Introduction
Hypnosis is a state of consciousness which can be self-induced or promoted through various procedures (“induction”) enacted by other persons (Elkins et al., 2015). It is described as different from the ordinary state of consciousness (Pekala et al., 2006) and cannot be defined independently from self-reports, although many cortical correlates have been observed by imaging (Landry et al., 2017; Wolf et al., 2022) and EEG studies (Baghdadi & Nasrabadi, 2012; Hiltunen et al., 2021; Rho et al., 2021; Yargholi & Nasrabadi, 2015). Neutral hypnosis (NH) is a state following hypnotic induction without specific suggestions, i.e., requests to imagine perception, memory and behavior different from the actual ones (for instance, analgesia, hallucination, movement) and to experience them as real and involuntary. According to the bio-psycho-social model of hypnosis (Jensen et al., 2015), the proneness to enter the hypnotic state and accept suggestions is influenced by contextual and individual factors, one of them being the psychophysiological trait of hypnotizability. It is substantially stable through life (Piccione et al., 1989), is measured by scales (Elkins et al., 2015) classifying high (highs, 15% of the general population), medium (mediums, 70%) and low (lows, 15%) hypnotizable individuals, and displays physiological correlates observable in the ordinary state of consciousness even in the absence of suggestions (Bocci et al., 2017; Ibanez-Marcelo et al., 2019; Rashid et al., 2022; Santarcangelo & Scattina, 2016; Spina et al., 2020).
Hypnotizability and hypnosis could be relevant to the integration of bodily signals with ongoing conscious and unconscious mental processes at high levels of the central nervous system (Quadt et al., 2018). Their integration, in fact, differs according to cognitive-emotional states (Gentsch et al., 2019; Kritzman et al., 2022) and traits (Judah et al., 2018; Zhou et al., 2022), and interoceptive information is conveyed to a few brain structures displaying hypnotizability-related morpho-functional differences, i.e., insula, cingulate cortex and cerebellum (Landry et al., 2017; Picerni et al., 2019). Moreover, the highs’ sensitivity to interoceptive signals - the self-reported mode of interpretation of bodily signals - is different and more “adaptive” than in lows and mediums, indicating a good relationship with the body and a tendency to positively interpret bodily signals (Diolaiuti et al., 2020). In contrast, the interoceptive accuracy - the ability to detect bodily signals -, which can be tested using the heartbeats count, is lower in highs than in lows, although only during the first of three heartbeats count trials (Rosati et al, 2021).
The results of heartbeat count have been associated with the amplitude of the cortical potential evoked by heartbeat (HEP) which has been related to cardiac afferents, despite the presence of some non-interoceptive information (Desmedt et al., 2018). It is obtained by averaging electrophysiological signals (such as the EEG and MEG) synchronized to R-peaks or T-peaks of a simultaneously recorded ECG signal (Park & Blanke, 2019b). In contrast to exteroceptive cortical potentials, the HEP amplitude increases during deep sleep (N3) with respect to light sleep, and is like N2 during REM sleep (Lechinger et al., 2015), thus overcoming the effect of the thalamic gate during N3 and of the disrupted brain activity during REM. The amplitude of the HEP earlier component, which reflects cardiac interoceptive accuracy (200-350 ms), has been associated with heartbeat counting scores, although not unanimously (Park & Blanke, 2019b), and is more pronounced at the medial-right fronto-central sites. The amplitude of the later component (400-600 msec) is related to the proneness to not worry about body sensations, to stress-induced changes in cardiac output, emotional arousal, dysregulation of emotions  and some clinical disorders (Baranauskas et al., 2021; Luft & Bhattacharya, 2015).
Since the heartbeat count indicated lower interoceptive accuracy in highs than in lows (Rosati et al., 2021), the aim of the present study was to assess whether this finding is supported by differences in the HEP amplitude, and whether the HEP amplitude changes during neutral hypnosis in highs and lows differentially.