than in baseline and relaxation independently from the group. Cz roughly corresponds to the dorsal anterior cingulus which is deactivated during hypnosis (Deeley et al., 2012; Demertzi et al., 2011; Jiang et al., 2017; McGeown et al., 2009). Thus, in highs the HEP reduction could be accounted for by reduced excitability of this region. In lows, who did not experience hypnosis, the HEP decrease could be accounted for by reduced arousal due relaxation. In this respect a limitation of the study is that the experience of relaxation was not scored.